Symposium Organizers
Zhenan Bao Stanford University
Anna B. Chwang Universal Display Corporation
Lynn Loo The University of Texas-Austin
Rachel A. Segalman University of California-Berkeley
M1: Novel Organic Materials
Session Chairs
Tuesday PM, April 18, 2006
Room 3001 (Moscone West)
9:30 AM - **M1.1
Novel Electroactive Star-shaped Molecules
Mary Galvin 1 , Yashpal Bhandari 3 , Olga Zolotarskaya 3 , Hermona Christian-Pandya 3 , Frederick Beyer 4 , Zukhra Niazimbetova 2
1 , Air Products, Allentown, Pennsylvania, United States, 3 Materials Science and Eng., University of Delaware, Newark, Delaware, United States, 4 Multifunctional Materials Branch, WMRD, Army Research Lab, APG, Maryland, United States, 2 , Rohm and Haas Electronic materials LLC, 455 Forest St., Massachusetts, United States
Show AbstractWhile electroactive organics are being used in light-emitting diodes and being considered for use in photovoltaic devices, the fundamental structure/property relationships that govern their performance are still poorly understood. One relationship our group is investigating is whether star molecules offer any advantage over linear polymers. Specifically, we are investigating the role of architecture, synthesizing some novel molecules that contain a central tetra substituted phenyl ring. These molecules have a tendency to pi-pi stack, further delocalizing the carriers. The synthetic strategy used to prepare these molecules is versatile so that the four arms in the molecules do not have to be identical. The placement of differing arms also affords the opportunity to study the effect of symmetry on the properties of these molecules. Recent results obtained with these materials and their applications in photovoltaics and light-emitting diodes will be described.
10:00 AM - M1.2
Synthesis and Device Performance of New Liquid Crystalline Fluorophenyl Thiophene Oligomers.
Michael Turner 1 , Franklin Jaramillo Isaza 1 , Jonathan Maunoury 1 , Rodney Baldwin 2 , Theo Kreouzis 2
1 School of Chemistry, University of Manchester, Manchester United Kingdom, 2 Department of Physics and Astronomy, Kings College , London United Kingdom
Show Abstract10:15 AM - M1.3
Fullerene (C60) Functionalized Polymers for Probing Organic Photovoltaic Performance
Kevin Sivula 1 , Zachary Ball 2 , Nobuhiro Watanabe 2 , Dave Kavulak 2 , Jean Frechet 1 2
1 Chemical Engineering, University of California, Berkeley, California, United States, 2 Chemistry, University of California, Berkeley, California, United States
Show AbstractThe morphology, and electrical properties of the active layer are two areas of great interest in organic bulk heterojunction photovoltaic devices made from a blend of poly(3-hexylthiophene) (P3HT) and the fullerene derivative PCBM. In an effort to control the nanoscale morphology and the electrical properties of these devices we have designed, prepared, and tested several new fullerene-containing polymers and copolymers. The approaches used have included direct polymerization of C60-containing monomers to create homopolymers and copolymerization with macromonomers with P3HT pendant groups. Solution processible materials with high fullerene content (>50 wt%) were obtained. We show that these new materials used in combination with P3HT and PCBM enable control over the length scales between blend components in the photovoltaic devices.
10:30 AM - **M1.4
Processing of Graphene Molecules for Device Applications.
Wojciech Pisula 1 , Klaus Mullen 1
1 , Max-Planck-Institute for Polymer Research, Mainz Germany
Show AbstractOrganic electronics are emerging as possible technology options for creating new, improved electrical devices. Components made from such organic materials could be cheaper and easier to manufacture than their silicon counterparts. Thereby, the utilization of conjugated oligomers or polymers is one major concept. The exploitation of molecules with extensive non-covalent interactions is an alternative to prepare large domains by supramolecular organisation. For this approach, it is a prerequisite to orient the molecules adequately on a substrate with respect to their function in the device and put them in electrical contact with electrodes. A prominent class of compounds, the polycyclic aromatic hydrocarbons (PAHs), utilizes aromatic stacking to form one-dimensional columnar superstructures, which possess semi-conducting properties along the stacking direction. The substitution of these disc-shaped molecules by aliphatic chains results in phase formation leading to mesophases. Discotic hexa-peri-hexabenzocoronenes (HBC), as mesogens, exhibit some of the highest values for the intrinsic charge carrier mobility, which allowed the successful implementation in photovoltaic devices and field-effect transistors (FET). Thereby, different device geometries necessitate a specific macroscopic alignment of the molecules with respect to the substrate. In FETs, the charges are migrating between source and drain parallel to the substrate, which requires an edge-on orientation of the molecules. On the other hand, a molecular face-on orientation, which leads to the formation of a homeotropic phase, is thought to improve the efficiency of photovoltaic cells or could be interesting as hole-injection layers for organic LEDs. The control of the molecular orientation and therefore the alignment of the charge carrier pathways constitute a task of major importance for material scientists, because it sensitively determines the performance of the electronic device.
11:30 AM - **M1.5
Synthesis and Design of Conjugated Polyelectrolytes for Applications in Biosensor Design and Novel Device Fabrication Techniques
Guillermo Bazan 1 , Bin Liu 1 , Janice Hong 1 , Xiong Gong 1 , Paramaswad Iyer 1 , Alan Heeger 1
1 Materials, University of California, Santa Barbara, California, United States
Show AbstractConjugated polyelectrolytes can be defined as polymers having a backbone with a π-delocalized electronic structure and pendant groups with functionalities capable of ionizing in a high dielectric medium. These materials have solution and solid state properties that are difficult to predict from simple molecular structure considerations since they combine the well-known complexity of polyelectrolytes, for which physicochemical properties depend on variable long-range electrostatic interactions, with the rigid and highly hydrophobic nature of conjugated polymers. Despite this complexity, it is possible to design cationic conjugated polyelectrolytes whose emission frequency responds to the degree of aggregation in solution. This property, coupled with the well established complexation of oppositely charged polyelectrolytes, can be used to design polymer structures that are responsive to very low concentrations of single stranded- or double stranded-DNA. The mechanism of action involves enhanced interchain energy transfer within the conjugated polyelectrolyte/DNA aggregate. Highly accurate determination of DNA concentration can be obtained in this manner and control at the synthesis stage is important for adjusting molecular features that achieve maximum sensitivity. Fine control of molecular structure can also be used to design conjugated polyelectrolytes with properties that make them useful in the fabrication of organic light emitting diodes (OLEDs) by solution methods. Multilayer structures can be constructed with little disturbance of underlying layers by using a casting technique that alternates solvent polarity. Cationic copolymers with the general poly(fluorene-alt-Ar) structure can be prepared for this purpose, where the Ar component can be chosen so that the material is suitable for either electron transport or hole transport (injection) layers. By extending the technique of casting from solvents of different polarities to oligomers and multicomponent conjugated polymer blends, it is possible to fabricate multilayer phosphorescent OLEDs with white emission, high efficiency and good color stability. The choice of materials includes a sulfonated electron transport oligomer, a blend containing a blue emitting poly(fluorene) with electron transport end groups and an anionic version of poly(vinylcarbazole) for the hole injection layer.
12:00 PM - M1.6
Nanoscale Charge Transport in Conjugated Polyelectrolytes.
Thuc-Quyen Nguyen 1 , Guillermo Bazan 1 , Andres Garcia 1 , Ranqiang Yang 1
1 Chemistry and Biochemistry, UCSB, Santa Barbara, California, United States
Show AbstractIn recent years, conjugated polyelectrolytes (charged conjugated polymers) have been used as the key optical component for the amplification of fluorescent biosensors and as electron transporting layers in Light-Emitting Diodes (LEDs). Conjugated polyelectrolytes have a hydrophobic conjugated backbone and anionic or cationic pendant groups. Therefore, these polymers are soluble in water and polar solvents such as methanol, dimethylformamide, and dimethylsulfoxide. In polar solvents, the polymer chains presumably pack in a way that minimizes exposure of the hydrophobic backbones, whereas in an organic solvent, they pack in a way to minimize exposure of the charged groups. Thus, the charge groups provide additional parameter for tuning the polymer conformation and chain packing that in turn influence the charge transport property. In this work, a combination of various scanning probe techniques (atomic force microscopy, electrostatic force microscopy, and conducting atomic force microscopy) is used to characterize the electrical properties at the nanoscale and thin film morphology of a charged conjugated polymer, poly[(9,9-bis(6'-N,N,N-trimethylammonium)hexyl)fluorene-alt-4,7-(2,1,3-benzothiadiazole)] (PFBT). Films cast from PFBT can carry much higher current than films cast from its neutral counterpart. Also, the charge mobility strongly depends on the type of counterion used.
12:15 PM - M1.7
Functionalised Poly(3,4–ethylenedioxythiophene) Derivatives For Sensing Applications.
Simon Higgins 1 , Fouzi Mouffouk 1
1 Chemistry, University of Liverpool, LIVERPOOL United Kingdom
Show AbstractPoly(3,4–ethylenedioxythiophene) (PEDOT) is a commercially-successful polythiophene derivative with a very low oxidation potential, favorable optical properties and extraordinary stability in its oxidised (p-doped) form. It therefore makes an idea platform for electrochemical (bio)sensing. We have synthesised EDOT derivatives bearing covalently-attached carboxylic acid, metal–tetraazamacrocycle complexes or biotin. These have been electrochemically copolymerised with EDOT itself, to give conducting polymer-modified electrodes, which have been characterized by electrochemical and in situ spectroscopic methods. The biotinylated PEDOT, on 10 μm radius Pt disk microelectrodes, responds electrochemically to the binding of avidin at the 10(–16) mole level. The carboxylated PEDOT has been used for in situ grafting of aminoalkyl-terminated oligonucleotides, and the electrochemistry of these polymers undergoes marked and reproducible changes upon hybridisation with the complementary oligos.
12:30 PM - **M1.8
Amplifying Fluorescent Polymers
Timothy Swager 1
1 Chemistry, MIT, Cambridge, Massachusetts, United States
Show AbstractThis lecture will describe the conceptual design and optimization of chemical/biological sensors based upon conjugated polymers. The ability of a conjugated polymer to produce gain (amplification) in a fluorescence- or resistance-based chemo-, bio-sensor stems from its ability to transport optical excitations or electrical charge, respectively, over large distances. These transport properties provide the increased sensitivity and versatility of conjugated polymers over small-molecule chemosensors. In a fluorescence sensor, the migration of an optical excitation increases the probability of an encounter with an occupied binding site. We originally demonstrated this scheme making use of analyte induced quenching. We have also been interested in using local reductions in the polymers bandgap (Eg) to produce wavelength shifts in the polymers emission. Previous designs described by us and others have focused on avoiding interchain interactions (3-D electronic interactions) with leads to self-quenching of fluorescence and inherent compromises in the mobility of charge and energy throughout the medium. Other new concepts will be reported including new sensors that derive additional sensitivity from attenuated lasing behavior. Methods for the detection of biological analytes including determining protease activity will also be presented.
M2: Organic Based Sensors, Self-Assembled Systems, and Organic Semiconducting Wires
Session Chairs
Tuesday PM, April 18, 2006
Room 3001 (Moscone West)
2:30 PM - **M2.1
Biosensors: The Next Big Thing for Organic Electronics?
George Malliaras 1
1 Materials Science, Cornell University, Ithaca, New York, United States
Show AbstractThe application of organic semiconductor devices to chemical and biological sensors seems to be a good idea. Organics offer facile processing, which can result into low fabrication cost, which is a requirement for disposable sensors. Chemical synthesis can be used to tune their electronic properties and to attach chemical and biological receptor sites, which can lead to high sensitivity and specificity. Low end performance, which prohibits organic transistors from competing with silicon, is not a limitation in sensors. Also, lifetime issues which plague usually organics, are avoided in disposable sensors. One of the most promising approaches towards organic based sensors involves the use of thin film transistors. The mechanism of operation of organic transistor-based sensors will be examined. The influence of device geometry on the sensitivity will be discussed. Various approaches for introducing specificity will be reviewed.
3:00 PM - M2.2
Sensitivity of Organic Field Effect Transistor (OFET) Chemical Vapor Sensors.
Cynthia Burham 1 , Ananth Dodabalapur 1
1 Electrical & Computer Engineering, The University of Texas at Austin, Austin, Texas, United States
Show AbstractSensors able to accurately detect trace chemical vapor (analyte) concentrations are important components of many military, medical, and industrial applications. OFETs are cost-effective, simply fabricated, and easily fabricable alternatives to commercially utilized inorganic sensors; an OFET’s suitability for a particular application will depend upon its selectivity, sensitivity, and stability in ambient. Low range analyte response must be established in order to determine an organic semiconductor’s suitability for an intended application. The research addressed in this talk establishes that OFETs fabricated with a copper phthalocyanine (CuPc) semiconducting layer are responsive to ethanol, a polar alcohol commonly produced by biological processes, in concentrations as low as 16 parts per million (16 PPM) with detection of lower concentrations probable. The devices completely recover their pre-exposure current when reverse biased for a period equivalent to half the initial exposure time. The detectable ethanol concentration is well below the concentration necessary for commercial applications such as the Breath Alcohol Concentration (BrAC) inebriety test and detection of dangerous bacteria levels in canned comestibles. Low concentration analyte response was determined using nitrogen as both a carrier and diluent gas in order to establish an optimal response range to which results obtained under less than ideal conditions may be compared. The talk will also address the use of receptors and semiconductor chain length variation to increase sensor selectivity and sensitivity.
3:15 PM - **M2.3
Alkoxy Substituted Conjugated Systems Thin-film Transistor Sensors.
Luisa Torsi 1 2 , M. Cristina Tanese 1 , G.M. Farinola 1 , B. Pignataro 3 , F. Babudri 1 5 , F. Naso 1 5 , L. Sabbatini 1 2 , P.G. Zambonin 1 2
1 Dipartimento di Chimica, Universita' degli Studi di Bari, Bari Italy, 2 Centro di Eccellenza TIRES, Universita' degli Studi di Bari, Bari Italy, 3 Dipartimento di Chimica Fisica "F. Accascina", Universita' degli Studi di Palermo, Palermo Italy, 5 CNR ICCOM, Dipartimento di Chimica , Universita' degli Studi di Bari, Bari Italy
Show AbstractThere is a growing interest in the use of Organic Thin-Film Transistors (OTFT) as chemical and biological sensors [1] as demonstrated by the number of groups presently contributing to this field. Such devices have been proposed as multi-parameter sensors and it has been demonstrated that the responses are very repeatable. Besides, it is generally recognized that a field-induced response enhancement takes place. These properties can make OTFTs better performing sensing devices then chemiresistors. Critically important is to have also a tunable selectivity toward target chemical or biological substances. One approach to confer selectivity to OTFT sensors involves the use of ad hoc chemically or biologically functionalized conjugated polymers or oligomers as active layers. A successful example of OTFTs chemical selectivity toward classes of homologous analyte was in fact recently demonstrated with poly(alkylthiophene)s [2]. Subsequently, a more versatile alkoxy phenylene-thienylene backbone substituted with glucose or aminoacid side groups has been investigated as chiral sensing layer. One of the major challenges of this approach is to obtain selectivity preserving backbone conjugation. Indeed, the presence of bulky substituents, or even of simple alkoxy side chains, can lead to the reduction of the active layer conductivity as well as of the field-effect properties. In this presentation a detailed study of the strategies adopted to face and partially solve this problem is presented. In particular it will be shown how deposition procedures such the Langmuir-Schäfer (LS) one can greatly enhance the field-effect properties of alkoxy substituted thin-films [3]. This will be evidenced by comparing thin-film deposited by cast and LS procedure. A rationale for this finding will be also proposed relaying on a detailed morphological, spectroscopic and electrical characterization of the sensors active layers. A multi layer configuration to realize OTFT biosensors will also be presented. [1] Luisa Torsi and Ananth Dodabalapur, Analytical Chemistry 70, 381A-387A (2005)[2] L. Torsi, M.C. Tanese, N. Cioffi, M.C. Gallazzi, L. Sabbatini, P.G. Zambonin, G. Raos, S.V. Meille, M.M. Giangregorio, J Phys. Chem B 107, 7589-7594 (2003). [3] M. C. Tanese, G. M. Farinola, B. Pignataro, L. Valli, L. Giotta, S. Conoci, P. Lang, D. Colangiuli, F. Babudri, F. Naso, L. Sabbatini, P. G. Zambonin and L. Torsi, Chemistry of Materials, submitted (2005).
3:45 PM - M2.4
Self-Assembled Host-Guest Materials for Organic Electroluminescence.
Pavel Anzenbacher 1
1 Chemistry, Bowling Green State University, Bowling Green, Ohio, United States
Show AbstractThe device performance of OLEDs based on host-guest materials depends dramatically on the distribution of the guest or dopant molecules in the bulk of the host. Unfortunately, precise control over guest/dopant distribution during processing of such materials is nearly impossible and the resulting materials often display phase separation, which, in turn, limits the device performance.Because the synthesis of host materials with covalently incorporated guests or dopants is relatively difficult to perform and one runs a relatively high risk of developing structural irregularities that may later act as energy traps, we designed materials capable of self-assembling from short segments in the solution to form alternating host-guest domains. The advantage of this approach is that one synthesizes only shorter components, which are less likely to develop structural defects that may act as exciton traps in the final material.The self-assembled host-guest materials are designed to process the energy of both singlet and triplet states of electroluminophores aimed at harvesting maximum energy of the electron-hole recombination events and improve the efficacy of electroluminescence in the resulting device. Recent advances in understanding the singlet/triplet exciton migration and diffusion are taken into account in the design of the materials components. This is particularly daunting as one must try to anticipate the structure and spatial orientation of materials components not only in the solution, but in the solid state. Our results show that the solid state is more conducive to triplet energy transfer than one would expect purely on triplet diffusion models. These results suggest that inter-component pathways are perhaps even more effective that intra-component ones as far as triplet-triplet energy transfer is concerned. In this presentation, we will describe our progress toward the synthesis, understanding the fundamental relationships between components, structure, and topology in self-organized electroluminescent coordination polymers that can be potentially useful in the development of organic light-emitting diodes (OLEDs).
4:30 PM - **M2.5
Self-assembled pi-conjugated Systems.
Albert Schenning 1
1 , Eindhoven University of Technology, Eindhoven Netherlands
Show AbstractSupramolecular chemistry, which makes use of self-assembling molecules offers an excellent tool to construct cheaply and easily components in the nanometer range. If pi-conjugated oligomers are used as building blocks one can expect new, unprecedented electronic properties. During my presentation, examples will be given on how pi-conjugated materials like oligo(p-phenylenevinylenes)s and oligothiophenes can be self-assembled, manipulated and applied in nanosized, supramolecular electronics.
5:00 PM - M2.6
Self-Organized Single-Crystal Polythiophene Microwires for High-Performance Supramolecular Electronics
Kilwon Cho 1 , Do Hwan Kim 1
1 Chemical Engineering, Pohang University of Science and Technology, Pohang Korea (the Republic of)
Show AbstractControl over supramolecular organization of electronically active π-conjugated polymers provides great opportunities to fine-tune and optimize their electrical properties for applications in organic field-effect transistors (OFETs) and sensors. Typically, obtaining high-quality single-crystals with conjugated polymers via facile solution processing is more challenging than with oligomers or small molecules which can be prepared by vacuum processes. However, one-dimensional (1D), high-quality single-crystal semiconductors comparable to inorganic single-crystals such as silicon nanowires have not hitherto been observed in conjugated polymers. Here we show a well-faceted, high-quality 1D single-crystal poly (3-hexylthiophene), P3HT microwire with unprecedented electrical characteristics such as a low resistance (0.5 MΩ), a channel current as high as 25 μA, and a well-resolved gate modulation via solution growth. We find that 1D single-crystal P3HT microwires are formed spontaneously through facile self-assembly of individual polymer chains, adopting preferential well-ordered inter-chain stacking along the wire axis. Our findings indicate that π-conjugated polymer single-crystals are capable of very efficient charge transport. This approach could lead to the development of chemical and biological sensors which are efficiently capable of electrical and /or optical monitoring. In short, we are quite optimistic that the findings presented in this research are capable of opening up new avenues in high-performance supramolecular electronics.Acknowledgement. This work was supported by the National Research Laboratory Program and ERC Program (R11-2003-006-03005-0) of the MOST/KOSEF, a grant (F0004022) from Information Display R&D Center under the 21st Century Frontier R&D Program of the Ministry of Commerce, Industry and Energy of Korea, and the BK21 Program of the Ministry of Education and Human Resources Development of Korea, and the Pohang Acceleratory Laboratory for providing the synchrotron radiation source at the 4C2, 3C2, and 8C1 beam lines used in this study.
5:15 PM - M2.7
Tuning Electrical Properties of PEDOT Nanowires with the Diameter.
Jean-Luc Duvail 1 , Yunze Long 1 2 , Zhaojia Chen 2 , Changzhi Gu 2
1 , Institut des Materiaux, Nantes France, 2 Institute of Physics, Chinese Academy of Sciences, Beijing China
Show AbstractIn this study, it is shown that electrical properties of single poly(3,4-ethylenedioxythiophene) (PEDOT) nanowires can be tuned through the critical regime from an insulating to a metallic state by changing the nanowire diameter. Properties of nanowires and nanotubes made of conjugated polymers are much investigated at present for both fundamental and applied reasons. The resulting nanomaterials are promising as support for bio- and chemical sensors, field emission displays, nanodevices like single electron tunneling devices and nanoLEDs. It is thus crucial to understand and to control the mechanisms responsible for their improved properties in comparison to the bulk materials. Among them, electrical properties are of importance as many applications are based on the electrical behavior. The electrical conductance of conjugated polymers depends strongly on the doping level, the molecular and supermolecular structure, all parameters which can be drastically modified by the method of synthesis. Previous works on template-prepared polymer nanowires have shown an improvement induced by the confined growth in the molecular and supermolecular structure for the smaller nanowire diameters. But the effects of the nanowire diameter on their electrical behavior are still a controversy because contrary observations have been reported. Part of this controversy is due to the fact that the conductivity was not directly measured on a single nanowire but measured on pellets or by two-probe measurements on thousands of nanowires simultaneously. Here, by four-probe measurements on single nanowires, we show unambiguously that the electrical behavior of PEDOT nanowires prepared by the template method depends dramatically on the nanowire diameter in the range [55 – 190] nm. Both the room temperature conductivity and the temperature dependence of the conductivity indicate that the electrical conduction is dramatically improved from an insulating to a metallic regime when the nanowire diameter decreases from 190 to 55 nm nanowires. Such an electrical behavior is attributed to an improvement in the molecular and supermolecular structure induced by the confined synthesis, as shown by a Raman spectroscopy study. Moreover, by Electron Spin Resonance study, we evidence a decrease in the spin/charge ratio when the nanowire diameter goes from 100 nm to 50 nm. This decrease attributed to an increase in the bipolaron population ratio, corroborates the improved chain ordering for the smaller nanowire diameters. These results are of importance for devices based on conjugated polymer nanostructures.
5:30 PM - **M2.8
Soluble Polyrotaxanes Based On Conjugated Oligomers/Polymers And Cyclodextrines: Towards Single Chain Semiconducting Wires And Devices Thereof.
Georges Hadziioannou 1 , Guillaume Bonnet 1 , Cyril Brochon 1 , Thomas Heiser 2 , Rony Bechara 2 1
1 , ULP-ECPM, Strasbourg France, 2 , INESS-CNRS, Strasbourg France
Show AbstractIn the field of electronic engineering, miniaturisation is one of the prime focal points. This will reach a limit in the near future due to physical limitations. Molecular electronics can be an alternative to these limitations. A molecular wire, the simplest and smallest possible electronic component can be represented by a one-dimensional molecule as a conjugated polymer chain.Synthesis of a soluble conjugated polymer is a first priority for their application in molecular electronics. The substitution of solubilising groups on the conjugated backbone is one of the most used strategies for obtaining soluble conjugated polymers. This can have serious undesirable consequences over conductivity.Instead of modifying the polymer backbone with substituents, the strategy is here to form polyrotaxane with a conjugated polymer as backbone and cyclodextrins (CD’s : cyclic oligosaccharides containing glucose unit, the symbols α, β and γ indicate CD’s with 6, 7 and 8 glucose units. CD’s are molecules with a toroidal shape, complexation properties, and hydrophilic exterior) as rings to form a compact sheath. This strategy not only leads to soluble conjugated polymers but also gives the opportunity to create insulated single chain semiconducting polymers. Previously, synthesis of soluble conjugated oligomers using a combination of the polyrotaxane with cyclodextrins and electrolyte side groups has been discussed in the literature. This elegant approach does not offer, however, the possibility to have a maximum coverage with the CD’s, an essential point towards the fully insulated single polymer semiconducting chain. Our approach differs on two points; first we expect that the good solubility of our molecular wire will only be due to a high coverage in cyclodextrin instead of adding to the chain some electrolyte substituents and few CD’s. Secondly, we use a totally different polymerization reaction which takes place in an organic solvent like DMF instead of water to obtain long soluble non substituted polymer chains.In this presentation the synthesis the visualisation and preliminary charge transport experiments for polythiophene (PT) based insulated semiconducting single polymer wires with a maximum coverage of β-CD’s as the insulating sheath will be described.
M3: Poster Session: Organic Conductors: Synthesis, Structure, and Morphology
Session Chairs
Wednesday AM, April 19, 2006
Salons 8-15 (Marriott)
9:00 PM - M.37
Electronic Properties and Device-forming Capabilities of terbium tris-(tetraphenylimidodiphosphinate) and Related Complexes.
Sondra Hellstrom 1 , Gaetano Mancino 1 , Andrew Ferguson 1 , Nicholas Long 1 , Tim Jones 1
1 , Imperial College London, London United Kingdom
Show AbstractOrganolanthanide complexes are promising for the development of lanthanide-based light-emitting devices for several reasons. Firstly, if a lanthanide ion is complexed to organic ligands capable of capturing charge carriers and transferring their energy to the ion, the selection rules forbidding the emissive f-f transition are relaxed, and a higher degree of luminescence can be obtained than from a lanthanide not sensitized in this manner.1 Secondly, devices based on organolanthanide complexes offer many benefits associated with organic light-emitting diodes (LEDs) in general, including flexibility and ease of manufacture. Lastly, provided adequate sensitization remains, different lanthanides can be used with similar ligands to alter the colour of a device without greatly affecting its charge transport properties.2 When designing an organolanthanide complex for use in LEDs it is important, especially for near-infrared (NIR) emitters, that the ligands shield the lanthanide ions from solvent coordination and keep C-H and O-H bonds, which quench NIR emission, out of the inner coordination sphere. Recent studies indicate that complexes of the form Ln(tpip)3, where tpip is tetraphenylimidodiphosphinate, serve these purposes well.3,4 The electronic structure and properties of Ln(tpip)3 are, however, still largely unknown. Further, few attempts to incorporate these materials into devices have been made; these attempts indicate difficulty keeping electron-hole recombination contained in the organolanthanide layer. Even with careful control over layer thicknesses, a significant portion of their output radiant energy originates from emissive charge-carrying layers.5 The first study undertaken here is an analysis of the electronic and structural characteristics of the Ln(tpip)3 complexes. Thin film morphology is characterized, and energy levels ascertained via cyclic voltammetry. A variety of device structures are then studied. These devices investigate the poor charge confinement specifically in Tb(tpip)3 and indicate possible remedies. An LED with emission exclusively from the lanthanide is obtained. Finally, Ln(tpip)3 devices are shown to work similarly for several different lanthanides, and the possibilities of colour tuning via blending of different Ln(tpip)3 complexes are explored. It is hoped that in the future this work will lead to optimization of device structures based on Ln(tpip)3, and therefore to development of inexpensive, efficient visible and near-infrared organolanthanide-based LEDs.References:1. Kido, J. et. al., Chem. Rev., 2002, 102, 2357-2368.2. Hong, Z. et. al., Appl. Phys. Lett., 2001, 79, 13, 1942.3. Magennis, S.W. et. al., Chem.-Eur. J., 2002, 8, 5761-5771.4. Mancino, G. et. al., J. Am. Chem. Soc., 2005, 127, 524-525. 5. Christou, V. et al., Synth. Met., 2000, 111, 7–10.
9:00 PM - M3.1
Pathways of Degradation in Solution Processable Pentacene Derivatives.
Paolo Coppo 1 , Stephen Yeates 1 , Gonzalo Rincon Llorente 1 , Khalid Alamry 1
1 Chemistry, University of Manchester, Manchester United Kingdom
Show AbstractPentacene is one of the most widely investigated organic semiconductors, showing hole mobility often exceeding that of amorphous silicon.[1] In addition, solution processable precursors and derivatives of pentacene have attracted tremendous interest for application in organic and flexible electronics, due to the possibility of low manufacturing costs via spin coating and printing techniques.[2] In spite of the very high performance achieved for pentacene and its derivatives as thin films in field effect transistors, commercial application is hampered by the lack of stability of pentacene and its derivatives to light and oxygen.[3] Although photo-oxidation of pentacene, via the formation of endo-peroxides is a well established pathway of degradation, little is known about the degradation of pentacene in the absence of oxygen. Pentacene derivatives are particularly appealing for application in photovoltaics, given the high absorption coefficients in the visible region of the spectrum and the exceptional semiconducting properties. Solar cells are generally encapsulated and protected from the environment, but clearly stability to light is an essential requirement for the semiconductor. In this contribution, the pathways of photo-degradation of solution processable pentacene derivatives are discussed, giving evidence that photo-induced dimerization in a “butterfly fashion” is responsible for loss of performance of the material when exposed to light only. Conclusions are supported by spectroscopic evidence, as well as NMR and Mass spectrometry.[4] Photo-induced dimerization was found to occur both in solution and in solution processed thin films. Strategies to improve the stability in this type of materials via control of film morphology are presented, with the aim of providing guidelines for the preparation of stable thin films. References[1] C. D. Dimitrakopoulos, P. R. L. Malenfant, Adv. Mater. 2002, 14, 99.[2] a) C. D. Sheraw, T. N. Jackson, D. L. Eaton, J. E. Anthony, Adv. Mater., 2003, 15, 2009. b) A. Afzali, C. D. Dimitrakopoulos, T. O. Graham, Adv. Mater. 2003, 15, 2066.[3] A. Maliakal, K. Raghavachari, H. Katz, E. Chandross, T. Siegrist, Chem. Mater. 2004, 16, 4980.[4] P. Coppo, S. G. Yeates, Adv. Mater., in press.
9:00 PM - M3.10
Solution-Processed Organic N-type Thin-Film Transistors Based on Alkyl Chain Substituted C60 Derivatives.
Masayuki Chikamatsu 1 , Shuichi Nagamatsu 1 , Atsushi Itakura 1 , Yuji Yoshida 1 , Reiko Azumi 1 , Noriyuki Yoshimoto 2 , Takeshi Kodama 3 , Koichi Kikuchi 3 , Kiyoshi Yase 1
1 Photonics Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba Japan, 2 , Iwate University, Morioka Japan, 3 , Tokyo Metropolitan University, Hachioji Japan
Show Abstract9:00 PM - M3.11
Functionalized Pentacenes for Solution-Based Fabrication: Air Stable Pentacene Precursors via Photocycloaddition-Thermolysis for Purification.
Alex Fallis 1 , Christophe Benard 1 , Matthew Heuft 1
1 Chemistry, University of Ottawa, Ottawa, Ontario, Canada
Show AbstractA versatile four-step synthesis of 2,9-di(triisopropylsilylethynyl)pentacene and 2,10-di(triisopropylsilylethynyl)pentacene from 1,4,5,8-anthradiquinone will be described. A double Diels-Alder reaction with Danishefsky's diene gave a mixture of silyl ethers (1:1), which were separated by fractional crystallization. Ditriflates were indepently obtained from a one-pot desilylation/triflation reaction of the corresponding silyl ether. Palladium catalyzed coupling with the triflate gave a diethynyl-diquinone followed by reductive aromatization to afford the pentacenes. These acenes were unstable in the open laboratory, and thus were converted to pentacene precursors upon exposure to light. Photolysis promoted the [4+4] photocycloaddition of the ethynyl-pentacenes independently to air stable crystalline solids. Thermally induced cycloreversion afforded pure pentacenes, with no residual contamination, for thin film electronic applications. This process may be observed visually as the white adducts sublime upon heating to generate the purple pentacene(s). These tetrakisnaphthotricyclo-[4.2.2.22,5]dodecane structures resemble biplanes due to their juxtaposed napthyl pi-components. Consequently, they hold promise as versatile building blocks for oligomers, organic field effect transistors (OFETs), organic light emitting diodes (OLEDs), cyclophanes, and solar cells.
9:00 PM - M3.12
Relationships Between Molecular Structure and Electronic Transport in Organic Electronics.
Sung-Yeon Jang 1 4 , Pramod Reddy 2 , Arun Majumdar 3 4 , Rachel Segalman 1 4
1 Chemical Engineering, University of California Berkeley, Berkeley, California, United States, 4 Materials Science Division, Lawrence Berkeley National Laboratories, Berkeley, California, United States, 2 Applied Science and Technology Program, University of California Berkeley, Berkeley, California, United States, 3 Department of Mechanical Engineering, University of California Berkeley, Berkeley, California, United States
Show AbstractWhile our understanding of organic electronic components has increased dramatically in recent years, basic understandings of the relationship between molecular structure and properties are unclear. Frequently, there could be interplay between molecular structure and mesoscale structure. We attempt to separate these effects by studying the charge transport behavior, conductance, and voltage current response of organic molecules at the single molecule level. A series of thiol terminated aliphatic and aromatic organic molecules were studied using a mechanically controllable break junction technique to ascertain the effects of molecular weight and chemical substitution on conductance. In the mechanically controllable break junction, nanometer-sized gaps between two metal electrodes are created and the distance between the electrodes can be controlled with great stability (< 1 Å). The nanoscale gaps are generated in the solution of organic molecules, such that the thiol-terminated molecules create a molecular bridge between the metal electrodes. The molecular bridges can be made and broken by controlling the distance between the electrodes so that the conductance of single molecules can be measured. The dramatic influence of chemical structure and composition on conductance will be discussed. As expected, conjugated molecules show larger conductance than non-conjugated molecules of similar length. We will also discuss energy transport and the measurement of thermopower of organic molecules. These measurements shed additional light on the electronic structure of conjugated organics.
9:00 PM - M3.13
Micropatterning of Conjugated Polymer Films through a “Green” Template Method.
Shuhong Liu 1 , Zhenan Bao 2
1 Materials Science and Engineering, Stanford University, Stanford, California, United States, 2 Chemical Engineering, Stanford University, Stanford, California, United States
Show AbstractMicropatterning of conjugated polymer films has become increasingly important in a variety of technical applications such as electronic and optical devices. Recently, sacrificial templating methods have been shown to provide an effective approach for the fabrication of structured materials that are difficult to produce by traditional patterning procedures (such as photolithography, dip-pen lithography, and holographic patterning). The templates usually used include organic or inorganic colloidal crystals, diblock copolymers, and anodized alumina layers etc.. However, harsh experimental conditions such as high temperature or strong basic solutions are usually needed to remove the templates afterwards. In our work, we develop a new approach that relies on “green” liquid templates to fabricate patterned P3HT films. Ordered structures with dimensions from several microns to sub-milimeters are demonstrated. Substrates with different designed patterns are fabricated through soft-lithography method. Self-assembled monolayers are used to functionalize different patterned regions. Patterning of liquid templates and the P3HT films are studied by controlling different parameters such as surface chemistry and topography of the substrates, solvents, polymer concentrations and flow conditions. Liquid templates are easily removed through evaporation at ambient conditions. High throughput processes such as stamping, dip-coating and spin-coating are employed here. After patterning of the films, other materials with different electric or optical properties can be potentially incorporated.
9:00 PM - M3.14
Optical studies of Poly[p-(2,5-didodecylphenylene)ethynylene] in Thin Films.
Lynn Rozanski 1 2 3 , David Vanden Bout 1 2 3 , Uwe Bunz 4
1 Chemistry and Biochemistry, University of Texas at Austin, Austin , Texas, United States, 2 Center for Nano and Molecular Science and Technology, University of Texas at Austin, Austin, Texas, United States, 3 Texas Materials Institute, University of Texas At Austin, Austin, Texas, United States, 4 School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia, United States
Show Abstract9:00 PM - M3.16
New Water-soluble cationic Poly(p-phenylenevinylene)s: Facile Synthesis and pH-dependent Tunable Optical Properties.
Yuan Gao 1 , Hsing-Lin Wang 1
1 Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show Abstract Water-soluble cationic poly(phenylenevinylene)s, PPVs, are of current interest because of their potential applications on the construction of light-emitting diodes (LED) via self-assembly and as highly sensitive fluorescent sensory materials in living bodies. We report here the facile synthesis of two new water-soluble cationic PPVs via quaternization of their neutral polymer using acetic acid or ethyl bromide. These PPV derivatives exhibit unique pH-dependent optical properties presumably due to mutual electrostatic repulsions of positive charges pendent on the benzene rings, which lead to an increased or decreased torsional angles along the conjugated backbone and thus affecting the effective conjugation length of the polymers. The transition between the highly charged repulsion state and the partially neutralized state was clearly manifested by a isosbestic point in the UV-Vis spectra as the pH values changes from 4.0 to 10.0. These polymers display very high quantum yields in organic solvents (~50%) using DCM as the standard. The cationic polymer quaternized with ethyl bromide is readily soluble in water and displays a very high quantum yield (14%). The unique combination of tunable optical properties, solubility in water, and high quantum efficiency can potentially enable further development of optical devices for detection of chemical and biological analytes.
9:00 PM - M3.17
Patterned Surface Modification of Silicon Oxide and Micropatterning of Organic Semiconductors.
Yutaka Ito 1 , Jason Locklin 1 , Zhenan Bao 1
1 Chemical Engineering, Stanford University, Stanford, California, United States
Show AbstractThe fabrication of solution-processable organic field-effect transistors (OFETs) is still a major challenge and potentially a limiting factor for manufacturing of low cost, large area devices based on organic semiconductors. Recently, we have demonstrated the use of polydimethylsiloxane (PDMS) stamps to fabricate patterns of different conjugated thiols on gold substrates that could be used to template the selective growth of single crystals from solution. Here, we extend this process to oxide surfaces that are used as the dielectric layer of OFETs. Microcontact printing was used to create patterns comprising of aliphatic and aromatic silanes on silicon oxide surfaces. Selective deposition of different organic semiconductors onto the aromatic silane templates was observed. The films were characterized using optical microscopy, AFM, contact angle measurements, and ellipsometry. Arrays of field-effect transistors using these patterned crystals are demonstrated.
9:00 PM - M3.18
Exploring the Structure-Property Relationships of Phenylene-Thiophene Organic Semiconducting Oligomers.
Andrew Sung 1 , Jason Locklin 1 , Stefan Mannsfeld 1 , Mang-Mang Ling 1 , Zhenan Bao 1
1 Chemical Engineering, Stanford University, Stanford, California, United States
Show AbstractOrganic field-effect transistors (OFETs) show promise as active components in flexible electronics and a continual effort has been directed towards optimizing their ease of production, oxidative stability, and field-effect mobility (µ). Several simple molecular structures based on oligothiophene or thiophene-phenylene co-oligomers have been synthesized previously that have shown good success in terms of mobility and stability. Here we report the synthesis and electronic properties of highly soluble phenylene-thiophene co-oligomers that have been substituted at the periphery with bulky sidechains. We have performed a systematic study by varying the side-chain structure and length of the core to observe changes in the vacuum-deposited thin film morphology and field-effect mobility of these oligomers. Various alkyl and alkoxy side groups of differing length and bulkiness were added to the core in order to observe their effects on solubility, crystallinity, and thin film morphology. Bottom contact devices processed from solution will also be reported.
9:00 PM - M3.19
Morphology of Polythiophene and Polyphenyl Films Produced via Surface Polymerization by Ion-Assisted Deposition.
Sanja Tepavcevic 1 , Adam Zachary 1 , Amanda Wroble 1 , Yongsoo Choi 1 , Luke Hanley 1
1 Chemistry, Univesity of Illinois at Chicago, Chicago, Illinois, United States
Show AbstractSurface polymerization by ion-assisted deposition (SPIAD) of conducting polymer films is achieved by simultaneous co-deposition of hyperthermal organic ions and neutral organic oligomers onto a substrate surface.1 Our previous work on SPIAD found that polymerization occurs in a defined region of ion/neutral ratio and ion energy space to form a distribution of species, rather than a single oligomer.2,3 Conducting polymer films formed by SPIAD display new valence band features resulting from a reduction in both their band gap and barrier to hole injection compared to evaporated neutrals only.4Control of film morphology is widely recognized as one of the limiting factors in the development of conjugated polymers for photonic and electronic applications. We show that SPIAD polymerization reaction is driving nanoscale morphology5. A wide variety of structures form in the SPIAD polythiophene films, including islands formed by sublimation, lamellar structures, nanoscale crystallites, and fractal-like growth patterns. The variation of these patterns deposition conditions clearly indicate that ion-induced polymerization mediates film morphology through control of ion energy and ion/neutral ratio. Furthermore, these ion-assisted events mediate thermal processes such as neutral deposition and sublimation. Morphology can be controlled for a combinatorial examination by variation of substrate temperature, ion energy, ion or neutral structure, and ion/neutral ratio.5 However, the above discussion does not consider all potentially significant phenomena, such as diffusion and dewetting. It is clear that a predictive understanding of how morphology develops in films during SPIAD is a complex interplay between thermal and ion-induced processes that will only avail itself through computer simulations of the controlling phenomenon. References:1.Tepavcevic, S.; Choi, Y.; Hanley, L. J. Amer. Chem. Soc. 2003, 125, 2396.2.Tepavcevic, S.; Choi, Y.; Hanley, L. Lang. 2004, 20, 8754.3.Choi, Y.; Zachary, A.; Tepavcevic, S.; Wu, C.; Hanley, L. Inter. J. Mass Spectrom. Ion Proces. 2005, 242, 139.4.Tepavcevic, S., Wroble, A., Bissen, M.,Wallace, D., Choi, Y., Hanley, L. J. Phys. Chem. B, 2005, 109, 7134.5.Tepavcevic, S., Zachary, A., Wroble, A., Choi, Y., Hanley, L., J. Phys. Chem. A, in print.
9:00 PM - M3.2
Application of Inductively Coupled Plasma - Mass Spectrometry (ICP-MS) for Analysis of Novel Organic Semiconductor Materials.
Svitlana Shcherbyna 1 , Vladimir Baranov 2 , Diethard Bohme 1
1 Physics & Astronomy, York University, Toronto, Ontario, Canada, 2 IBBME, University of Toronto, Toronto, Ontario, Canada
Show AbstractICP-MS is a well established analytical technique in the semiconductor industry for essential trace metal characterization in chemical reagents and silicon wafers. Tetracene, Antracene, Pentacene and Rubrene are among the most interesting organic semiconductors for use in molecular electronic devises such as single-crystal Organic Field Effect Transistors (OFETs). One of the most important parameter for fabrication of single-crystal OFETs is the purity of the starting material. As the crystal growth process also results in the chemical purification of the material, several re-growth cycles may be required for improving the field-effect mobility, with the grown crystals used as the starting material for the subsequent re-growth. The number of required re-growth cycles depends strongly on the purity of starting material. We report the application of ICP-MS for impurity analysis in the organic semiconductors. The ICP plasma, where samples are atomized and ionized, generates strong ion current which is instrumental in achieving exceptional detection limits. Some elements can be measured down to part per trillion range. This study can provide a better characterization of the trace metal impurities in starting material.
9:00 PM - M3.20
Synthesis and Properties of Polythiophene Derivatives Containing Alkyl-sulfone Group or Dialkyl Phoshonate Group.
Naoki Otani 1 , Nobuyuki Kakiuchi 1 , Hitoshi Furusho 1 , Yoshio Ohara 1 , Tatsuo Okauchi 2 , Naoki Nakaie 2 , Toru Minami 2
1 Chemical Research Laboratories, Nissan Chemical Industries, LTD., Funabashi, Chiba, Japan, 2 Department of Applied Chemistry, Kyushu Institute of Technology, Kitakyushu, Fukuoka, Japan
Show Abstract9:00 PM - M3.21
Blue Phosphorescent Host materials with Twisted Structure Induced by 1, 7 Hydrogen Interaction.
Tae-Hyuk Kwon 1 , Jongchul Kwon 1 , Myoung Ki Kim 1 , Kyoung-Sik Lee 1 , Su-Jin Park 2 , Dae Yeup Shin 2 , Jong-In Hong 1 3
1 Chemistry, Seoul National University, Seoul Korea (the Republic of), 2 Corporate R&D Center , Samsung SDI, Yongin Korea (the Republic of), 3 Center for Molecular Design and Synthesis , KAIST , Daejon Korea (the Republic of)
Show AbstractAlthough triplet energy (3.0 eV) of carbazole is suitable for blue phosphorescent host materials, there are several problems as host materials for phosphorescent light-emitting devices, such as low molecular weight and flat structure. To address the above issues, we designed new host materials with higher energy band gap by the modification of known carbazole derivatives by DFT calculations. The developed hosts, TECTEP and TECTMP, were synthesized by coupling carbazoles with 1, 3, 5-tris(bromomethyl)-2, 4, 6-triethylbenzene and 1, 3, 5-tris(bromomethyl)-2,4,6-trimethylbenzene. Phosphorescence spectrum at 77 K of the designed host materials, TECTEP, shows the higher triplet energy gap compared to that of CBP host and TECTMP. Interactions between hydrogens at ethyl groups and benzylic positions of TECTEP caused the steric hindrance to form the up (carbazole units) and down (ethyl groups) structure. But, there is a no 1,7-hydrogen interaction between the methyl group and benzylic position in the case of TECTMP. Therefore, TECTEP with twisted structure by 1, 7-hydrogen interaction would be more useful to prevent the self-aggregation of the host molecules, which will be a trigger to increase the efficiency and durability of EL device. Herein, we report the synthesis of host materials, UV, solution PL (298 K and 77 K ) and EL data.
9:00 PM - M3.22
Synthesis of the First Thiophene Based Dendronized Macromonomers and Polymers.
Prashant Sonar 1 , A. Dieter Schluter 1
1 Institute of Polymers, Department of Materials, Swiss Federal Institute of Technology, Zurich Switzerland
Show AbstractDendronized polymers (denpols) have attracted considerable scientific attention in recent years due to their unique structures and properties. [1] They attain a rodlike, cylindrical shape with the polymer backbone encapsulated into the dendritic envelope. High generation denpols with an appreciable degree of polymerization (DP) could be considered as “nanoobjects” because they attain cylindrical shape in both solution and solid state. [2] Synthesis of thiophene dendrons and dendrimers have been reported, their band gap (ΔEg) and broad absorbance can be tuned by varying number of thiophene units in the dendrimer molecule. [3] These molecules are also known to form interesting nanostructures on mica and graphite surfaces due to π-π and van der Waals interactions. [4] The development of these novel families (thiophene dendron and dendrimers) with structural features combines their intrinsic electronic properties for the application of solar energy conversion. [5] Other thiophene based structures like dendrimers, their macrocycles, tubular and linear chain analogues have been studied; no report has yet occurred on thiophene dendronized polymers. Denpols with a conjugated dendron are of special interest due to their potential applications as “nanowires” and light-emitting materials. We report here the synthesis of a new family of methacrylate macromonomers containing pendant thiophene dendrons of generations two (G2) and three (G3) respectively. Polymerization of macromonomer G2 was carried out by using free radical polymerization method and initial attempts gave the corresponding polymers with Mn = 5.3 x 104 and Mw = 1.2 x 105 whereas polymerization of G3 is presently under progress. References[1] Schlüter, A. D.; Rabe, J. P. Angew. Chem. Int. Ed. 39 (2000), 864.[2] Bo, Z.; Rabe, J. P; Schlüter, A. D. Angew. Chem. Int. Ed. 38 (1999), 2370.[3] Xia, C.; Fan, X.; Locklin, J.; Advincula, R. Org. Lett. 4 (2002), 2067. [4] Xia, C.; Fan, X.; Locklin, J.; Advincula, R.; Gies, A.; Nonidez, W. J. Am. Chem. Soc. 126 (2004), 8735.[5] Locklin, J.; Patton, D.; Deng, S.; Baba, A.; Millan, M.; Advincula, R. Chem. Mater. 16 (2004), 5187.
9:00 PM - M3.23
Synthesis of Block Copolymers Based on a Stiff Conjugated Segment and Investigation of Their Self-assembly in Aqueous or Organic Media.
Anne de Cuendias 1 , Sebastien Lecommandoux 1 , Eric Cloutet 1 , Henri Cramail 1
1 , LCPO-University Bordeaux 1, Pessac France
Show Abstract9:00 PM - M3.24
Design and Use of PEO-based Reactive Stabilizer for the Preparation of Latexes of Conjugated Polymers.
Eric Cloutet 1 , Anne de Cuendias 1 , Muhammad Mumtaz 1 , Henri Cramail 1
1 , LCPO-University Bordeaux 1, Pessac France
Show Abstract9:00 PM - M3.25
Synthesis and Characterization of Novel Platinum Acetylide Oligomers.
Thomas Cooper 1 , Douglas Krein 1 , Aaron Burke 1 , Daniel McLean 1 , Joy Rogers 1 , Jonathan Slagle 1
1 , Air Force Research Laboratory, WPAFB, Ohio, United States
Show Abstract9:00 PM - M3.27
Impact of Film Morphology on Electrical Properties of Thin Films of Oligofluorene-thiophene Derivatives.
Quan Yuan 1 , Stefan Mannsfeld 1 , Mike Ling 1 , Ming lee Tang 1 , Jan Luning 1 , Zhenan Bao 1
1 , Stanford University, Palo Alto, California, United States
Show Abstract9:00 PM - M3.28
Phosphono- and sulfonyl-substituted oligothiophens: Synthesis and Electropolymerization.
Tatsuo Okauchi 1 , Naoki Nakaie 1 , Toru Minami 1 , Naoki Otani 2 , Nobuyuki Kakiuchi 2 , Hitoshi Furusho 2 , Yoshio Ohara 2
1 Department of Applied Chemistry, Kyushu Institute of Technology, Kitakyushu Japan, 2 Chemical Reserch Laboratories, Nissan Chemical Industries, LTD., Funabashi Japan
Show Abstract9:00 PM - M3.29
A Novel σ-Conjugated Hyperbranched Polysilane Polymethylphenylsilane-co-methylsilane (PMPS-co-MS)
Jiang Yu 1 , Weijian Xu 1 , Zhenghua Zhang 1 , Yu Ni 1 , Yanbing Lu 1
1 College of chemistry and chemical engineering, Hunan University, Changsha, Hunan, China
Show Abstract9:00 PM - M3.3
Polarization-sensitive Photovoltaic Cell Utilizing Uniaxially Oriented Polyfluorene Thin Films.
Yuji Yoshida 1 , Masahiro Misaki 1 , Shuichi Nagamatsu 1 , Masayuki Chikamatsu 1 , Nobutaka Tanigaki 1 , Reiko Azumi 1 , Kiyoshi Yase 1
1 Photonics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Japan
Show AbstractWe report on the polarization-sensitive photovoltaic cells utilizing uniaxially oriented polyfluorene (PFO) thin films. The highly oriented PFO layer formed by using friction transfer methods. The dichroic ratio of absorption of PFO thin films is more than 10 at the wavelength of maximum absorption peak. The configuration of the photovoltaic device is Al/In/PTCDI/PFO/ITO. As a results, the cell exhibit the dependence of the photocurrent on the polarization of the incident light, yielding a highly polarization sensitivity of a factor of 10 at the onset of the absorption.
9:00 PM - M3.30
Polymeric Nanocomposite Dielectrics for Organic Thin-film Transistors.
Fang-Chung Chen 1 , Chiao-Shun Chuang 2 , Yung-Sheng Lin 2 , Li-Jen Kung 1 , Dong-Sian Chen 1
1 Department of Photonics and Display Institute, National Chiao Tung University, Hsinchu Taiwan, 2 Department of Photonics and Institute of Electro-optical Engineering, National Chiao Tung University, Hsinchu Taiwan
Show Abstract9:00 PM - M3.31
A Very High-Spin (S > 20/2) and Purely Organic Polymer: Poly(aminium triphenylene)s
Takeshi Ibe 1 , Eiji Fukuzaki 1 , Hiroyuki Nishide 1
1 Department of Applied Chemistry, Waseda University, Tokyo Japan
Show AbstractWe have succeeded in, for the first time, synthesizing a purely organic-derived, room temperature-durable, and very high-spin polymer.High molecular weight and networked aromatic polyamines were synthesized by the palladium-catalyzed polycondensation of 2,6,10-tribromo-3,7,11-tris(hexyloxy)triphenylene and 2,6,10-tris(hexyloxy)-3,7,11-tris(4-methoxyphenyl)triphenylene. The polymers were soluble in common organic solvents and the molecular weight was >104. The π-conjugated, but non-Kekulé and non-disjoint structure of the polymers were precisely characterized by NMR. The polyamines were converted to the corresponding poly(aminium cationic radical)s. The stepwise and quantitative radical formation was electrochemically studied. The polyaminium radicals were stable even at room temperature under air atmosphere: lifetime > 1 month at ambient conditions.The SQUID measurement indicated a very high-spin alignment with S = 20/2 on the polyaminium radical at low temperature. It was discussed that the quartet triaminium triphenylene unit was effectively connected through the π-conjugated backbone to realize the very high-spin ground state.Performance of the very high-spin organic polymer is also described.
9:00 PM - M3.32
Design, Synthesis, and Properties of Push-Pull Conjugated Organic Materials Containing Phenylene Bridge
Dong Wook Kim 1 , Hyun Kyung Ju 1 , Jin Joo Choi 1 , Jong Sun Lim 1 , Sung Cheol Yoon 1 , Changjin Lee 1
1 Advanced Materials Division, Korea Research Institute of Chemical Technology, Daejeon Korea (the Republic of)
Show AbstractSince the first nolinear optical (NLO) polymer was reported in 1982, most of the researchers have focused on synthesizing the organic chromophores possessing high molecular hyperpolarizability and dipole moment in order to enhance their second-order nonlinear susceptibility. The figure of merit (μβ) is highly dependent upon characteristics of the conjugation bridge in the chromophores. Polyene structure has been commonly adopted for the bridge due to its efficient conjugation. However such polyene chromophores commonly have poor thermal and photo stability, which is a critical drawback in a commercial application. Aromatic rings such as thiophene or phenylene are considered better alternatives, which are efficient and thermally stable conjugation bridge. Recent experiment and molecular simulation have suggested that high μβ chromophores are difficult to align due to their high ground-state dipole moment interactions. The intermolecular electrostatic interaction prevents aligning of the chromophore molecules because the interaction becomes strong enough to resist the external electric poling field at a high chromophore loading density. Hence preventing the antiparallel interaction of the chromophores at the molecular level is highly required to achieve an efficient poling and hence high macroscopic NLO activity. The recent results have shown that introducing a bulky side chains is one of the promising approaches to achieve chromophore separation and to minimize intermolecular interaction of the chromophores. In this work, we designed and synthesized a push-pull type chromophore containing phenylene ring as a conjugation bridge. Bulky alkyl chain was substituted into the phenylene bridge to minimize intermolecular interaction of the neighbor molecules and to enhance solubility of the chromophore in solvents and polymer host. The chemical structure and purity were verified by 1H NMR, 1H-1H COSY, 13C NMR, DEPT and Mass spectroscopy. The NMR spectra showed that we obtained the chromophore in a trans conformation. In spite of the bulky alkyl chains, St-TCF exhibited fairly high melting point of 167 oC. The chromophore was thermally stable up to 200 oC in the nitrogen atmosphere. The relatively high melting point and decomposition on-set may be attributable to the rigid phenylene ring. The chromophore was blended with an optically transparent polymer host and then the macroscopic electro-optic activity was evaluated. In a preliminary result, the polymer films containing St-TCF as a concentration of 50 wt% exhibited as large as 20-24 pm/V electro-optic coefficient at 1.55 μ
9:00 PM - M3.34
A Morphology Controllable Hole-transporting Material Containing binaphthyl and triphenylamine chromophores.
Qingguo He 1 2 , Jian-Gong Cheng 1 , Fenglian Bai 2
1 Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai China, 2 Institute of Chemistry, Chinese Academy of Sciences, Beijing China
Show AbstractAn organic compound with two triphenylamine moieties linked with binaphthyl at 3, 3’-positions (2,2'-dimethoxyl-3,3'-di-(phenyl-4-yl-diphenyl-amine)-[1,1']-binaphthyl, abbreviated as TPA-BN-TPA) was synthesized by Suzuki coupling. The amorphous and homogeneous films were obtained by either vacuum deposition or spin coating from its solution in good solvents, while the single crystal could be cultivated under the induced growth of proton solvent. The crystal structure of TPA-BN-TPA by X-ray crystallography showed a multi-channel structure including solvent molecules in the channel. The intramolecular charge transfer state was observed by solvatochromic experiments resulting from amino conjugation effects. High Tg (130oC) and Td (439oC) in combination with the reversible oxidation and reduction process make it a promising candidate for the LED devices as a hole transport material. With TPA-BN-TPA as hole-transporting layer in the device of ITO/TPA-BN-TPA/Alq3/Mg-Ag, the brightness of the device is about 10100 cd/m2 at 15.6 V with a maximum efficiency of 3.85 cd/A, which is superior to the device with NPB as counterpart under the same conditions. Other devices with TPA-BN-TPA as blue light emitting layer or host for blue dye emitter were also studied.
9:00 PM - M3.35
New Host Materials with High Triplet Energy Level for the Electrophosphorescent Device.
Joo Hyun Kim 4 , Do Yoon 1 , Ji Whan Kim 2 , Jang-Joo Kim 2 3
4 Division of Applied Chemical Engineering, Pukyong National University, Busan Korea (the Republic of), 1 Department of Chemistry, Seoul National University, Seoul Korea (the Republic of), 2 School of Materials Science and Engineering, Seoul National University, Seoul Korea (the Republic of), 3 Center for Organic Light Emitting Diode, Seoul National Unviersity, Seoul Korea (the Republic of)
Show Abstract9:00 PM - M3.38
Negative Capacitance in Organic Small Molecule Copper Phthalocyanine (CuPc) Single Layer Films.
Ming ZHU 1 , Xiaojiang YU 1 , Jin An 1 , Jianbin XU 1
1 Electronic Engineering, The Chinese University of Hong Kong, Hong Kong Hong Kong
Show AbstractAlthough negative capacitance phenomenon has been observed in many material systems, the basic physical mechanism is still unknown. Blom et al. had observed such phenomenon in Ca/OC1C10-PPV/ITO by impedance spectroscopy and they believed that it is due to the redistribution of space charges [1].In this work we report on a study of hole transport and trap filling effect in copper phthalocyanine (CuPc) thin films by admittance spectroscopy. The single layer CuPc films were fabricated in sandwiched structures using indium-tin-oxide (ITO) as anode; Al, Ag and Au as cathode respectively. We measured the capacitance of these devices as a function of frequency, and found that at high frequency region, the capacitance is independent of frequency and equals to geometrical value. At low frequency region, the negative capacitance phenomenon is easier to appear with the decreasing of the work function of cathode. With the high dc bias voltage which leads to high electrical field strength in the organic single layer films, the negative capacitance effect can be largely increased. We ascribed these experimental phenomenons to the electron-hole recombination [2, 3] and the slow trap filling process [4] in the organic CuPc single layer films.[1] H. C. F. Martens, J. N. Huiberts, and P. W. M. Blom, Appl. Phys. Lett. 77, 1852 (2000).[2] E. Tutis, D. Berner, and L. Zuppiroli, J. Appl. Phys. 93, 4594 (2003).[3] F. A. Castro, P. R. Bueno, C. F. O. Graeff, F. Nuesch, L. Zuppiroli, L. F. Santos, and R. M. Faria, Appl. Phys. Lett. 87, 013505 (2005).[4] M. Ershov, H. C. Liu, L. Li, M. Buchanan, Z. R. Wasilewski, and A. K. Jonscher, IEEE Trans. Electron Devices, 45, 2196 (1998).
9:00 PM - M3.4
Transport Properties of Stretch-Oriented PPV Films.
F. Doty 1 , A. Talin 1 , Luke Hunter 1 , Frank Jones 1 , Douglas Chinn 2 , Tiffany Wilson 2 , Christine Cuppoletti 1 , Hagop Rouchanian 1 , Christel Munoz 2
1 , Sandia National Laboratories, Livermore, California, United States, 2 , Sandia National Laboratories, Albuquerque, New Mexico, United States
Show AbstractOrganic semiconductors are under investigation for radiation sensors at Sandia National Laboratories. The wide band gaps, high resistivities, low dielectric constants, and high dielectric strengths of conjugated polymers suggest these materials may be suitable for solid-state particle counting detectors. A range of solution cast materials have been evaluated for this application, including PPVs and polythiophenes. Films were prepared by novel solution casting and mechanical stretching methods. Device structures including planar ITO/PPV/metal, interdigital metal electrodes on glass, and TFT structures on silicon were fabricated by drop casting and lamination of the stretched films, and transient DC responses were recorded and analyzed. Experiments included laser stimulus TOF measurements to determine the feasibility of single particle detection. Calculated and experimental leakage current and capacitance, as well as photoconductive pulse response characteristics, and sensitivity and radiation tolerance to pulsed fast protons were used to predict SNR for optimal device geometries. The effect of traps on charge transport will be discussed in light of both pulse counting and current-mode detection of fast neutrons and other particles.
9:00 PM - M3.40
Tuning the Photoluminescence of Water Soluble Conjugated Poly[2-(3-thienyl)ethyloxy-4 butylsulfonate)]
Enrique Cabarcos 1 2 , Veronica Sholin 1 , Sue Carter 1
1 Physics, University of California Santa Cruz, Santa Cruz, California, United States, 2 Physical Chemistry, UCM, Madrid Spain
Show AbstractThe photoluminescence of Poly[2-(3-thienyl)ethyloxy-4-butylsulfonate)] (PTE-BS) in aqueous solution results in a 3-fold increase by addition of the surfactant tetrabutylammonium perchlorate (TBA). On the other hand, the luminescence of the PTE-BS/TBA system is reduced by more than 5 times by addition of small amounts of the cationic electron acceptor methyl viologen (MV2+). The Stern-Volmer constant KSV =1.4x104 M-1 for the quenching of the polymer surfactant complex by MV2+ is approximately 60 times smaller than the KSV = 8.4x105 M-1 obtained in water polymer solutions without surfactant. Thus, the luminescence of PTE-BS in aqueous solution can be modulated by complexing the polymer either with a surfactant or with a quencher. In this contribution we show that the surfactant/quencher tuning effect found in polymers of the phenylene vinylene family, such as poly(2,5-methoxy-propyloxysulfonate phenylene vinylene), also appears in polymers of the thiophene family such as PTE-BS. When used in biosensor applications, this effect has to be taken into account since the presence of some amount of surfactants could easily modify the luminescence of the conjugated polymer.
9:00 PM - M3.41
Versatile Synthesis and Characterization of Pentacene Derivatives
Toshihiro Okamoto 1 , Abhijit Mallik 1 , Ming Tang 1 , Mang Ling 1 , Zhenan Bao 1
1 Chemical Engineering, Stanford University, Stanford, California, United States
Show AbstractLinear fused polycyclic aromatic hydrocarbons have been widely studied as semiconducting compounds for organic field effect transistors (OFETs). To date, thin film OFETs with the best reported performance are based on pentacene. However, pentacene has poor solubility in common organic solvents, preventing the use of low cost large area deposition methods such as solution processing. Moreover, it is difficult to functionalize pentacene due to its poor solubility and lack of reactive groups. Therefore, the preparation of soluble pentacene derivatives and their precursors would make them more attractive candidates. Our goal is to develop new synthetic routes that can allow us to prepare a variety of soluble pentacene derivatives readily. In this work, we report the synthesis and electrical characterization of several pentacene derivatives with symmetrical and asymmetrical substitutions. We designed a versatile soluble pentacene synthon that can be easily converted into other pentacene derivatives. This allowed us to prepare a variety of new substituted pentacene. Thin film transistor devices are fabricated and their device performance will be reported.
9:00 PM - M3.43
Synthesis and Characterization of Low Bandgap Conjugated Polymer Precursors
Paul Armstrong 1 , Carine Edder 2 , Christine Luscombe 1 , Jean Frechet 1 2
1 Department of Chemistry, University of California, Berkeley, California, United States, 2 The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show AbstractTo improve the efficiencies of photovoltaic devices made from solution-processable organic semiconductors, there is a need to design soluble low bandgap polymers that absorb light near the maximum of the solar emission spectrum. Towards this end, several novel polymers based on modifications to the structures of PTPTB (1) and BBL (2) have been synthesized. The new polymers bear solubilizing groups attached to the main chain via thermally labile linkers. These polymers, which can be viewed as precursors to low bandgap polymers, are air stable and soluble in common organic solvents. Upon thermal annealing, films of these polymers are converted into insoluble, low bandgap materials. This accomplishes a solubility switch, which allows for the simple fabrication of devices with architectures based on either multiple polymeric layers or bulk heterojunction blends. The polymers are thus attractive for use as light-absorbing materials in a variety of organic solar cells. The synthesis of these polymers, as well as the detailed characterization of the thermal treatment of their films, will be presented. (1) Brabec, C. J.; Winder, C.; Sariciftci, N. S.; Hummelen, J. C.; Dhanabalan, A.; van Hal, P. A.; Janssen, R. A. J. Adv. Funct. Mater. 2002, 12, 709. (2) Alam, M. M.; Jenekhe, S. A. Chem. Mat. 2004, 16, 4647.
9:00 PM - M3.44
Pivaloylnitroxide Radical Polyacetylene as a n-type Doping Active Material.
Masashi Fujinaga 1 , Hiroyuki Nishide 1
1 Department of Applied Chemistry, Waseda University, Tokyo Japan
Show AbstractA nitroxide radical molecule involving an electron-withdrawing group is able to be electrochemically reduced to the corresponding aminoxy anion which is reversible and ascribed to a n-type doping of the molecule. This redox capability of the unpaired electron of the electron-withdrawing group-substituted nitroxide molecule is suitable as an electro-active material of a next generation secondary battery. This paper describes the molecular designing and the n-type doping performance of the pivaloylnitroxide-compounds, 4-(N-oxyl-N-pivaloylamino)-t-butylbenzene and poly((4-N-oxyl-N-pivaloylamino)acetylene). The redox potential of these compounds was -0.73 and -0.76 V, respectively, and the redox was reversible and stable. Electron transfer rate of the redox reaction was 5.80 × 10-3 and 4.36 × 10-3 (cm/s), respectively, which was faster than those of n-type doping for the previously reported nitroxide compounds such as 4-(N-t-butyl-N-oxylamino)-benzotrifluoride with k0 = 1.1 × 10-3. The polymer with molecular weight of 2.6 × 104 was soluble in THF, DMF and methanol, insoluble in electrolyte solutions, and displayed a thin- film formability besides its n-type redox activity.
9:00 PM - M3.45
Effect of a Long Alkyl Side Chain of Gate Insulator on the Performance of Pentacene Organic Thin-film Transistors.
Seungmoon Pyo 1 , Yoonjeong Lee 1 , Jihyun Jeon 1 , Mi Hye Yi 1 , Soon-Ki Kwon 2 , Jae Heung Lee 1
1 Polymeric nanomaterials laboratory, Korea research institute of chemical technology, Daejeon Korea (the Republic of), 2 Polymer Science and Engineering, Gyeongsang National University, Chinju Korea (the Republic of)
Show AbstractThis presentation discusses some recent findings regarding the effect of a long alkyl (C18) group on the performance of organic thin-film transistors (OTFTs). Two different types of polymer gate insulators, BTDA-DACM (PI-1) and BTDA-DACM-DA18IM (PI-2), were prepared by a condensation polymerization. DA18IM with C18 alkyl group was used as a co-monomer for the preparation of PI-2. We found that the C18 alkyl group of PI-2 protruded to the surface and made the surface more hydrophobic. The modified surface strongly affects the initial growth mechanism of pentacene on PI-2, and subsequently the device performance. The performance of pentacene OTFTs with the PI-2 was superior to that of the OTFTs with the PI-1. The OTFTs with PI-2 exhibited a carrier mobility of 0.4 cm2/Vs, current on/off ratio of 1.5 x 10-5, subthreshold slope of 2.1 V/dec. This suggested that the C18 alkyl group over the surface of PI-2 contributed to the better interface formation between PI-2 and pentacene and resulted in more favorable initial growth of pentacene and much improved performance.
9:00 PM - M3.46
Silver Embedded Water-Soluble Polyfluorene Derivatives
Seung-Hwan Oh 1 , Won San Choi 1 , DooJin Vak 1 , Yoon-Chae Nah 1 , Dong-Yu Kim 1
1 Heeger Center for Advanced Materials, Dept. of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju Korea (the Republic of)
Show Abstract9:00 PM - M3.47
Temperature Dependent Built-in Potential in Organic Semiconductor Devices.
Martijn Kemerink 1 , Jan-Maarten Kramer 1 , Hans Gommans 1 , Rene Janssen 1
1 Applied Physics, Eindhoven University of Technology, Eindhoven Netherlands
Show AbstractOptimal performance of bipolar devices like light-emitting diodes (LEDs) and photo-voltaic (PV) cells generally requires the presence of ohmic electron and hole contacts. Since an ohmic electron contact is usually non-ohmic for holes, and vice versa, different contacts for electrons and holes are used in most optimized devices. This results in asymmetric current-voltage (I-V) characteristics. In the field of organic electronics this asymmetry is commonly described in terms of a built-in voltage, Vbi. [1].We have experimentally investigated the built-in potential of polymer- and fullerene-based diodes, as well as of a photo-voltaic device based on a blend of these materials using temperature dependent I-V characterization. For all devices, a strong, non-linear T-dependence of Vbi was found, which cannot be understood in the common picture of a linear distribution of the potential between the two electrodes. Therefore, we developed a simple analytical model to describe the band bending at a metal/insulator or metal/semiconductor contact. It is based on the notions that (a) diffusion may cause a significant charge density in the entire device and (b) at zero device current, diffusion and drift currents must cancel each other. The model is successfully used to describe both magnitude and temperature dependence of the built-in potential of all investigated devices. Moreover, the effects of a thin LiF layer between cathode and active layer are consistently described by the model.Finally, it is verified that our model exactly reproduces the outcome of more complex models that require numerical evaluation [2].References[1] G.G. Malliaras, J.R. Salem, P.J. Brock, and J.C. Scott, Phys. Rev. B 58, 13411 (1998).[2] J.G. Simmons, J. Phys. Chem. Solids 32, 1987 (1971).
9:00 PM - M3.48
Surface Morphology of poly(p-phenylenevinylene) Multilayer Films.
Rodrigo Bianchi 1 , Maria Vega 2 , Alexandre Marletta 3 , Roberto Faria 2
1 Departamento de Engenharia de Sistemas Eletronicos - LME, Escola Politecnica da Universidade de Sao Paulo, Sao Paulo, SP, Brazil, 2 Instituto de Fisica de Sao Carlos, Universidade de Sao Paulo, Sao Carlos, SP, Brazil, 3 Faculdade de Fisica, UFU, Uberlandia, MG, Brazil
Show Abstract9:00 PM - M3.50
Excitons and Lattice Modes in Pentacene Monolayers*.
Rui He 1 , Nancy Tassi 2 , Graciela Blanchet 2 , Aron Pinczuk 1 3
1 , Columbia University, New York, New York, United States, 2 , Dupont, Central Research and Development, Wilmington, Delaware, United States, 3 , Bell Labs, Lucent Technologies, Murray Hill, New Jersey, United States
Show Abstract9:00 PM - M3.51
Interface Energetics Of Derivatives Of Polyflourene.
Jaehyung Hwang 1 , Eung-Gun Kim 2 , Jean-Luc Bredas 2 , Antoine Kahn 1
1 Electrical Engineering, Princeton University, Princeton, New Jersey, United States, 2 School of Chemistry and Bio Chemistry, Georgia Insitute of Technology, Atlanta, Georgia, United States
Show AbstractConjugated polymers are being actively researched as materials for various organic electronic devices. The energetics of these materials interfaces, and the molecular levels relevant to carrier transport, are key factors that determine charge injection into, and ultimately the performance of, the devices. Yet, it has been difficult to precisely define the energy of the electronic transport levels and the interface energetics of these materials. One reason is the structural and electronic complexities of the polymers. Another is the perennial difficulty of making interface measurements on films formed ex-situ by spin casting from solution. We address some of these issues here by comparing experimental and theoretical densities of states (DOS), which leads to a precise definition of molecular levels, and by spectroscopic measurements on ultra-thin films. Films of poly(9,9-dioctylfluorenyl-2,7-diyl) (F8) and poly(9,9 '-dioctylfluorene-co-bis-N,N '-(4-butylphenyl)-diphenylamine) (TFB) with thickness ranging from 3 to 18 nm are prepared by spinning from solution. Filled and empty states are measured using ultraviolet photoelectron spectroscopy (UPS) and inverse photoelectron spectroscopy (IPES). UPS and IPES spectra are compared to theoretical calculations based on density functional theory. Very good agreement is obtained, which allows a clear determination of the molecular transport levels and provides a firm basis for the studies of interface barriers. The transport, or conductivity, gaps of F8 and TFB are found equal to 3.5 eV and 3.3 eV, respectively. The ionization energy (measured by UPS) and electron affinity (measured by IPES) of the materials are also found to be in good agreement with cyclic-voltammetry measurements.Interface energy barriers are investigated for films of the two polymers on substrates with work function ranging from 4.3 eV to 5.1 eV. Measurement on ultra-thin films (3-4 nm) spun from highly diluted solutions show that the energetics of all these interfaces are characterized by vacuum level alignment. When the interface hole barrier is larger than 0.6 eV, e.g. for F8 on PEDOT:PSS or TFB on Au, molecular levels remain flat throughout the film. However, when the interface barrier is equal to, or smaller than, 0.4 eV, e.g. for TFB on PEDOT:PSS, molecular level bending occurs with the HOMO level moving away from the Fermi level in the bulk of the film. This is presumably due to an accumulation of holes at the interface in the polymer due to the relatively small interface barrier. The resulting field in the polymer film moves the molecular levels down away from the interface (band bending), to prevent excess charge from extending into the bulk of the film.
9:00 PM - M3.52
The Influence of Post-deposition Treatment on the Conductivity of PEDOT:PSS films.
Alexandre Nardes 1 , Nicole Kiggen 2 , Albert van Breemen 2 , Margreet de Kok 3 , Martijn Kemerink 1 , Rene Janssen 1
1 Applied Physics , Eindhoven University of Technology, Eindhoven Netherlands, 2 , TNO Science and Industry, Eindhoven Netherlands, 3 , Philips Research Laboratories Eindhoven, Eindhoven Netherlands
Show AbstractIt is well-known that the electrical properties of conducting polymers are strongly dependent on the film morphology and chemical and physical structure, which in turn can be strongly modified via a variety of post-deposition treatments. A typical example of such a system is Poly(3,4-ethylenedioxythiophene):poly(styrenesulphonic acid) (PEDOT:PSS) which is commonly used as anode in polymer light-emitting diodes (PLED) and as electrical interconnect. Here, we report a detailed investigation to the relation between conductivity and morphology of spin-coated PEDOT:PSS films and the influence of various heat and humidity treatments. Moreover, the effects of the addition of high-boiling solvents to the spin-casting solution and subsequent annealing are investigated.Regarding the annealing, we show that annealing of freshly cast films typically yields a conductivity enhancement by 30 %. Heating and cooling rates make no significant difference in the resulting conductivity. When the thin film is exposed to an ambient with 30 % relative humidity, the conductivity is typically reduced by one order of magnitude. Subsequent removal of all water from the film by pumping or purging of dry N2 does not lead to a full recovery of the conductivity, from which we conclude that the changes in conductivity are due to morphological changes. These changes can be fully reversed by annealing.Enhancements in conductivities are also obtained by addition of high-boiling solvents, e.g. ethylene glycol/glycerol or sorbitol, and subsequent annealing. Such compounds cause a clustering of the polaronic islands (i.e. the PEDOT-rich clusters) which can directly be related to the observed enhancement in conductivity via the reduced charging energy of bigger particles.By measuring the temperature dependence of the conductivity of PEDOT:PSS films after and before post-deposition treatments (annealing, exposure to humidity and the addition of solvents) information on the microscopic mechanism of the conductivity enhancement is obtained.Finally, changes in the macroscopic conductivity are correlated to morphologies as observed by Scanning Tunneling Microscopy.
9:00 PM - M3.53
Substitution and Conjugation-Length Dependence of Electronic Structures and Transport Properties in Fluorene-Based Organic Materials
Chih-I Wu 1 2 , Chan-Tin Lin 2 , Yu-Hung Chen 2 , Yun-Hua Hung 3 , Chung-Chih Wu 1 2 3 , Ken-Tsong Wong 4
1 Electrical Engineering, National Taiwan University , Taipei, Taiwan, Taiwan, 2 Graduate Institute of Electro-Optical Engineering, National Taiwan Universtiy, Taipei, Taiwan, Taiwan, 3 Graduate Institute of Electronics, National Taiwan Universtiy, Taipei, Taiwan, Taiwan, 4 Department of Chemistry, National Taiwan Universtiy, Taipei, Taiwan, Taiwan
Show AbstractIn this paper, we present a systematic study of electronic structures and transport properties of oligofluorenes with different substituents and conjugated lengths. Terfluorenes with dialkyl substituents (E-series), diaryl substituents (T-series), and spiro-linked conjugated substituents (B-series) and with different conjugation lengths were studied in this work. The electronic structures of these materials were studied with both photoemission spectroscopy (UPS) and theoretical calculation. The solid-state IE’s for B-type, T-type, and E-type oligofluorenes, are 5.9, 6.1, and 6.2 eV, respectively. Along with the gas-phase ionization potentials calculated with Hartree-Fock method (7.0 eV for all three terfluorenes), the polarization energies, representing electronic screening and intermolecular relaxation in molecular thin films, are estimated to be 1.1, 0.9, and 0.8 eV for B-, T-, and E-type, respectively. The stronger polarization effect of B-series oligofluorenes can be attributed to more delocalized frontier MO’s and electronic distribution, and thus give rise larger dielectric constants and optical refractive indices. Furthermore, different types of substitutions and consequent effects on MO’s/electronic structures of oligofluorenes also have strong influences on their carrier-transport properties [1]. These results indicate that the substitutions could considerably change the energy levels, refractive indices, and carrier-transport properties of oligofluorenes that would substantially influence device operation in terms of carrier injection, optical out-coupling, and driving voltages etc. The conjugation length, on the other hand, has little influences on the electronic structures of the oligofluorenes. The influence of the length on the carrier transport properties is due to other physical properties, such as density, of the materials, rather than the variation in electronic structures.[1]C.C. Wu, W.G. Liu, W.Y. Hung, T.L. Liu, K.T. Wong, Y.Y. Chien, R.T. Chen, T.H. Hung, T.C. Chao, and Y.M. Chen, Appl. Phys. Lett., 87, 052103 (2005)
9:00 PM - M3.54
Interfacial Chemistry and Carrier Transport of Organic Thin Films Doped by Cs2CO3: Interfacial Doping vs. Bulk Doping
Chih-I Wu 1 2 , Chan-Tin Lin 2 , Yu-Hung Chen 2 , Mei-Hsin Chen 2 , Chung-Chih Wu 1 2
1 Electrical Engineering, National Taiwan University , Taipei, Taiwan, Taiwan, 2 Graduate Institute of Electro-Optical Engineering, National Taiwan University, Taipei, Taiwan, Taiwan
Show AbstractAn effective cathode using thin cesium carbonate (Cs2CO3) layers can drastically improve the electron injection efficiency of OLEDs without using the low work-function metals or LiF thin layers. In this paper, we investigate the energy structures and electron injection mechanisms of cathode layers consisting of Cs2CO3. The current-voltage (I-V) characteristics show that the electron injection efficiency is significantly improved with an ultra-thin layer of Cs2CO3 inserted between Alq3 and cathode metals. Compared to the conventional LiF thin layer, Cs2CO3 can work with not only Al, but also with other cathode metals, such as Ag. Our studies show that the electron injection efficiency can be further improved with co-evaporation of Alq3 and Cs2CO3 as the electron transport layer (ETL).To understand the mechanisms of the improvement on electron injection, the interfacial chemistry was studied with X-ray photoemission spectroscopy (XPS) and the energy levels of metal and organic materials were measured with ultraviolet photoemission spectroscopy (UPS). From UPS spectra, we found that a thin layer of Cs2CO3, as thin as 0.5 A, at the metal and organic ETL interface can bring the Fermi level of Alq3 from mid-gap to less than 0.2 eV below the lowest unoccupied molecular orbital (LUMO), indicating that the Alq3 film at the interface is heavily n-type doped with Cs2CO3 . The smaller gap between the Fermi level and LUMO with Cs2CO3 reduces the electron injection barrier. Strong dipole fields are also found at the surface, which also affects the electron injection considerably. The XPS data further show that Cs ions are dissociated at the interface as soon as Cs2CO3 is deposited on Alq3. The result is different from the case of LiF, in which Al metal is needed in order to release the Li ions. With co-evaporation of Cs2CO3 with Alq3 in the bulk as n-doping ETL, electron injection barrier is similar to that of the interfacial doping with Cs2CO3. The further improvement of electron injection efficiency shown in the I-V characteristics with bulk doping is presumably due to the enhancement of the electron transport in the Alq3 films.
9:00 PM - M3.55
Optimizing Electro-Optic Coefficients in Dendrimer-Based Amorphous Thin-Films.
Philip Sullivan 1 , Andrew Akelaitis 1 , Yi Liao 1 , Jingdong Luo 2 , Tae-Dong Kim 2 , Tom Baehr-Jones 3 , Michael Hochberg 3 , Dong Hoon Choi 1 , Bruce Robinson 1 , Alex Jen 2 , Axel Scherer 3 , Larry Dalton 1
1 Chemistry, University of Washington, Seattle, Washington, United States, 2 Materials Science and Engineering, University of Washington, Seattle, Washington, United States, 3 Applied Physics, California Institute of Technology, Pasadena, California, United States
Show AbstractHigh performance materials for 2nd-order nonlinear optical applications are of much current interest. In order for these materials to be adopted for widespread application they must meet certain performance requirements. High thermal stability of poling induced molecular-scale order, as well as high Electro-optic coefficients (r33) are required. In this paper synthesis and fabrication of dendrimer based amorphous thin-films is explored for potential improvements in r33 values. Large tuning figures (> 5 Ghz/V) are demonstrated in silicon-organic hybrid microring resonator based optical modulators. Lattice hardening of these high r33 materials through the use of Diels-Alder “Click” chemistry in order to improve thermal behavior is also demonstrated. Material properties are evaluated by way of an in-situ pole-and-probe single beam polarization interferometry reflection apparatus for real-time material data.
9:00 PM - M3.56
New Polythiophenes with Large Domains and High Mobilities
R. Kline 1 , Michael Chabinyc 2 , Martin Heeney 3 , Iain McCulloch 3 , Michael Toney 4
1 Polymers Division, National Institute of Standards and Technology, Gaithersburg, Maryland, United States, 2 Large-Area Electronics, Palo Alto Researc Corporation, Palo Alto, California, United States, 3 Organic Semiconductors, Merck Chemicals Ltd., Southampton, Hants, United Kingdom, 4 Stanford Synchrotron Research Laboratory, Stanford Linear Accelerator Center, Menlo Park, California, United States
Show AbstractIncreasing the charge carrier mobility of conjugated polymers has been an intense focus of the development of polymer-based thin-film transistors for several years. Currently used polymers such as regioregular poly(3-hexylthiophene) (P3HT) have a mobility of about 0.1 cm^2/Vs. These mobilities are sufficient for low-end applications, but a higher mobility would greatly increase the impact of this technology. We report the results of a study of the recently developed liquid crystalline polymer, poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophenes), having mobilities of 0.2 – 0.6 cm^2/Vs. We use atomic force microscopy and x-ray diffraction rocking curves to study the domain structure and orientation to determine why this polymer is better than P3HT. Annealing in the liquid crystalline phase produces terraced domains greater than 200 nm in diameter, substantially larger than any previously observed in conjugated polymers. Rocking curves show that these domains grow from the substrate and extend through the entire thickness of the film. Furthermore, the domains are highly oriented (within 0.03 degrees of the substrate normal). The intensity of the rocking curve peaks is at least an order of magnitude greater than that observed for the best P3HT films, indicating a significant increase in crystallinity. We will show how the combination increased order and domain size result in the high mobilities.
9:00 PM - M3.57
Chemical Analytes Identification Using Organic Semiconductor Thin Films
Andrew Kummel 1 , Richard Yang 2 1
1 Chemistry and Biochemistry, UC San Diego, La Jolla, California, United States, 2 Material Science and Engineering, UC San Diego, La Jolla, California, United States
Show AbstractTwo strategies have been developed for chemical analyte identification using organic semiconductor thin films. (A) In the first approach, the ac conductance of Metallophthalocyanine (MPc) thin films is monitored with and without chemical analytes using impedance spectroscopy. Above a critical concentration, the magnitudes of the high frequency conductivity changes are invariant with concentration but distinct for different analytes. Therefore, these impedance magnitudes can be used for analyte identification. The analyte induced AC conductivity changes above 5 kHz have been converted to frequency shifts in a circuit resonance and used to differentiate methanol, ethanol, and isopropanol vapors in a nitrogen carrier gas. The analyte induced changes in the conductivity as a function of frequency have similar response patterns compared to charge injection by DC bias, suggesting the analyte induced conductance changes may be due to analyte induces variation in the charge relaxation time. (B) In the second approach, low-voltage CuPc thin-film transistors are used for chemical sensing (chemFETs). In general, changes in the threshold voltages are associated with changes in the fixed charge at the semiconductor-oxide interface while changes in the subthreshold slope and mobility are associated with changes in the trap density in CuPc. Analytes can be differentiated based on how they interact with fixed charges versus traps. Volatile analytes such as methanol are found to strongly change the threshold voltages of the device but only weakly affect subthreshold slope. Conversely, low volatility analytes such as diisopropyl methylphosphonate (DIMP) strongly change the subthreshold slopes. The selectivity and sensitivity of the chemFET is tunable with both drain and gate voltages. For low gate voltage, the chemFET sensitivity is dominated by analyte induced changes in the threshold voltage but at high gate voltage the sensitive is dominated by analyte induced changes in the mobility. Therefore, the chemFET can be tuned to operate in different sensing mechanisms and identify chemical analytes in a single device. The two methods hold great promise for chemical selective detectors which will not require the complicated recalibration process used with electronic noses.
9:00 PM - M3.58
Electrochemical Controlled Synthesis of Micro/Nanostructured Conducting Polymer Polypyrrole and Characterization
Sanju Gupta 1 , N Smith 1
1 Physics and Materials Science, Missouri State University, Springfield, Missouri, United States
Show Abstract9:00 PM - M3.59
Energy Transfer in Bichromophoric Systems based on Alq3 and Oligofluorene moieties.
Victor Montes 1 , Pavel Anzenbacher 1
1 Chemistry and Center for Photochemical Sciences, Bowling Green State University, Bowling Green, Ohio, United States
Show AbstractEnergy transfer processes play an important role in organic electronic materials, particularly in the OLEDs field. Recently, significant advances have been achieved through understanding of exciton generation as well as the energy transfer mechanisms taking place in the systems of interest. An appealing strategy for obtaining OLED devices with high performance is based on combining different types of electroluminescent materials, hosts, and dopants that undergo efficient energy transfer. The effective energy transfer allows for lowering the amount of the dopant in the final material, which would not only be cost effective but it would also alleviate potential problems with phase separation during material processing.In this presentation, we report the synthesis, photophysics and semiconductor properties of a series of conjugated metallodendrimer materials which comprise two electroluminophores well known in OLED applications: tris(8-hydroxyquinoline) Aluminum(III) and oligofluorene moieties of a various number of fluorene units (n=1-9). Solution photoluminescence studies of these bichromophoric systems exhibit energy transfer between the conjugated fragment and the luminescent Alq3 centers depending on the size of the oligofluorene segment. Our experimental results support recent theoretical descriptions put forward by Bredas and Beljonne for singlet excitons migration in polymer chains, which incorporate acceptor moieties as energy traps. According to these models, computed values for transfer rates between conjugated segments in the polymer (exciton hopping) are maximized for relatively short donor and acceptor lengths. When the donor and acceptor sizes surpass these optimal lengths (around 2-3 units), the overall energy transfer mechanisms become less efficient compared to the competing radiative decay from the donor.In the solid state, however, only emission from the quinolinolate centers is observed regardless of the excitation wavelength, an indicative of a more effective energy transfer processes. Incorporation of these bichromophoric materials into OLED devices suggests that these materials can behave as both good hole- and electron-transporting layers allowing the use of simple configurations. In fact, external quantum efficiencies as high as 5% have been recorded for unoptimized devices, which suggests potential application in OLEDs
9:00 PM - M3.6
Oxidatively Initiated Chemical Vapor Deposition of PEDOT.
Sung Gap Im 1 2 , Karen Gleason 1
1 Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Institute For Soldier Nanotechnologies, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractThin film applications of conducting conjugated polymers, such as poly 3,4 ethylenedioxy thiophene (PEDOT) are often limited due to their insolubility in common solvents and infusibility with heat, While aqueous dispersions of PEDOT can be created by the addition of polystyrene sulfonate(PSS) [e.g. Baytron P®], the solubility is obtained at the expensive of lower conductivity (<10 S/cm). By contrast, the chemical vapor deposition (CVD) method reported here, enables the deposition of PEDOT without the PSS matrix and conductivities of >100 S/cm can be achieved. The CVD method requires introducing both the oxidant, iron(III) chloride, and EDOT monomer in the vapor phase. The CVD reactor allows for sublimation of the iron chloride directly into the reactor chamber operating at 150 mtorr. Spontaneous reaction of the oxidant with the monomer introduced though a feedback controlled mass flow system, results in the rapid (> 200nm thick film in 30 min) formation of π-conjugated PEDOT films directly on the substrate. The films are transparent(> 90%) and uniform over a 150 mm diameter area. The PEDOT step growth polymerization reaction consists of three main steps: 1) oxidation of monomer or oligomer forming cation radicals, 2) combination of cation radicals, and 3) deprotonation from the polymer backbone. Fourier transform infrared spectroscopy (FTIR) reveals an increasing degree deprotonation reaction and increasing conjugation length as substrate temperature is increased from 25 °C to 140 °C. We observed a corresponding increase in the conductivity and calculated apparent activation energy of 27.1 ± 1.1 kcal/mol. With the addition of pyridine as a base in the vapor phase, the oxidation strength could be systematically controlled. Proper tuning of the oxidation strength of enables formation of high molecular weight PEDOT and enhances the rate of deprotonation step that forms conjugated PEDOT. The maximum obtained conductivity of >100 S/cm is very promising and further optimization is under way.
9:00 PM - M3.60
Molecular Photoconductive and Photorefractive Systems. From Macro-scale Towards Nano-scale Molecular Devices.
Irena Kratochvilova 1 , Karel Kral 1 , Stanislav Nespurek 1
1 Dielectrics, Institute of Physics, Prague 8 Czech Republic
Show AbstractWe study experimentally and theoretically relations between photochemical activity and electronic properties of disordered molecular solids, in particular polymers, on various levels of aggregation, from molecular scale to macroscopic samples. The interest is focused on photo-induced micro- and macroscopic structural changes. A parallel area of interest is the charge carrier transport in molecular structures. The carrier mobility is studied by several techniques: time-of-flight, field-effect transistor, drift of temporal gratings. Charge movement in molecular wires is modeled by quantum mechanical methods (energy structure, excitation energy decay, polaron formation, electron-phonon coupling, formation of quantum dots). Carrier trapping by photochromic additives can be used in molecular opto-electronic switches. The results will be utilized in the device engineering of thin film FET transistors, light detectors, electroluminescent diodes and gas sensors.
9:00 PM - M3.61
Self-Assembled Monolayers in Organic Diodes.
Bert de Boer 1 , Hylke B. Akkerman 1 , Afshin Hadipour 1 , Magda M. Mandoc 1 , Dago M. de Leeuw 2 , Paul W. M. Blom 1
1 Molecular Electronics, Materials Science Centre, University of Groningen, Groningen Netherlands, 2 , Philips Research Laboratories, Eindhoven Netherlands
Show Abstract9:00 PM - M3.62
Synthesis and Structural Elucidation of Disordered Two-Dimensional Nitrogen-Rich Carbon Nitride Materials.
Edward Gillan 1 2 , Dale Miller 1 2 , James Holst 1 2
1 Department of Chemistry, University of Iowa, Iowa City, Iowa, United States, 2 Optical Science and Technology Center, University of Iowa, Iowa City, Iowa, United States
Show AbstractMolecular and polymeric structures based on carbon and nitrogen containing heterocycles have a long and colorful history. For example, some serve as macrocyclic ligands to sequester transition metals and others find use as organic dyes and light emitting materials. One recent and somewhat controversial chapter in C-N materials development involves postulated nitrogen-rich three-dimensional solid state structures, specifically ones at or near a C3N4 composition, To date, there have been no verifiable reports of the synthesis of a theoretical superhard diamond-like sp3 C3N4. In contrast, several precursor routes to kinetically stable nitrogen-rich carbon nitrides have produced disordered network structures with two-dimensional (sp2 carbon) bonding more akin to graphite. These extended structures comprise a varied and ill-defined class of solids generally called carbon nitrides. Our research program has developed a range of thermally unstable symmetric triazine (C3N3) and heptazine (C6N7) precursors that produce “carbon nitride” powders and thin films with compositions in the C3N4 range using thermolysis and chemical vapor deposition methods. One key component of nearly all materials in this class is that they very often contain residual hydrogen provided by precursor components or by passivation of reactive bonds in the as-synthesized structure. Many published “carbon nitride” reports ignore hydrogen components, even though most of these materials are better formulated as CxNyHz materials. Several powders synthesized in our program have formulas near C3N4.5H1.5. These extended solids are thermally stable to 500 °C, some exhibit optical absorption and emission in the blue-green spectral region, and powders can be thermally sublimed to deposit films. The powders are unreactive in concentrated acids and bases and are insoluble a variety of organic solvents. Recent progress with “chemical deconstruction” structural analysis on these insoluble chemically stable carbon nitrides will be presented. This process involves chemically degrading the carbon nitride network and identifying CN molecular fragments as possible monomer building blocks for the extended structure. Preliminary indications are that thermally decomposed triazine precursors likely lead to network structures containing larger heptazine components. Experiments in utilizing these carbon nitrides as metal support materials will also be presented.
9:00 PM - M3.63
Nanoscale Characterization Of Solution-Cast Poly(Vinylidene Fluoride) Thinfilms Using Atomic Force Microscopy.
Taekwon Jee 1 , Theodore Eisenbach 1 , Hong Liang 1
1 Mechanical Engineering, Texas A&M University, College Station, Texas, United States
Show AbstractThis paper focuses on the characterization of thinfilms made of poly(vinylidene fluoride) (PVDF). Surface analysis techniques include an atomic force microscope (AFM), profilometer, and Fourier Transform Infrared Spectroscopy (FTIR). Thinfilms of PVDF were fabricated by a using a spin coating method with different viscosities and spin speeds. Subsequently, in situ corona poling with annealing of spin-cast PVDF was conducted to enable a phase change from α to the mixture of β and γ phases. The surface roughness was evaluated using the AFM and profilometer. The phase and conformational changes of thinfilms were investigated using the FTIR. In this paper, the surface properties-process relationships are investigated based on experimental results. Polymeric behavior and spherulites will be discussed. It is found that the change of microstructure not only affect piezoelectric and other physical properties, but also influence the surface forces such as nanoscale friction and adhesion.
9:00 PM - M3.8
High Performance and Light Emission Mechanism Study for RGB Phosphorescent Organic Light Emitting Diodes.
Jun Yeob Lee 1 , Oh Young Kim 1
1 Department of Polymer Science and Engineering, Dankook University, Seoul Korea (the Republic of)
Show AbstractThe light emission mechanism of phosphorescent light emitting diodes(OLED) was investigated by changing the doping profile of phosphorescent emitting material in the light emitting layer. In the red phosphorescent devices, both the hole and electron trapping had the same effect on the charge injection in the devices, while in the green devices the hole trapping played a major role in determining the charge injection of the devices. In the blue case, the hole or electron trapping effect was not observed. The efficiency of the red and green devices could be improved by multilayer structure due to high current density at the same driving voltage and the lifetime of phosphorescent devices could be improved by more than 50% using multilayer structure.
9:00 PM - M3.9
Efficient Hole Injection in Top-emitting Organic Light Emitting Diodes Using C60 as a Buffer Layer for Al Reflective Anodes.
Jun Yeob Lee 1
1 Department of Polymer Science and Engineering, Dankook University, Seoul Korea (the Republic of)
Show AbstractThe hole injection of the organic light emitting diodes with Al as a reflective anode for top emitting devices was improved by using C60 as a thin buffer layer between Al and a hole transport layer. The driving voltage of the devices with C60 buffer layer was 5.5 V compared with 11 V for the devices without C60 buffer layer. The decrease of interfacial energy barrier by interface dipole formation between Al and C60 contributed to the low driving voltage of the devices. In addition, the driving voltage of the devices with LiF/C60 interfacial buffer layer could also be improved greatly by ohmic contact between Al and LiF/C60. The efficiency of the devices could also be improved considerably by inserting interfacial C60 containing buffer layer on top of Al. This work will enable the application of Al anode for top-emitting devices because the critical high driving voltage problem of Al top-emitting devices can be improved by this work.
Symposium Organizers
Zhenan Bao Stanford University
Anna B. Chwang Universal Display Corporation
Lynn Loo The University of Texas-Austin
Rachel A. Segalman University of California-Berkeley
M4: Charge Transport Through Organic Semiconductors
Session Chairs
Wednesday AM, April 19, 2006
Room 3001 (Moscone West)
9:30 AM - **M4.1
Charge Transport in pi-Conjugated Materials: A Molecular Viewpoint
Jean-Luc Bredas 1
1 School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractConjugated organic oligomer and polymer materials are being increasingly considered for incorporation as the active semiconductor elements in devices such as light-emitting diodes, photo-voltaic cells, or field-effects transistors. In the operation of these devices, electron-transfer and energy-transfer processes play a key role, for instance in the form of charge transport, energy transport, charge separation, or charge recombination. Here, we provide a theoretical description of electron-transfer phenomena based on electron-transfer theory, which allows us to provide a molecular, chemically-oriented understanding. In this presentation, we focus on the parameters that impact the mobility of charge carriers, that is the electronic coupling within chains and between adjacent chains and the reorganization energy of the chains upon ionization. Materials under study include conjugated oligomers such as oligoacenes, oligothiophene-acenes, oligothiophenes, and oligothienacenes.
10:00 AM - M4.2
High Performance n-type Organic Field-effect Transistors.
Mang-mang (Mike) Ling 1 , X. Mo 2 , M. Shi 2 , M. Wang 2 , Hongzheng Chen 2 , Zhenan Bao 1
1 Chemical Engineering, Stanford University, Stanford, California, United States, 2 Department of Polymer Science and Engineering, Zhejiang University, Hangzhou, Zhejiang, China
Show AbstractOrganic thin film transistors (OTFTs) play an important role in applications such as large-area electronics, light-emitting diodes, smart cards, and sensors (1,2,3). For realization of organic complementary metal-oxide semiconductor (CMOS) circuits, both p-type and n-type semiconductors with suitable physical, chemical, electrical, and/or photochemical properties are required (4). In this work, a series of n-type organic semiconductors based on perylene diimides are synthesized. These materials are terminated on the imide position of perylene diimides core by phenyl groups with different substituents (i.e., F, Cl, CF3, etc.) at different position. We systematically studied the effect of substitution of the ending phenyl group on material stability transistor charge carrier mobility. Substituents with stronger electron-withdrawing ability was found to have higher field-effect mobility. TFT with fluorinated perylene diimides showed excellent air-stability. Mobility as high as 0.068 (cm2/Vs), on/off current ratio over 100000, and threshold voltage as low as 0.4 V were obtained. Mechanisms of the correlation between the fluorination and the TFT air-stability was studied. Effects of surface chemical treatments, substrate temperature and thin film morphology were also discussed.1) Kelley, T.; Boardman, L. D.; Dunbar, T. D.; Muyres, D. V.; Pellerite, M. J.; Smith, T. P. J. Phys. Chem. B 2003, 107, 58772) Dimitrakopoulos, C. D.; Malefant, P. R. L. Adv. Mater. 2002, 14, 99. 3) Katz, H. E.; Bao, Z. J. Phys. Chem. B 2000, 104, 671.(4) Nelson, S. F.; Lin, Y. Y.; Gundlach, D. J.; Jackson, T. N. Appl. Phys. Lett. 1998, 72, 1854.
10:15 AM - M4.3
Influence of Impurities and Structural Properties on the Electronic Transport of Pentacene Thin Film Transistors.
Dietmar Knipp 1 , Tobias Muck 1 , Amare Benor 1 , Veit Wagner 1
1 , International University Bremen, Bremen Germany
Show AbstractDespite the realization of polycrystalline pentacene transistors with high mobility the electronic transport is not fully understood. In particular the influence of impurities like oxygen and moisture on the charge transport is still under investigation. Electrical in-situ measurements of pentacene TFTs were carried out to study the influence of oxygen on the device operation. The polycrystalline pentacene films transistors were prepared by Organic Molecular Beam Deposition (OMBD) with hole mobilities of >0.5cm2/Vs in air and under ambient conditions. Exposing the transistor to oxygen leads to a significant change of the sub-threshold characteristic of the transistors. Under vacuum conditions the onset of the drain current is observed for small negative gate voltages. Unintentional doping of the pentacene film leads to a shift of the onset of the drain current towards positive gate voltages. As a consequence the sub threshold slope is increased from 0.2V/decade for 50nm thick thermal oxide dielectrics to 1.5V/decade. The onset of the drain current shifts towards positive gate voltages for increasing exposure time and oxygen concentration. The threshold voltage and the mobility of the transistors are only slightly affected by the oxygen contamination. Only for high concentrations of oxygen or long exposure times the threshold voltage of the transistors shifts towards positive gate voltages and the mobility is affected. Numerical simulations of pentacene transistors using a density of state charge transport model were carried out. The simulations exhibit that the onset of the drain current for positive gate voltages is caused by acceptor-like defect states deeper in the bandgap. These acceptor-like defects are mainly located at the drain and source contacts as the structural order of the pentacene film is reduced in this area and the grain boundary density is increased. With increasing grain boundary density the influence of oxygen on the device operation is increased.
10:30 AM - **M4.4
Charge Transport at Organic-organic Heterointerfaces.
Henning Sirringhaus 1
1 Cavendish Laboratory, University of Cambridge, Cambridge United Kingdom
Show AbstractPolymer transistors are becoming an enabling technology for realising electronic circuits on large-area flexible substrates, but many aspects of their device physics remain to be explored. In this work we will present recent advances towards realizing ambipolar organic transistors through control of interface electronic structure and realization of ambipolar light-emitting field-effect transistors, in which the position of the recombination zone can be moved to any position along the transistor channel. We will also discuss recent progress towards manufacturing of organic transistor circuits using the self-aligned inkjet printing technique which is capable of defining channel lengths with sub-100 nm dimensions.
11:30 AM - **M4.5
Microstructure / Mobility Correlations in Organic Semiconductors.
Dean DeLongchamp 1 , Joseph Kline 1 , Brandon Vogel 1 , Youngsuk Jung 1 , Sharadha Sambasivan 2 , Marc Gurau 3 , Lee Richter 3 , Daniel Fischer 2 , Eric Lin 1
1 Polymers Division, National Institute of Standards and Technology, Gaithersburg, Maryland, United States, 2 Ceramics Division, National Institute of Standards and Technology, Gaithersburg, Maryland, United States, 3 Surface and Microanalysis Science Division, National Institute of Standards and Technology, Gaithersburg, Maryland, United States
Show AbstractThe crystalline microstructure of organic semiconductor films critically influences carrier transport in transistors because it controls the persistence and quality of π overlap in the source-drain plane. Important aspects of this microstructure are the size and connectivity of crystals, the packing arrangement within crystals, and the surface-relative crystal orientation. Because organic semiconductor films are deposited from vapor or solution, this microstructure develops in a dynamic assembly process that is influenced by the semiconductor primary chemical structure, its crystallization rate, and substrate characteristics.We employ synchrotron-based Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy to investigate crystalline microstucture development in several types of organic semiconductors. Microstructure is revealed by the bond orientation within films; one or more bond orientations specify a molecular orientation, from which we assess the surface-relative crystal orientation and the limiting extents of crystallinity or order.The NEXAFS technique is particularly valuable for examining the crystalline microstructure of polymer semiconductors, which are often inscrutable by atomic force microscopy. We examine the effect of drying rate on the microstructure of regioregular poly(3-hexylthiophene) (P3HT) by changing the speed of the spin-coating process. The preferential packing of P3HT varies from an edge-on motif achieved at slow drying rates to a plane-on motif achieved at fast drying rates. The edge-on motif presumably allows better π overlap in the film plane, which results in six-fold higher field-effect mobility over plane-on oriented films. Casting solvent choice causes the same effect; less volatile solvents dry more slowly and exhibit less plane-on packing.The influence of substrate chemistry on the assembly of crystalline microstructure is particularly challenging to understand or predict. A nonpolar dielectric surface typically enhances transistors by increasing field effect mobility, improving sub-threshold behavior, or reducing bias stress. These enhancements are often attributed to improved order at the buried dielectric interface, but evidence of microstructure improvement is sparse. To search for evidence, we delaminate semiconductor films from the dielectric and expose the buried interface. By confining the NEXAFS sampling volume to within 6 nm of the free surface, we evaluate the molecular orientation of the semiconductor in the mobile channel region ex situ. We compare P3HT films cast upon bare silicon and silicon modified with hexamethyldisilazane, octyltrichlorosilane, and octadecyltricholorosilane. These nonpolar dielectric modifications do not dramatically enhance the edge-on packing of P3HT in the mobile channel. These measurements will be extended to next-generation materials to illustrate the microstructural basis underlying performance enhancement.
12:00 PM - M4.6
High Mobility in Single-crystal Field-effect Transistors of Rubrene Derivatives With and Without Pronounced π-stacking.
Arno Stassen 1 , Simon Haas 1 , Goetz Schuck 1 , Bertram Batlogg 1 , Ulrich Berens 2 , Hans-Joerg Kirner 2
1 Laboratory for Solid State Physics, ETH, Zurich Switzerland, 2 Ciba Specialty Chemicals Inc., Group Research, Basel Switzerland
Show AbstractIn the field of organic semiconductors, rubrene is at this moment the material with the highest reported mobility in single crystal field-effect transistors. The excellent transport properties of rubrene are attributed to the co-planar π-stacking along the crystallographic b-axis, in contrast to the herringbone stacking found in unsubstitued polyacenes such as tetracene and pentacene. We have investigated two rubrene derivatives, which differ only in the position of one of the substituents. Single crystals have been grown by physical vapor transport. The quality of the crystals has been probed by SCLC spectroscopy, revealing deep-trap densities as low as 1015 cm-3 eV-1, comparable to that in the best rubrene crystals. The crystal structures of the two derivatives differ substantially. The structure of the first derivative is almost identical to that of unsubstituted rubrene, with even more closely spaced naphthacene backbones (3.54 Å vs. 3.70 Å in rubrene). In the second material no co-planar stacking of naphthacene backbones is present, with a minimal distance between the naphthacene backbones of over 8 Å.This difference in crystal packing does not affect the good mobility of holes: in ‘flip crystal FETs’, device mobilities over 8 cm2/Vs have been measured for both derivatives. These values are comparable to identically fabricated rubrene devices. This observation of high charge mobility not associated with close-spaced co-planar π-electron stacking highlights the potentially beneficial role of side-groups in charge transport.
12:15 PM - M4.7
Probing the Anisotropic Field Effect Mobility of Solution-Deposited Single Crystal Thiophene Oligomers.
Stefan Mannsfeld 1 , Jason Locklin 1 , Zhenan Bao 1
1 Chemical Engineering, Stanford University, Stanford, California, United States
Show AbstractOrganic thin-film transistors (OFETs) are attractive for microelectronic applications such as sensor arrays or small-scale displays. The so-called field effect mobility μ is an important quantitative figure, describing the electrical performance of an OFET device. However, the understanding of charge transport in such devices is still incomplete. A OFET device comprising a single organic crystal as active layer is highly suitable for studying the impact of the crystal morphology on the charge carrier mobility. In such a device, the resulting device performance will strongly depend on the orientation of the crystal with respect to transistor electrodes, reflecting the anisotropy in the packing of the organic molecules in a single crystal and the resulting anisotropy in μ. This is in contrast to thin film devices where the experimentally determined value of μ is averaged over numerous crystal grains.We prepare single crystal OFET devices by drop-casting a solution of novel, highly soluble thiophene oligomers onto a bottom contact silicon-based device with large arrays of pre-deposited gold electrodes. After evaporation of the solvent, numerous small single crystals (50-100 μm) are randomly deposited on the device, enabling us to measure statistical amounts of differently oriented crystals.Here, we present results from electrical characterization of the OFET devices, discuss the obtained μ-anisotropy, and correlate it with the in-plane crystal structure.
12:30 PM - **M4.8
Electronic Transport In Polycrystalline Polymer Tfts.
Robert Street 1 , M. L. Chabinyc 1 , A. Salleo 1 , A. C. Arias 1 , J. Northrup 1
1 , Palo Alto Research Center, Palo Alto, California, United States
Show Abstract Solution-deposited polycrystalline polymers have reached mobility of 0.1-1 cm2/Vsec, coming close to the mobility of amorphous silicon and the vapor deposited organic small molecules. These improvements result from the development of new materials and in the understanding of the interaction between surface energy, film structure and mobility. In high mobility materials, electronic conduction tends to be band-like and we discuss multiple trapping transport based on the calculated electronic structure calculations and the polycrystalline structure. One enduring issue with polymer electronics is the expected lifetime of devices as a result of both exposure to the ambient and the application of electrical stress. To replace amorphous silicon in display backplanes or in other applications, the polymers must be sufficiently stable. Pulsed measurements of electrical stress in polythiophene (PQT-12) provide information about the performance of TFT backplanes and we describe increasingly long term lifetime studies. Electrical stress proves to be a complex process with stress and recovery occurring at different timescales. The results are discussed in terms of bipolaron trapping and other mechanisms. The lifetime studies are consistent with the operation of TFT backplanes, and we describe recent results showing the performance of jet-printed polymer TFT arrays.
M5/L8: Joint Session: Organic Field Effect Transistors I
Session Chairs
Wednesday PM, April 19, 2006
Room 3001 (Moscone West)
2:30 PM - **M5.1/L8.1
Vapor and Solution Deposited Organic Thin Film Transistors
Tom Jackson 1
1 Department of Electrical Engineering, Center for Thin Film Devices and Materials Research Institute, Penn State University, University Park, Pennsylvania, United States
Show AbstractThe history of the microelectronics industry over the past four decades or more has been largely an elaboration of Moore’s Law, but many future electronic opportunities will be in non-Moore’s law applications. Organic thin film electronics is well suited to non-Moore’s-law electronics applications because of the diversity and flexibility of the devices, function, and processing it allows. Organic thin film transistors (OTFTs) are particularly interesting because they can act as enabling devices to allow the integration of other organic devices in displays, sensor arrays, low cost memories, and other applications in addition to use in standalone OTFT applications.For practical device and system use, OTFTs must demonstrate the uniformity, reproducibility, reliability, and integration with other devices needed for realistic applications. As one possible application, we have considered the integration of vapor-deposited OTFTs with organic light emitting diodes (OLEDs) and have fabricated small test displays that allow us to investigate device characteristics and passivation and isolation requirements for integrating these organic devices.Many organic device applications are likely to be cost sensitive and solution-deposited organic semiconductors may offer important advantages for low-cost processing. However, solution processed semiconductors typically lack the molecular-level order which may be important for good carrier transport and large field-effect transistor mobility. Working with J. Anthony (University of Kentucky) we have investigated functionalized pentacenes and pentacene derivatives. These materials use bulky molecular side groups to control molecular packing and allow solubility in a range of common solvents. Surprisingly, solution-deposited films of some of these materials show good molecular ordering and, using these materials, we have been able to fabricate OTFTs with mobility > 1.5 cm2/V-s.The technological landscape appears ripe for an explosion of organic electronic applications. Details of device structure, function, and performance are critically related to the success of various application possibilities and organic thin film electronic offers unique advantages. The wide range of device and application possibilities as well as physical phenomena makes this one of the most interesting and exciting areas of device physics and engineering with potentially large economic and societal impact.
3:00 PM - M5.2/L8.2
Hot Wall Epitaxially Grown Fullerene Films for High-Mobility Organic Thin-Film Transistors.
Birendra Singh 1 , Nenad Marjanovic 1 , Helmut Neugebauer 1 , Niyazi Sariciftci 1 , Alberto Ramil 2 , Andrei Andreev 3 , Helmut Sitter 2 , Siegfried Bauer 4 , Thomas Anthopoulos 5 , Dago Leew 5
1 Linz Institute of Organic Solar Cells (LIOS), Physical Chemistry, Johannes Kepler University of Linz, Linz Austria, 2 Institute of Semiconductor and Solid State Physics, Johannes Kepler University of Linz, Linz Austria, 3 Institute of Physics, University of Leoben, Leoben Austria, 4 Soft Matter Physics (SOMAP), Johannes Kepler University of Linz, Linz Austria, 5 Philips Research Laboratories, Prof. Holstlaan 4 (WAG11), Eindhoven Netherlands
Show AbstractHot wall epitaxy (HWE) is a well known technique for growing highly ordered thin films of C60 and para-hexaphenyl nano-needles.1 Here we report on HWE grown fullerene based n-channel organic field-effect transistors (OFETs) with electron mobility in the order of 6 cm2/Vs and current on/off ratio >105. Optimisation of these devices includes use of solution-processed high-glass transition temperature organic dielectrics and accurate control of substrate temperature during film growth. To demonstrate the quality of our HWE C60 films we fabricate an unipolar (n-type) seven-stage ring oscillator with oscillating frequency as high as 30 KHz. The HWE technique is also used to grow crystalline nano-needles of para-hexaphenyl on crystalline substrates.1 The average needle length and width are in the order of 30 µm and 100 nm respectively. The anisotropic charge carrier transport of para-hexaphenyl nano-needles based high mobility OFETs and from the measurement of space charge limited current (SCLC) are also presented. 1A. Andreev, G. Matt, C. J. Brabec, H. Sitter, D. Badt, H. Seyringer and N. S. Sariciftci, Adv. Mat. 12, 629 (2000).
3:15 PM - M5.3/L8.3
Optical Effects in Rubrene OFETs and Related Optical Properties of Rubrene Single Crystals.
Vitaly Podzorov 1 , Hikmat Najafov 2 , Matt Calhoun 1 , Ivan Biaggio 2 , Michael Gershenson 1
1 Physics Department, Rutgers University, Piscataway, New Jersey, United States, 2 Physics Department, Lehigh University, Bethlehem, Pennsylvania, United States
Show AbstractOptical absorption, luminescence, and charge carrier photoexcitation mechanisms of rubrene single crystals will be briefly discussed [1], followed by a discussion of novel optical effects observed in rubrene OFETs. Photo-induced charge transfer of positive or negative charges across the interface between the ordered organic crystal and a polymeric insulator is observed in the field-effect experiments [2]. The effect occurs when OFET is illuminated through a semi-transparent gate electrode with photons of energy exceeding the absorption edge of rubrene. The transferred charge is trapped in the insulator, resulting in a shift of the field-effect onset that can be controlled by light intensity and by the sign and magnitude of the gate voltage applied during illumination. 1. H. Najafov, et al. http://arxiv.org/abs/cond-mat/05107682. V. Podzorov and M. E. Gershenson, Phys. Rev. Lett. 95, 016602 (2005);
3:30 PM - **M5.4/L8.4
Solution-Processed Polymer Ferroelectric Field-Effect Transistors.
Paul Blom 1 , Ronald Naber 1 , Gerwin Gelink 2 , Albert Marsman 2 , Dago de Leeuw 2
1 Materials Science Centre, University of Groningen, Groningen Netherlands, 2 , Philips Research Laboratories, Eindhoven Netherlands
Show AbstractWe demonstrate a rewritable, nonvolatile memory device with flexible plastic active layers deposited from solution. The memory device is a ferroelectric field-effect transistor (FeFET) made with a ferroelectric fluoropolymer and a bisalkoxy-substituted poly(p-phenylene vinylene) semiconductor material. The on- and off-state drain currents differ by several orders of magnitude, have a long retention time, a high programming cycle endurance and short programming time. The remanent semiconductor surface charge density in the on-state has a high value of 18 mC/m2, which explains the large on/off ratio. Application of a moderate gate field raises the surface charge to 26 mC/m2, which is of a magnitude that is very difficult to obtain with conventional FETs because they are limited by dielectric breakdown of the gate insulator. In this way, the present ferroelectric/semiconductor interface extends the attainable field-effect band bending in organic semiconductors.Using an ambipolar bulk heterojunction semiconductor the polarity of the ferroelectric FET can be remanently switched from n- to p-type and back. The high charge density of 28 C/m2 induced by the ferroelectric combined with a high mobility in a regioregular polythiophene in a top-gate transistor layout yield a record conductance value of 0.3 μS for conjugated polymer based FETs.
M6: Organic Field Effect Transistors II
Session Chairs
Wednesday PM, April 19, 2006
Room 3001 (Moscone West)
4:30 PM - **M6.1
Organic Semiconductors Towards a-Si Performance.
Janos Veres 1 , Simon Ogier 1 , Stephen Leeming 1 , Giles Lloyd 1 , Domenico Cupertino 1 , Richard Williams 1 , Munther Zeidan 1 , Bev Brown 1
1 Organic Semiconductors, Merck Chemicals Ltd., Manchester United Kingdom
Show AbstractIn this talk recent advances in developing improved organic semiconductors (OSC) towards performance levels of a-Si are described. Evaporated organic materials such as pentacene have already proved that such performance is possible, however, solution coated OSC have been lagging behind either in mobility or stability. Merck have been developing a range of solution coated OSC which now reproducibly break the mobility barrier of 0.5 cm2V-1s-1. The organic layer can be coated uniformly over a large area, affording good yield on flexible or rigid substrates. The devices are fabricated and operated in air, exhibiting high on/off ratios. The excellent stability of these compounds is also discussed in terms of mobility, on/off ratio and bias stress. The potential of these materials will be discussed for a variety of flexible electronic applications.
5:00 PM - M6.2
Controlling the Growth and Orientation of Molecular Crystals on Patterned Self-Assembled Monolayers for High Performance Organic Transistors.
Alejandro Briseno 1 , Mang Mang Ling 2 , Mark Roberts 2 , Joanna Aizenberg 3 , Zhenan Bao 2
1 Chemistry and Biochemistry, UCLA, Los Angeles, California, United States, 2 Chemical Engineering, Stanford University, Stanford, California, United States, 3 , Bell Laboratories, Lucent Technologies, Murray Hill, New Jersey, United States
Show AbstractOrganic single crystals are important for the fundamental understanding of charge transport in field-effect transistors. Although polycrystalline thin film devices are easier to fabricate compared to organic single crystal devices, their performance are limited by structural imperfections. Mobilities as high as 20 cm2/Vs have been reported for single crystal rubrene transistors [1]. Despite the high mobilities reported for single crystal devices, there are many factors limiting their applications. In addition to the difficulties of fabricating good electrical contacts to single crystals, the most challenging undertaking is handling the fragile crystals. Currently, single crystals are handpicked and made into an individual device but this method is impractical for fabricating a high density of devices over a large area. Furthermore, controlling the nucleation sites, sizes and orientation of organic semiconductor single crystals remains a challenge. In this presentation we report a method for inducing site-specific growth of large oriented organic semiconductor crystals using self-assembled monolayers (SAMs) as nucleation templates [2]. We show for the first time that molecular crystals can be patterned in well-ordered arrays for applications in large-area electronics. This method of patterning eliminates the need to hand-pick organic single crystals and enables the fabrication of several dozen transistors on a common device platform. Most importantly, our crystal growth method now makes it possible to use organic single crystals in applications other than intrinsic charge transport studies.1. Sundar, V. C.; Zaumseil, J.; Podzorov, V.; Menard, E.; Willett, R. L.; Someya, T.; Gershenson, M. E.; Rogers, J. A. Science 2004, 303, 1644-1646.2. Briseno, A. L.; Aizenberg, J.; Han, Y-J.; Penkala, R.A.; Moon, H.; Lovinger A.J.; Kloc, C.; Bao, Z. J. Am. Chem. Soc., 2005, 127, 12164 -12165.
5:15 PM - M6.3
Control of Channel Conductivity of Rubrene Single Crystal Field Effect Transistors.
Magnus Wikberg 1 , Woo-Young So 1 , Dave Lang 1 , Theo Siegrist 1 , Arthur Ramirez 1 , Christian Kloc 1
1 , Bell Labs, Lucent Technologies, Murray Hill, New Jersey, United States
Show AbstractCarrier mobility higher than 1 cm2/Vs has been measured in numerous organic single crystals FETs, making them interesting for microelectronic applications. The understanding why some organic pi-electron systems show high mobility and others, very similar molecules, show much lower mobility is crucial for design of efficient and robust organic semiconductor devices. It seems that transistor properties measured on FETs are extrinsic properties limited by technology used for transistor fabrication. However, to evaluate the applicability of organic semiconductors, intrinsic properties need to be assessed. We have carried out a program to purify and grow low defect density single crystals and fabricate FETs on their surfaces. Using graphite as electrodes and parylene as an insulator we measured maximal mobility in rubrene of 13 cm2/Vs and significant anisotropy of transport properties. To control the transistor properties, we chemically modify the channel area and measured the conductivity of transistor channels before covering it with dielectrics and gate electrode. We found that the channel area of rubrene is very sensitive on reduction and oxidation and that the transistor properties may be modified by performing chemical reactions on the crystal surfaces before finishing transistor structure.
5:30 PM - **M6.4
Structural and Electrostatic Complexity in Pentacene Films and Implications for OTFT Performance
C. Daniel Frisbie 1
1 Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota, United States
Show AbstractThe properties of organic semiconductor/insulator (O/I) interfaces are critically important to the operation of organic thin film transistors (OTFTs) currently being developed for printed flexible electronics. In this talk, I will describe our observations of structural defects and correlated electrostatic potential variations at the interface between the benchmark organic semiconductor pentacene and a common insulator, silicon dioxide. Using an unconventional mode of lateral force microscopy, we have generated high contrast images of the grain boundary (GB) network in the first pentacene monolayer. Concurrent imaging by Kelvin probe force microscopy (KFM) reveals localized surface potential wells at the GBs, indicating that GBs will serve as charge carrier (hole) traps. Scanning probe microscopy and chemical etching also demonstrate that slightly thicker pentacene films have domains with high line dislocation densities. These domains produce significant changes in surface potential across the film. The correlation of structural and electrostatic complexity at O/I interfaces has important implications for understanding electrical transport in OTFTs and for defining strategies to improve device performance.
M7: Poster Session: Organic Optoelectronics
Session Chairs
Thursday AM, April 20, 2006
Salons 8-15 (Marriott)
9:00 PM - M7.1
Electron Energy Level Alignment of Fluorene-based Polymer Light-Emitting Diodes.
Man Keung Fung 1 , Chun Sing Lee 1 , Shuit Tong Lee 1
1 , City University of Hong Kong, Hong Kong Hong Kong
Show AbstractThe energy level alignment of the fluorene-based polymer light-emitting diodes (LEDs) with a device configuration of Ca/poly(9,9-dioctylfluorene) (PFO)/ poly[9,9-dioctylfluorene-co-(bis-N,N'-(3-carboxyphenyl)-bis-N,N'-phenylbenzidine)] (BFA) /indium tin oxide (ITO) was measured using ultraviolet photoelectron spectroscopy and a photoluminescence spectrometer. Unlike some small molecule-based LEDs, we found that in the PFO-based LEDs, charge transfer occurs at the polymer/polymer interface as indicated by the difference in the vacuum energy levels, which may have some influence on the carrier injection. Moreover, the electron injection barrier can be much lower than the hole injection barrier by using low work-function metals as the cathode in the devices. Thus, the balancing of carrier recombination in the PFO-based LEDs may be limited by the hole current. Therefore, reducing the hole injection barrier should be an important consideration for the improvement of device performance in PFO-based LEDs.
9:00 PM - M7.10
Understanding the Impact of Morphology in a Rod-Coil Block Copolymer System on Light Emission in Organic LEDS.
Yuefei Tao 1 2 , Bradley Olsen 1 2 , Rachel Segalman 1 2
1 Department of Chemical Engineering, UC Berkeley, berkeley, California, United States, 2 Materials Science Division, Lawrence Berkeley Laboratory, Berkeley, California, United States
Show AbstractPlastic electronics have received great attention for potential applications such as light emitting diodes, photovoltaics and lasers. Despite a great deal of progress in improving the performance of organic optoelectronic devices, the fundamental relationship between nanoscale morphology and charge transport, brightness, efficiency remains unclear. The self-assembly of block copolymers provides an attractive means to control morphology and access a variety of nanostructured phases. Rod-like semiconducting polymers, however, do not follow classical block copolymer self-assembly patterns. To understand the impact of the rod-like chain morphology of semiconducting block copolymers, we recently demonstrated the self-assembly of a series of monodisperse rod-coil block copolymers in the weak segregation limit. This model polymer system self-assembles into lamellar, nematic, and isotropic phases and provides a basis for understanding the impacts of both microphase separation and liquid crystallinity on charge transport. Another rod-coil block copolymer system consisting of poly(phenylene vinylene) rod blocks (hole transport materials) and polyoxadiazole coils blocks (electron transport materials) is now used to explore the relationships between nanoscale structure and light emission properties. The bulk and thin film nanoscale properties will be compared to the performances of the devices.
9:00 PM - M7.11
Controlling Self-Assembly of Conjugated Rod-Coil Block Copolymers for Optoelectronic Devices.
Bradley Olsen 1 2 , Yuefei Tao 1 2 , Rachel Segalman 1 2
1 Chemical Engineering, University of California Berkeley, Berkeley, California, United States, 2 Materials Science Division, Lawrence Berkeley National Lab, Berkeley, California, United States
Show AbstractBlock copolymer self-assembly is an attractive route towards controlling nanometer scale order and several block copolymers have been suggested for the optimization of organic photovoltaic and light emitting diode devices. Conjugated block copolymers, however, follow non-classical thermodynamics and patterning rules. We have recently demonstrated the first equilibrium phase diagram of a model weakly-segregated rod-coil block copolymer system which self-assembles into lamellae with long-range order, nematic, and isotropic phases. Notably, many of the more intriguing structures of classical block copolymers including cylindrical and double gyroid morphologies are not observed when a rod-shaped conjugated block is present.The rodlike nature of a conjugated block also controls the thin film structure which is applicable to devices. Very thin films of polymers self-assemble into lamellae whose interfaces lie predominantly parallel to the substrate. The lamellar width is determined by the molecular size of the block copolymer. If the film thickness does not match an integral number of lamellae, an incommensurability exists. Rather than stretch the lamellae (and therefore the chains) to resolve the mismatch, the film forms surface structures to accommodate excess material (forming islands or holes). When the film thickness is closer to commensurate, the film can also accommodate mismatches by forming defects where the lamellar interfaces are oriented nearly perpendicular to the film surface. Within these defects the chains do not need to stretch to accommodate incommensurability. These defects form unusual patterns within the film. As the film thickness increases, the number of defects increases until all the lamellae are oriented perpendicularly from the substrate. We use a variety of depth profiling techniques including grazing incidence X-ray scattering and secondary ion mass spectroscopy to study the influence of substrate interactions on lamellar orientations. We will also demonstrate the application of this controlled self-assembly to a bipolar charge conducting block copolymer and discuss the effects of controlled nanoscale structure on a light emitting device.
9:00 PM - M7.12
Development of Digital Eletrochromic Window for Independent Control of Light Passing Degree.
Rei Furukawa 1 , Fengyu Su 2 , Chunye Xu 2 , Minoru Taya 2 1
1 Materials Science and Engineering, University of Washington, Seattle, Washington, United States, 2 Mechanical Engineering , University of Washington, Seattle, Washington, United States
Show AbstractIndependently controllable electrochromic (EC) polymer windows were developed in transparent substrate for optical application. The EC polymer, poly[3,3-dimethyl-3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepine] (PProDOT-Me2) which can switch color between blue and transparent, was deposited on patterned Indium Tin Oxide (ITO) layer on glass substrate. Microfabrication method was used to achieve the ITO pattern through the steps of mask design, photolithography and ITO etching. Different channels of EC window were connected to electric source separately, as a result, dark / transparent switching can be controlled independently for each channel, thus light beam can selectively pass through different sampling spots or reflect back. Such a device has potential in system integration with various optical sensors due to its precise and simple structure. It could be used to replace physical optical shutters in terms of scaling down.
9:00 PM - M7.13
Leading-edge Organic Electronic and Optoelectronic Devices: High-mobility thin-film Transistors with Operation Voltage Below 5V and Organic Photodiodes on Newspaper.
Barbara Stadlober 1 , Ursula Haas 1 , Anja Haase 1 , Hannes Maresch 1 , Heinz Pichler 1 , Valentin Satzinger 1 , Josef Krische 1 , Martin Zirkl 1 , Bernhard Lamprecht 1 , Elke Kraker 1 , Roland Thuenauer 1 , Georg Jakopic 1 , Guenther Leising 1
1 Institute of Nanostructured Materials and Photonics, Joanneum Research, Weiz Austria
Show AbstractOrganic thin film transistors (OTFTs) with high performance can be achieved by carefully controlling all relevant parameters determining the function of the transistor device, as there are: the morphology of the organic semiconductor thin film, its thickness and density, the dielectric constant of the gate dielectric and the magnitude of the contact resistance. In the past period we have produced a large body of pentacene OTFTs. We present our studies on different material families for the gate dielectric and examples were we successfully decreased the thickness of the dielectric layer to values below 150nm. The reduction of the contact resistance is essential for stable and high-performance OTFTs. As a key element in our OTFT manufacturing process development, we have quantitatively studied the growth of pentacene thin films (by physical vapour deposition) on different dielectrics for different temperature regimes by using scaling and rate equation theory. It turned out, that high-mobility pentacene OTFTs (µ~1 cm2/Vs) can be produced with operation voltages below 5V by minimizing the dielectric film thickness, minimizing the contact resistance and optimizing the pentacene morphology. A mobility model for OTFTs has been considered that fully accounts for the effects of grains and grain boundaries of the organic layer. Comparison between our simulation results of top contact devices and experimental data show a strong dependence of the mobility as a function of grain size - the carrier mobility increases with increasing grain size. We present results on the miniaturization of the critical dimensions of OTFTs to below 1µm in conjunction with investigations of short-channel effects as another brick in the wall for a deeper understanding of OTFTs.One major advantage of organic semiconductor devices over their inorganic counterparts is the possibility to manufacture organic devices on almost any user defined substrate. To demonstrate the enormous potential of this integration concept, we fabricated organic photodiodes on conventional newspaper sheets. Parylene acts as water and chemical barrier layer, an organic-inorganic hybrid dielectric layer serves as an excellent smoothing and base layer for the organic photodiodes, which were constructed as Tang-type bilayer cells [1]. The photodiodes show very good stability and excellent diode behaviour in current-voltage measurements of the devices under dark and illumination conditions. The spectral response covers the UV and visible spectral range.[1]C.W. Tang, Appl. Phys. Lett. 48, 183 (1986)
9:00 PM - M7.14
Characterization of Materials and Multilayer Structures of OLED by Spectroscopic Ellipsometry.
Christophe Defranoux 1 , Laurent Kitzinger 2 , Yannick Guinche 1 , Ludovic Jolivet 1 , Jean Philippe Piel 1
1 Application, SOPRA, BOIS COLOMBES France, 2 , SOPRA Inc., Menlo Park, California, United States
Show AbstractSpectroscopic Ellipsometry (S.E.) is a well adapted optical technique widely used for the characterisation of all types of thin films for thickness and optical indices on glass or plastic substrates. Flat Panel display (standard AMLCD or Low Temperature Polysilicon) manufacturers are routinely using this method to monitor and control the production line by taking advantage of the high throughput, high accuracy, and the limited measurement probe size of this technique. S.E. is also applied to the characterization of materials and multi-layer structures of organic light-emitting diodes (OLEDs); where all parameters of each layer are given in one single measurement. The main parameters of interest are the thickness and dopant values that are relatively small (lower than 50 nm and a few %). Complex organic materials can be analyzed accurately from their absorption bands in the visible and UV range; therefore, dopant concentrations can be measured, monitored and controlled in these films. It has already been demonstrated that a sensitivity of better than 0.1% for AlQ3 and a Hole Transport Layer can be achieved. As the OLED materials are sensitive to moisture and pollution, it is required to measure the materials optical properties and thickness values through an encapsulated layer. Only S.E. is able to measure these parameters through this encapsulated structure. We will demonstrate how we can measure single layer and multi-layers through encapsulated samples from the front side or from the back side of the substrate. This technique can be applied to monitor and control an OLED process in a production line. As demonstrated by our results, “Backside” and “through the cap” measurements can also be applied for the determination of the ITO resistance, without contact, by using Near Infra Red ellipsometry. Additionally, we will study the effect of the change of refractive index versus the resistance of the ITO. Correlation measurements with 4 Point Probe will also be presented. An OLED structure is not only comprised of Organic materials and ITO, but thin metallic layers like MgAg also need to be characterized during the process development and manufacturing stage of the display. As these layers are thin, S.E. can also measure their thickness. Correlation measurements with Grazing X Ray reflectometry will be presented as well.
9:00 PM - M7.15
Development of Light-Harvesting System using Green Phosphorescent Dyes coupled Dendrimer Host.
Tae-Hyuk Kwon 1 , Myoung Ki Kim 1 , Jongchul Kwon 1 , Su-Jin Park 2 , Dae Yeup Shin 2 , Kyung-Sik Lee 1 , Jong-In Hong 1 3
1 Chemistry, Seoul National University, Seoul Korea (the Republic of), 2 Corporate R&D Center, Samsung SDI, Yongin Korea (the Republic of), 3 Center for Molecular Design and Synthesis ., KAIST, Daejon Korea (the Republic of)
Show Abstract9:00 PM - M7.16
The Stretched Exponential Decay as a statistical tool to describe the OLED degradation mechanisms
Cina Salvatore 1 , Helene Cloarec 1 , David Vaufrey 1 , Tony Maindron 1 , Henri Doyeux 1
1 , Thomson R&D France , Cesson Cevigne France
Show Abstract9:00 PM - M7.17
Phenothiazine-labeled Dendrimers Encapsulated Dye for Light Emitting Diodes.
Dong Hoon Choi 1 , Jung-Il Jin 1 , Go Woon Kim 1 , Min Ju Cho 1
1 Chemistry, Korea University, SEOUL Korea (the Republic of)
Show AbstractLight-emitting devices (LEDs) made of organic low molar mass materials or polymers have drawn considerable attention over the past decade in view of their practical applications in flat-panel or flexible display devices. In polymer light emitting diodes (PLEDs), interchain aggregation of π-conjugated molecules often results in low luminance and low quantum efficiency. The extent of the interchain interactions in the solid films is dependent on the kind of solvent and the concentration of as-prepared polymer solution. As the solution is diluted, the interchain interaction is highly suppressed. Dendrimers having numerous functional chain ends all emanating from a single core have attracted considerable attention as novel luminescence materials for applications in organic light-emitting diodes (OLEDs). Compared to well known conjugated polymers, the dendrimers have the ability to spatially control the active components and thus the electroluminescence (EL) properties, and improve the stability of devices. Recently, light emission from dendrimer encapsulated chromophore have been studied for achieving the site isolation effect. In this work, we have synthesized the phenothiazine-labeled dendrimer encapsulated the emitting chromophore. Phenothiazine moieties were placed at the periphery and DCM derivative was used as a core dye. Light-emitting devices were fabricated in an ITO(indium-tin-oxide)/PEDOT/dendrimer/Li:Al alloy configuration. The properties of dendrimer encapsulated dye were measured by the photoluminescence (PL), electroluminescence (EL), current-voltage characteristics, respectively. The spectrum and the intensity of EL and PL are varied with the number of generation. Significant difference of the I-V-L characteristics and the external quantum efficiency in the dendrimer devices reveals that the EL performance of the higher-generation dendrimers is higher than that of the lower-generation ones. These results suggest that the site isolation between the chromophores suppresses the non-radiative decay process and reduces the interchain molecular excimer emission from the aggregated dendrimers.
9:00 PM - M7.18
Rod/Coil Conjugated Diblock-copolymers for Photovoltaic Applications: Design, Synthesis and Characterization.
Cyril Brochon 1 , Karin van de Wetering 1 , Rachel Segalman 2 , Chheng Ngov 1 , Sophie Barrau 3 , Thomas Heiser 3 , Georges Hadziioannou 1
1 Laboratoire d'Ingenierie des Polymeres pour les Hautes Technologies (LIPHT, UMR 7165), Centre National de la Recherche Scientifique, University Louis Pasteur, Strasbourg France, 2 Department of Chemical Engineering, University of California, Berkley, California, United States, 3 Institut d'Electronique du Solide et des Systemes (UMR 7163, INESS), Centre National de la Recherche Scientifique, University Louis Pasteur, Strasbourg France
Show Abstract9:00 PM - M7.19
Spectroscopic Study on Sputtered PEDOT:PSS : Role of Surface PSS Layer.
Jaehyung Hwang 1 , Fabrice Amy 1 , Antoine Kahn 1
1 Electrical Engineering, Princeton University, Princeton, New Jersey, United States
Show Abstract9:00 PM - M7.2
Spatial Control of the Recombination Zone in Ambipolar Light-Emitting Polymer Field-Effect Transistors.
Jana Zaumseil 1 , Carrie Donley 1 , Ji-Seon Kim 1 , Richard Friend 1 , Henning Sirringhaus 1
1 , University of Cambridge, Cambridge United Kingdom
Show Abstract9:00 PM - M7.20
Photocrosslinkable Nonlinear Optical Maleimide Copolymers for Electro-optics.
Dong Hoon Choi 1 , Sang Kyu Lee 1 , Min Ju Cho 1 , Jung-Il Jin 1
1 Chemistry, Korea University, SEOUL Korea (the Republic of)
Show AbstractOrganic materials exhibiting large electro-optic response (r33), have drawn considerable attention over the past two decades. These materials have extensive potential for use in telecommunications, digital signal processing, phased array radar, and many other applications as active materials in photonic micro-devices.Organic materials present clear advantages over inorganic materials that have been employed for similar applications. These advantages include lower cost, ease of processing and device integration, and low dielectric constant. A low dielectric constant creates the potential for highly increased bandwidth relative to analogous inorganic based devices. Materials of synthetic origin also present the possibility of application specific material tailoring. A stable and high second-order nonlinear optical (NLO) response is required for the polymers. A number of researches have focused on improving the NLO stability by synthesizing high Tg polymers, such as NLO polyimides, or by creating a thermo- or photocrosslinked polymer networks while poling. For photocrosslinkable polymers, there is a great advantage because these polymers can be directly patterned by illuminating the UV light through a mask for making channel waveguides and other integrated optical devices.Chromophore active materials that exhibit large first-order molecular hyperpolarizability (b) values typically also possess high ground state and transition dipole moments. We selected 2-(1-allyl-3-cyano-4-methyl-5-oxo-1,5-dihydro-pyrrol-2-ylidene)-malononitrile (TCP) as an acceptor to synthesize the chromophore. Electrochemical method and UV-Vis absorption spectroscopy were employed to study their energy states for estimating the microscopic nonlinearity. A new photocrosslinkable polymer with 2-(3-cyano-4-(2-{4-[hexyl-(6-hydroxy-hexyl)-amino]-phenyl}-vinyl)-5-oxo-1-{4-[4-(3-phenyl-acryloyl)-phenoxy]-butyl}-1,5-dihydro-pyrrol-2-ylidene)-malononitrile as a second-order nonlinear optical chromophore has been synthesized to investigate the thermal stability of the electro-optic effect. Photocrosslinking for electro-optics was achieved via either a [2+2] cycloaddition of chalcone moieties, or a cationic polymerization of oxetane moieties. Thin film of the polymer was deposited atop ITO-coated glass substrates, and evaluated for differences in EO activity. Using an in-situ, pole and probe, reflection apparatus, the dynamic behavior of EO response was observed. Signal decay times were employed to estimate the activation energies associated with relaxation of the dipolar, chromophore lattice.
9:00 PM - M7.21
Optical Nonlinearities of Tricyanopyrrolidene Chromophores in Amorphous Polycarbonate: Effect of Molecular Kink Moiety in the Conjugative Structure.
Dong Hoon Choi 1 , Min Ju Cho 1 , Sang Kyu Lee 1 , Jung-Il Jin 1
1 Chemistry, Korea University, SEOUL Korea (the Republic of)
Show AbstractOrganic chromophores having large molecular hyperpolarizabilitiy values, as well as polymeric composites containing them have drawn much attention over the past two decades. To achieve good device functionality, the NLO chromophore has to simultaneously possess the following criterion: high microscopic molecular nonlinearity, good thermal stability and photostability, low absorption, and weak molecular electrostatic interaction in the polymer matrix.As the intermolecular electrostatic interactive force becomes higher than the poling force, we could observe a decreasing behavior over a certain chromophore density at cooling process even in the presence of the poling electric field. Additionally, when the dipole moment of the chromophore is quite high enough to order in an antiparallel way, the molecules are easily packed each other even under poling field. In order to disrupt the geometrical planarity of the chromophore itself and reduce the electrostatic attraction between the chromophores, they were suggested to modify adding a kink moiety to distort the molecular structure slightly. Although the microscopic nonlinearity of the planar and highly polar chromophore is relatively high, the macroscopic nonlinearity in the polymer matrix induced by electric field poling process may not be exhibited comparably. In this work, we described the syntheses of three chromophores and the EO properties of the doped APC samples. We selected 2-(1-allyl-3-cyano-4-methyl-5-oxo-1,5-dihydro-pyrrol-2-ylidene)-malononitrile (TCP) as an acceptor to synthesize the chromophore. Electrochemical method and UV-Vis absorption spectroscopy were employed to study their energy states for estimating the microscopic nonlinearity. In order to compare the macroscopic nonlinearities of three kinds of APC samples precisely, we doped the chromophore with an identical molar concentration and poled them to measure the EO coefficient at 1300 nm.Real time poled and probe technique permits us to monitor the EO signal. The effect of the geometry of the chromophore was investigated on the EO properties. EO dynamic and isothermal stabilities of the APC samples were investigated for comparing the NLO performance of the TCP chromophore.
9:00 PM - M7.22
Effect of Interfacial Exciton Blocking Layer in Blue Emitting Electrophosphorescent Organic Light Emitting Diodes.
Ji Whan Kim 1 , Joo Hyun Kim 2 , Do Yeung Yoon 2 , Jang-Joo Kim 1
1 Materials Science and Engineering, Seoul National University, Seoul Korea (the Republic of), 2 Department of Chemistry, College of Natural Sciences, Seoul National University, Seoul Korea (the Republic of)
Show Abstract Newly synthesized cabazole derivatives were employed as host material and interfacial exciton blocking layer between hole transport layer and light emitting layer in blue electrophosphorescent organic light emitting diodes. Iridium(III) bis [(4,6-di-fluorophenyl)-pyridinato-N,C2']picolinate (FIrpic) was used as blue phosphorescent dopant. The high triplet (~3.01eV) of the derivatives suppress electron and energy transfer quenching pathways between the phosphorescent dopant and the host as well as between the adjacent HTL and EML, resulting in improvement of external quantum efficiency.
9:00 PM - M7.23
Structure Property Relationship Studies Of Fluorene-Thiophene Oligomers.
Ming Tang 1 , Stefan Mannsfeld 2 , Quan Yuan 2 , Mang-mang Ling 2 , Zhenan Bao 2
1 Chemistry, Stanford, Stanford, California, United States, 2 Chemical Engineering, Stanford, Stanford, California, United States
Show AbstractFlourene-thiophene oligomers show great promise as the active material in p-type thin-film transistors. These oligomers have a relatively low HOMO level, and are stable for several months in ambient conditions. Charge mobilities as high as 0.12 cm2/V have been reported for the symmetrically substituted dihexyl flourene-thiophene-flourene oligomer. In this study, a series of fluorene-thiophene-fluorene oligomers were asymmetrically substituted with an alkyl chain at one end of the oligomer. The alkyl groups are n-hexyl, n-octyl, and n-dodecyl respectively. These oligomers were characterized by 1H NMR, elemental analysis, mass spectrometry, TGA and DSC, while the thin films of these oligomers were characterized by UV, XRD and GIXD. The effect that the chemical structure has on the packing of these oligomers, both in the thin-film phase and bulk phase is examined, and its effect on charge mobility deduced.
9:00 PM - M7.24
Triplet Organic Radical Molecules as a New Hole-Transporting Material
Fumiaki Kato 1 , Takashi Kurata 1 , Hiroyuki Nishide 1 , Junji Kido 2
1 Applied Chemistry, Waseda University, Tokyo Japan, 2 Department of Polymer Science and Engineering, Yamagata University, Yonezawa Japan
Show Abstract Hole-transporting in organic semiconductors usually bases on electron-transfer among the highest occupied molecular orbitals (HOMO) of the organic materials, which corresponds to the valence band of inorganic semi-conductors. A new concept to utilize charge-transporting based on the redox among singly occupied molecular orbitals (SOMO) of radical molecules could realize an electric and magnetic property such as spin-polarized current or magnetoresistance. This paper describes triplet organic radical molecules bearing unpaired electrons on SOMOs as a new hole-transporting material. We designed and have synthesized novel nitroxide radical derivatives, 2-[4’-{N-4’’-methoxyphenyl-N-(4’’’-N’-t-butyl-N’-oxyaminophenyl)amino}phenyl]-1-yl-4,4,5,5-tetramethylimidazolin-3-oxide-1-oxyl (NNNOTPA) and poly{4-(N-t-butyl-N-oxyamino)triphenyamine-alt-5-hexyloxy-1,3-phenylenevinylene} (polyNOTPA). These radicals were extremely stable at ambient condition including against moisture. The ionization potential was 0.28 and 0.39 V (vs. Ag/AgCl) for NNNOTPA and polyNOTPA, respectively, and these redox processes of the radicals were completely reversible. The first oxidation was ascribed to the removal of the SOMO electron of the nitroxide moiety, and all the radical molecules were converted to the corresponding cations. The ionization potential for NNNOTPA and NOTPA measured from photoelectron spectroscopy was 5.1 and 5.4 eV, suggesting an effective hole-injection from ITO or common metals with high work function. Current density (J) − electric field (E) characteristics of the typical ITO/radical/Al and ITO/radical/Al/Ni devices revealed the effective hole-injection and transporting caused by the redox of the SOMO electrons of these radical molecules.
9:00 PM - M7.25
Rod/Coil Conjugated diblock-copolymers for Photovoltaic Applications : Self-assembling Process and Thin Film Properties.
Thomas Heiser 1 , Karin van de Wetering 2 , Sophie Barrau 1 , Martin Brinkmann 3 , Cyril Brochon 2 , Rony Bechara 1 , Georges Hadziioannou 2
1 Institut d'Electronique du Solide et des Systemes, University Louis Pasteur / Centre National de la Recherche Scientifique, Strasbourg France, 2 Laboratoire d'Ingenierie des Polymeres pour les Hautes Technologies, University Louis Pasteur / Centre National de la Recherche Scientifique, Strasbourg France, 3 Institut Charles Sadron, Centre National de la Recherche Scientifique, Strasbourg France
Show AbstractFunctionalized Rod/Coil diblock-copolymers are able to self-assemble into nanostructured thin films with novel opto-electronic properties. With a semiconducting hole-transporting conjugated polymer as rigid block and a flexible polymer grafted with electron accepting fullerene moities as coil block, nanometer-sized electron donor (D) and electron acceptor (A) domains can be obtained. Depending on the domain morphology and orientation, such an interpenetrating D/A network, used as active layer in a bulk heterojunction photovoltaic cell, can be advantageous. In turn, understanding the self-assembling process of these copolymers will allow a better control of the thin film morphology in organic optoelectronic devices.In this work we investigate self-organization in solution and in solid phase of diblock copolymers, composed of poly(p-phenylenevinylene) (PPV) derivatives and various coil blocks (poly[styrene-stat-chloromethylstyrene], poly[butylacrylate-stat-chloromethylstyrene] and poly[methylacrylate-stat- chloromethylstyrene]). The CMS groups are introduced to allow subsequent grafting of fullerenes. Different mixtures of good and poor solvents are used to tune the aggregation behavior in solution. The solid phase self-assembling process of block copolymers and related homopolymers are studied in as-deposited and in annealed thin films.Small angle neutron scattering (SANS), UV-Vis absorbance and Photoluminescence measurements are reported. The aggregation type and particle shape in solution is discussed as a function of the solvent mixture.The block copolymer thin film morphology is investigated by atomic force microscopy (AFM) and transmission electron microscopy (TEM). ITO/PEDOT:PSS/Polymer/Au hole-only devices are used to study the thin film charge transport properties.On as-deposited films AFM in tapping mode reveals the presence of fribrillar structures or “nanorods”. The size and ordering of these structures are found to change upon annealing, leading in some cases to 0.5 µm sized rods. The lateral nanorod ordering is seen to depend on the coil bloc length. TEM investigations show that these nanorods are most probably out-of plane oriented lamellae. The correlation between the self-assembled nanostructures and the thin film electrical properties is discussed.
9:00 PM - M7.26
Ultimate Efficiency of Polymer/fullerene Bulk Heterojunction Solar Cells.
Lambert Jan Anton Koster 1 2 , Valentin Mihailetchi 1 , Paul Blom 1
1 Molecular Electronics, Materials Science Centre Plus, University of Groningen, Groningen Netherlands, 2 , Dutch Polymer Institute, Eindhoven Netherlands
Show AbstractWe present model calculations to explore the potential of polymer/fullerene bulk heterojunction solar cells. As a starting point, devices based on poly(3-hexylthiophene) (P3HT) and 6,6-phenyl C61-butyric acid methyl ester (PCBM), reaching 3.5% efficiency, are modeled. Lowering the polymeric band gap will lead to a device efficiency exceeding 6%. Tuning the electronic levels of PCBM in such a way that less energy is lost in the electron transfer process enhances the efficiency to values in excess of 8%. Ultimately, with an optimized level tuning, band gap and balanced mobilities polymeric solar cells can reach power conversion efficiencies approaching 11%
9:00 PM - M7.29
Charge Injection into Bottom-Contact Pentacene Organic Thin-Film Transistors
Yi Hong 1 , Feng Yan 1 , Piero Migliorato 1 , Seung Han 2 , Jin Jang 2
1 Department of Engineering, University of Cambridge, Cambridge CB2 1PZ United Kingdom, 2 Advanced Display Research Center, Kyung Hee University, Seoul 130-701 Korea (the Republic of)
Show AbstractWe investigate the contact effect in pentacene organic thin-film transistors with Au source/drain bottom contact. The device output characteristics are affected by the poor contact between electrode and semiconductor, which could be due to the presence of an amorphous region of semiconductor next to the electrode or injection barrier between electrodes and channel. Our 2D device simulation shows that in the latter case the contact effect near the source is much bigger than that near the drain, which is consistent with the result from scanning Kelvin probe microscopy. The I-V characteristics of the contact at different temperatures and different gate voltages are extracted by separating the voltage drop at the source contact and the channel according to the gradual channel approximation. It shows a nonlinear injection characteristic as a function of contact voltage. No apparent gate voltage dependence is found within the voltage range investigated, indicating that the gate voltage is screened by the surface charge in the channel. The injection current at the source can be fitted well with the hopping injection model [ V. I. Arkhipov, et al. J. Appl. Phys. 84, 848 (1998) ] which describes the carrier injection as a thermally assisted tunnelling from the metal contact into localized states in the semiconductor with a Gaussian distribution of density of states. The yielded energy difference between the Fermi level of Au and the center of Gaussian distribution, and Gaussian width of the density of states are 0.17eV and 0.028eV, respectively. The comparison of fitting to the conventional Schottky emission model was also presented.
9:00 PM - M7.30
Probing Ultrafast Conformational Dynamics of Conjugated Polymers.
Sebastian Westenhoff 1 , Villy Sundstrom 2 , Arkady Yartsev 2 , Wichard Beenken 3 , Silva Carlos 4 , Richard Friend 1 , Neil Greenham 1
1 Cavendish Laboraory, Univeristy of Cambridge, Cambridge United Kingdom, 2 Chemical Physics, University of Lund, Lund Sweden, 3 Theoretical Physics, Technical University of Ilmenau, Ilmenau Germany, 4 Department of Physics, University of Montreal, Monteal, Quebec, Canada
Show AbstractWe present a novel approach to probe conformations of conjugated polymers by using a combination of femtosecond time-resolved spectroscopy and Monte-Carlo simulation of resonant energy transfer [1]. Control over chain morphology is critical for the design of devices made of semiconducting polymers. Our method allows us to determine chain conformations from ultrafast polarization anisotropy data. This gives insight into the entropy-driven morphology characteristics of macromolecules in the solid state on a microscopic level. A molecular model yields the molecular parameters that drive polymer chain aggregation and our findings are thus directly applicable to optimizing device performance. We also report the real-time observation of conformational dynamics of (poly[3-(2,5-dioctylphenyl)thiophene] (PDOPT) in solution after excitation with an ultrashort laser pulse. Conformational motion is fundamentally important for the photophysics of conjugated marcromolecules because it directly influences spectral properties, i.e. by causing a large Stokes shift. We show that torsional motion occurs on a picosecond timescale. It leads to isomerization and planarization of the initially excited conjugated segment and we present a detailed molecular model illustrating the conformational changes. Interestingly, the timescale of the torsional motion varies by an order of magnitude with the choice of (good) solvent and can be as short as 500 fs for PDOPT in chloroform. The results suggest that the dynamics depend on the specific solvent-solute interactions (‘solvent cage’) and the parameters that drive torsional motion are discussed. [1] Westenhoff, S., Daniel, C., Friend, R.H., Silva, C., Sundström, V., and Yartsev, A., J. Chem. Phys. 122, (2005) 094903.
9:00 PM - M7.32
Photo-Conductive Electron Transport Material Used for Black Cathode OLEDs
Kai-Hsiang Chuang 1 , Yu-Hsuan Ho 1 , J. H. Lee 1 , Chun-Chieh Chao 2 , Man-Kit Leung 2
1 Graduate Institute of Electro-Optical Engineering, National Taiwan University , Taipei Taiwan, 2 Chemistry Department, National Taiwan University , Taipei Taiwan
Show AbstractIn this work, we demonstrated a Black Cathode organic light-emitting device (OLED) by using a new n-type organic material. Black Cathode OLEDs are used interference structures to enhance contrast ratio and often faced with increasing electrical loadings while inserting an interference layer. The new n-type material here we used exhibits high extinction ratio, low resisitivity and easy-fabrication which is a very suitable candidate for the Black Cathode application. Besides, photoconductivity phenomenon was observed in such a device which can be used to improve the electrical properties of the Black Cathode device and also used as an ambient-light sensor or a photovoltaic device. The absorption spectrum of the thin-film (60 nm) shows a peak at 557 nm with the absorption of 38.2%. With suitable device design, the reflectance of an OLED is less than 10% at 555 nm by using this material. Due to the low bandgap of this material, free carriers exist and contribute to carrier transport. The slope of the log(J)-log(V) is 1.82 and 1.21 when the thickness of the organic material is 90 and 110 nm, respectively. Such a value is between 1 and 2 that means the current transport mechanism in this material is a combination of space charge limited current and ohmic conduction. From simple calculation, the resistivity of the material is about 10^7 ohm-cm which is 7 orders of magnitude higher than that of conventional used electron transport material tris-(8-hydroxyquinoline) aluminum (Alq3). Such an organic material can be thermally evaporated without doping other organic or inorganic material that is compatible to the existing equipment system of thin-film deposition. J-V characteristic in the black OLED is nearly the same as the control device. Due to the photoconductivity of interference organic layer, the light is not only absorbed by interference layer and also feed excess carriers into the emitting layer. And the current efficiency of the black device is higher than half of that of the control device which is the theoretical limit in high contrast OLEDs. Operation lifetime is also comparable with the control device. A 0.5 V voltage reduction was observed at 100 mA/cm^2 when the black OLED is illuminated by a fluorescent lamp with the luminance of 500 cd/m^2 which shows a pronounced photoconductivity of this material.
9:00 PM - M7.33
Improved Efficiency in Polymer Light-emitting Diodes by Interfacial Engineering Using Polymer Brushes.
Saghar Khodabakhsh 1 , Gregory Whiting 1 , Yajun Xia 2 , Peng Wang 2 , Wilhelm Huck 1 , Richard Friend 2
1 Chemistry, University of Cambridge, Cambridge United Kingdom, 2 Physics, University of Cambridge, Cambridge United Kingdom
Show AbstractWe report on the improvement of the current efficiency of polymer light emitting diodes (PLEDs) based on poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT), upon inclusion of a thin layer of poly(triphenylamine acryalte) (PTPAA) brushes between the indium-tin oxide (ITO) anode and the F8BT active layer. The observed improvements are attributed to electron-blocking properties of PTPAA brushes. These brushes are surface-tethered and therefore are robust and compatible with solution-processed systems. Additionally, the thickness of these brushes can be easily tuned using different synthetic conditions.
9:00 PM - M7.34
Microscopic Understanding of the Anisotropic Conductivity of PEDOT:PSS Films.
Alexandre Nardes 1 , Martijn Kemerink 1 , Nicole Kiggen 2 , Albert van Breemen 2 , Margreet de Kok 3 , Rene Janssen 1
1 Applied Physics , Eindhoven University of Technology, Eindhoven Netherlands, 2 , TNO Science and Industry, Eindhoven Netherlands, 3 , Philips Research Laboratories Eindhoven, Eindhoven Netherlands
Show AbstractPoly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonic acid) (PEDOT:PSS) is a chemically doped polymer blend that is widely used as a buffer layer to increase the stability and the charge injection and extraction characteristics in organic light emitting diodes (PLEDs). Despite the huge industrial interest and its widespread application, the charge transport in PEDOT:PSS films is rather poorly understood.In this work, the dc conductivity of various (commercial and non-commercial) batches of PEDOT:PSS has been studied as a function of temperature in the range from 80 K to 300 K. Electrical measurements of the dc conductivity were performed in top-bottom (normal) and coplanar (lateral) contact geometries. Depending on temperature, differences between the normal and lateral conductivity of up to three orders of magnitude were found. This shows that the spin coated material is highly anisotropic, which may cause an enhanced cross-talk between neighbouring pixels in organic displays. The temperature dependence was found to follow σ=σ0exp[-(T0/T)α], with different T0 and α values for the lateral and normal electrode configurations.In order to interpret the data, we developed a theoretical model that describes the hopping between non-spherical metallic or polaronic islands in the presence of charging effects. The polaronic islands are formed by PEDOT-rich clusters that are separated from each other by PSS-rich barriers. By performing Scanning Tunnelling Microscopy (STM) experiments on the same layers, the particle size distribution and the anisotropy in the particle shape could be determined. Using these data as input in our theoretical model, the T-dependence in both directions could be well reproduced. These results now allow us to relate macroscopic observables (i.e. the conductivity) to nanometre-scale morphology (i.e. particle size and shape).
9:00 PM - M7.37
Photoluminescence and Electroluminescence Studies of Wide Band Gap Organic Diphosphine Sulfide (PS) Materials
Asanga Padmaperuma 1 , Paul Vecchi 1 , Hong Qiao 1 , Linda Sapochak 1 , Paul Burrows 1
1 Materials Science Division, Pacific Northwest National Laboratory, Richland, Washington, United States
Show AbstractMany organic materials based on aromatic amines, boranes, and silanes have been reported to exhibit important functionality for applications in organic light-emitting devices (OLEDs). For example, amine derivatives are preferentially hole-transporting and boranes electron transporting, whereas tetraaryl silanes are typically used as wide band gap inert matrices for high energy blue emitting devices. Interestingly, there have been few studies of organic materials incorporating phosphorous. We previously reported that aromatic diphosphine oxide (PO) materials can serve as wide band gap and charge conducting host materials for sky blue organometallic complexes used in electrophosphorescent devices. The PO compounds were designed by using the P=O functionalities as points of saturation between an “active” chromophore (i.e., biphenyl, fluorene, carbazole) bridge and outer higher energy aryl groups. In this way, materials with wide band gaps and high triplet energies representative of the “active” chromophore and physical properties amenable to device fabrication were achieved. More recently we have studied the corresponding aromatic diphosphine sulfide (PS) materials, where the P=S functionality, although less polar compared to P=O also acts as a point of saturation. Unlike the phosphine oxides which exhibit efficient fluorescence the PS materials are barely emissive at room temperature. However, upon cooling to 77 K, the PS materials show significantly greater phosphorescence emission. Preliminary device results indicate that aromatic diphosphine sulfides also serve as charge transporting hosts for blue phosphorescent dyes. The design concept, chemical and photophysical properties and preliminary device data of PS materials will be presented and compared to the corresponding PO compounds.
9:00 PM - M7.38
Ultrafast Charge Carrier Generation in Bulk Heterojunctions of P3HT and PCBM Studied by Time-resolved THz Spectroscopy
Xin Ai 1 , Matthew Beard 1 , Kelly Knutsen 1 , Sean Shaheen 1 , Randy Ellingson 1 , Garry Rumbles 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractBulk heterojunctions of regioregular poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) have been broadly used to fabricate organic solar cells. The power-conversion efficiency based on P3HT/PCBM cells has been reported to be as high as 4.4%. The precise composition of the blended films plays an important role in controlling the performance of the cell. The cell performance and charge carrier mobility as a function of the composition have been studied by time-resolved microwave conductivity (TRMC) and time-of-flight (TOF) techniques at ns to ms time scales. These studies provide insight into the carrier recombination processes. However, the charge generation process which generally occurs on the subpicosecond to picosecond time scale is not well understood. Time-resolved THz spectroscopy (TRTS) is a noncontact electrical probe with subpicosecond temporal resolution. We have employed TRTS to investigate the charge carriers in PCBM/P3HT films with PCBM weight fractions from 0 to 0.8. The dynamics of the complex photoconductivity in those films were measured and compared. This study not only demonstrates a new method to monitor the charge carrier generation in these materials, but also provides new information that will help understand and optimize the parameters affecting the efficiency in fabricating polymer solar cells.
9:00 PM - M7.39
Photovoltaic Properties of Small Bandgap Conjugated Polymers Based on Thiophene Building Block.
Chenjun Shi 1 , Qibing Pei 1 , Yan Yao 1 , Yang Yang 1
1 Materials Science and Engineering, UCLA, Los Angeles, California, United States
Show AbstractHomo- and co-polymers containing substituted thiophenes were synthesized as high molecular-weight, solution-processable p-type polymer semiconductors. The chemical structures and regioregularity of the polymers were confirmed by NMR. All the long side chain containing polymers, including P3DOT, POT-co-DOT, and PF-co-DBT, are soluble in organic solvents such as chlorobenzene and dichlorobenzene. All three polymers exhibit narrow bandgaps resulting from regioregular side chains. Bulk heterojunction photovoltaic devices based on these polymers and PCBM were fabricated. The open circuit voltage is as high as 0.68 V, and power efficiency is up to 1.45%.
9:00 PM - M7.40
Failure Mode Investigation for Blue Polymer EL Device
Mu-Gyeom Kim 1 , Sand-Yeol Kim 1 , Tae-Woo Lee 1 , Sand-Hun Park 1 , Ju-Cheol Park 1 , Jong-Jin Park 1 , Lyong-Sun Pu 1
1 OLED team, Samsung Advanced Institute of Technology, Gyeonggi-Do Korea (the Republic of)
Show AbstractSince discovering polymer materials utilizing for electroluminescent(EL) devices in 1990, there’s been great efforts pouring into device’s performance till now. After more than a decade of such period, we are now on the verge of real application for display. But, in reality, world-wide scientists, engineers, and decision makers does not draw clear picture of future display using polymer materials, especially blue one as a bottle neck due to its relatively short lifetime comparing to red and green. Considering above, we thought that we are on a situation to figure out what’s the cause of short lifetime in polymer EL blue devices. There’s been a lot of reports related with lifetime, mostly for green and red polymer devices, less for blue one. As a matter of fact, in spite of reduced number of multilayer, polymer device bears quite a complicated failure modes inside. Thus, even well-known degrading factors are sometimes treated as minor or preventable ones like dark spot. And, degrading mechanism depends highly on materials and device structures. Therefore, it has been pushed us to deeply investigate the failure mechanism/mode of currently developing blue devices in SAIT. Experimentally, we first prepared polymer EL blue devices like this structure : ITO/PEDOT 50nm/SB1 80nm/BaF2 5nm/Ca 3nm/Al 200nm, where SB1 is a SAIT blue material. We approached the failure mode via two ways. The non-destructive way is measure both electron and hole carriers’ mobility from transient EL according to life curve. The destructive way is to investigate cross-sectional/compositional variation using EDX filtered TEM, TOF-SIMS, FT-IR, and morphology by STM of so called insoluble layer after reverse engineering.In a non-destructive way, we measured transient EL along lifetime curve to obtain electron and hole mobility. Interestingly, rather than electron mobility, variation of hole mobility showed similar behavior of luminance drop. It, thus, suggests that the hole transport and charge balance are one of factors to affect stability of emitting polymer (SB1). In a destructive way, insoluble layer showed physically ordered domains through polymer rearrangement and cross-linking features via electrochemical reaction. Such generation of insoluble layer at the anode side possibly induced lowered hole mobility, its related reduced EL efficiency due to unbalanced EL operation, and luminance drop eventually. Furthermore, as an insoluble layer forming mechanism, we did not, within experimental errors of equipments, recognize any sulfur migration, rather diffusion into cathode.
9:00 PM - M7.41
Origin of Lower Efficiency of Phosphorescent Dye Doped Polymer LEDs than Organic LEDs.
Won-ik Jeong 1 , Jae Wook Kang 2 , Jang-Joo Kim 1 2
1 Materials science & engineering, Seoul National Univ., Seoul Korea (the Republic of), 2 Center for Organic Light Emitting Diodes, Seoul National Univirsity, Seoul Korea (the Republic of)
Show AbstractPhosphorescence dye doped light emitting device (PhOLED) in polymer host shows lower efficiency than in small molecule host. To find out the origin of the lower efficiency in the polymer host, we have investigated the optical properties of phosphorescence dye doped film using Ir(ppy)3 as a dopant and CBP and PVK as hosts. Absolute photoluminescence (PL) efficiency measurement and time resolved technique were employed for the purpose. PL efficiency of Ir(ppy)3 doped CBP film (Ir(ppy)3 :CBP) decreases monotonically from 100% with increasing doping concentration. In contrast, PL efficiency of Ir(ppy)3 doped PVK film (Ir(ppy)3 :PVK) increases with doping concentration when the doping concentration was low, reaches the maximum efficiency of 70% at the doping concentration of 6%, and decreases monotonically with further increment of the doping concentration. The PL efficiency of Ir(ppy)3 :PVK is lower than Ir(ppy)3 :CBP for whole range of doping concentration. This low PL efficiency may be the reason of the lower efficiency of polymer based PhOLED than small molecule PhOLED. The origin of the lower efficiency of polymer PhOLED is investigated using time resolved spectroscopy technique and will be discussed from the view point of triplet energy level and the exciton dynamics in polymer and organic molecules.
9:00 PM - M7.42
Contributions of Metallophthalocyanines in Organic LEDs
Wallace Choy 1 , Y.J. Liang 1
1 Department of Electrical & Electronic Engineering, the University of Hong Kong, Hong Kong Hong Kong
Show AbstractMetallophthalocyanines (MPcs) have been widely used to improve the efficiency of organic LEDs (OLEDs), particularly copper phthalocyanine(CuPc). In this article, various MPcs of CuPc, magnesium Pc (MgPc) and zinc Pc (ZnPc) have been investigated as the hole injection layer (HIL) and hole transporting layer (HTL). Through studying single layer and dual layer MPc HIL, our results shows that dual layer MPc HIL can improve the efficiency of OLEDs while single layer ZnPc can offer better the current efficiency by 22% and power efficiency by 67% as compared to that of the single layer CuPc in the typical OLEDs structure of ITO/HIL/HTL/Alq3/LiF/Al. The single peak EL emission is from Alq3 with a peak wavelength at ~545nm. The reasons of the improvement will be discussed.For HTL, a co-host system of MPc with N,N'-bis-(1-naphthyl)-N,N'-diphenyl-1,1'-bipheny1-4,4’-Diamine (NPD) has been investigated. The efficiency of OLEDs enhances when the concentration of ZnPc increases and the efficiency peaks at the 50%:50% ZnPc:NPD co-host structure. In this case, the current efficiency increases over 30% as compared to that of prsitine NPD HTL. As compared to the co-host HTL of 50%:50% CuPc:NPD, the ZnPc one has 7% higher current efficiency and 60% increment in the maximum brightness. However, when the concentration of ZnPc in the co-host structure is further increased, the efficiency reduces. The I-V characteristics of single layer devices sandwiched between two high workfunction electrodes show that the turn-on voltage reduces when the concentration of ZnPc increases to 50% and thus the performance of the OLED improves. When the concentration of ZnPc is further increased, the turn-on voltage continuously reduces. However, the brightness and efficiency of the OLEDs reduce. This can be explained by the crystallization of ZnPc observed from the peak spectrum in XRD while there is no peak found, i.e. amorphous layer, from the XRD spectrum of prsitine NPD and 50%:50% ZnPc:NPD. From the AFM results, the morphology of the ZnPc is also rougher that that of the prsitine NPD and 50%:50% ZnPc:NPD.As a consequence, MPc single layer and dual layer HIL and cohost HTL with NPD have been investigated. Our results show that ZnPc can be used in HIL and cohost HTL to enhance the efficiency and brightness of the OLEDs as compared to other commonly used MPc such as CuPc.
9:00 PM - M7.43
Disentangling Bipolar Transport in Organic Devices by Admittance Spectroscopy.
Hans Gommans 1 , Martijn Kemerink 1 , Rene Janssen 1
1 Applied Physics, Eindhoven University of Technology, Eindhoven Netherlands
Show Abstract9:00 PM - M7.44
Low-Voltage and High Field-Effect Mobility Organic Transistors with a Polymer Insulator
Kilwon Cho 1 , Yunseok Jang 1 , Minkyu Hwang 1
1 Chemical Engineering, Pohang University of Science and Technology, Pohang Korea (the Republic of)
Show AbstractOver the last decade, a considerable number of studies have been devoted to the improvement of the characteristics of organic field-effect transistors (OFETs). However, OFET devices still require rather high operating voltages, often exceeding 20 V. This is mainly controlled by the properties of the gate insulator rather than by those of the semiconductor. At operating voltages of about 2 V, to obtain high-performance pentacene FETs with mobilities similar to those of amorphous Si TFTs, a good on/off current ratio, and an excellent subthreshold slope, the dielectric layers must provide a relatively high capacitance, which can be achieved by using ultra-thin gate insulators. In this study, we report the fabrication of a pentacene thin film transistor on an ultra-thin layer (9 nm) of polyvinyl alcohol (PVA) and the electrical properties of this device. The device we fabricated with a PVA insulator was found to have a field-effect mobility of 1.05 cm2/Vs (close to the best mobilities reported for pentacene TFTs), a threshold voltage of -0.98 V, an exceptionally low subthreshold slope of 180 mV/decade and an on/off current ratio of 106. The performance of such devices is significantly improved by using an ultra-thin gate insulator of PVA. And this combination of favorable properties demonstrates that OTFTs can be operated successfully at voltages below 2 V.Acknowledgement. This work was supported by the National Research Laboratory Program and ERC Program (R11-2003-006-03005-0) of the MOST/KOSEF, a grant (F0004022) from Information Display R&D Center under the 21st Century Frontier R&D Program of the Ministry of Commerce, Industry and Energy of Korea, and the BK21 Program of the Ministry of Education and Human Resources Development of Korea.
9:00 PM - M7.45
Surface Characterization of Thin Polyfluorene Films Prepared by Ink-jet Technique for Use in Gamma Radiation Sensors.
Rodrigo Bianchi 1 , Alex Frazatti 1 , Helton Santos 1 , Bruno Nowacki 2 , Leni Akcelrud 2
1 Departamento de Engenharia de Sistemas Eletronicos - LME, Escola Politecnica da Universidade de Sao Paulo, Sao Paulo, SP, Brazil, 2 Departamento de Quimica, Universidade Federal do Parana, Curitiba, PR, Brazil
Show Abstract9:00 PM - M7.46
CW-Amplified Spontaneous Emission From An Organic Solid-State Film.
Chihaya Adachi 1 , Hajime Nakanotani 1
1 Center for Future Chemistry, Kyushu University, Fukuoka Japan
Show AbstractWe realized a CW organic solid-state dye laser. Under CW excitation, continuous wave amplified spontaneous emission (CWASE) was successfully obtained from an organic solid-state thin film. We used a 500nm-thick co-deposited organic thin film composed of a 4,4’-bis[(N-carbazole)styryl]biphenyl (BSB-Cz) as an active material and 4,4’-bis-(N-carbazole)-biphenyl (CBP) as a host. Under CW excitation, we observed a TE-mode ASE at a wavelength of L(ASE)=462nm with a very narrow full width at half maximum of (L)FWHM=3.4nm from an edge of the co-deposited thin film. The 6wt%-BSB-Cz:CBP co-deposited thin film showed not only fluorescence quantum efficiency of F(PL)~100% but also no excited state absorptions at the ASE wavelength, both providing large net gain for the CWASE.
9:00 PM - M7.48
About the Function of Buffer Layers in Phthalocyanine/C60 Solar Cells
Mirko Vogel 1 , Serge Doka 1 , Martha Christina Lux-Steiner 1 , Konstantinos Fostiropoulos 1
1 SE 2, Hahn-Meitner-Institut Berlin, Berlin Germany
Show AbstractIn this work new explanations for the function of the buffer layers PEDOT:PSS and Bathocuproine in Phthalocyanine/C60 solar cells were found. The PEDOT:PSS buffer layer is situated between the indium-tin oxide electrode and the Phtalocyanine/C60 photoactive film. By quantum efficiency measurements of solar cells with different device architectures it can be shown, that an important function of the PEDOT:PSS buffer layer consists in its selectivity in respect to the transport of holes from a co-sublimated Phthalocyanine/C60 film into the ITO electrode. The Bathocuproine buffer layer is inserted between the photoactive film and the aluminium back electrode. We demonstrated by examining the photoluminescence of C60, that non-radiative recombination is reduced by introduction of the Bathocuproine buffer layer between aluminium and C60 by more than one order of magnitude. However, a similar effect can be obtained without buffer layer by depositing C60 on aluminium instead of aluminium on C60. This suggests, that the non-radiative recombination is due to a large degree to defects created by the deposition procedure of the aluminium electrode on C60. Evaluation of the current-voltage-characteristics of solar cells revealed, that the insertion of the Bathocuproine layer leads to a dramatic increase in efficiency of about one order of magnitude. This increase can be mostly attributed to the fact, that electron transport from C60 via Bathocuproine to the aluminium electrode is efficient, while without buffer layer C60 and aluminium form a blocking contact. Therefore the more important function of the Bathocuproine buffer layer can be seen in its ability to establish an ohmic contact between C60 and aluminium.
9:00 PM - M7.49
Ink-Jet Printed Conjugated Polymer Light Emitting Devices
Gernot Mauthner 1 2 , Sigi Psutka 1 2 , Katharina Landfester 3 , Michael Kast 4 , Christoph Stepper 4 , Anton Koeck 4 , Hubert Brueckl 4 , Emil List 1 2
1 Christian Doppler Laboratory Advanced Functional Material, Institute of Solid State Physics, Graz University of Technology, Graz Austria, 2 Christian Doppler Laboratory Advanced Functional Material, Institute of Nanostructured Materials and Photonics, JOANNEUM RESEARCH, Weiz Austria, 3 Organic Chemistry III / Macromolecular Chemistry, University of Ulm, Ulm Germany, 4 Nano System Technologies, ARCsr, Vienna Austria
Show AbstractInkjet printing as a means of creating electronic devices such as polymer light emitting diodes (PLED) or light emitting electrochemical cells (LEC) was investigated. In particular a wide variety of conjugated light emitting polymers dissolved in several different solvents have been studied with respect to the jet ability and operation properties. The processes of wave propagation, droplet formation and drop impact at different parameters such as temperature, frequency and molecular weight of conjugated light emitting polymers were studied to improve the performance of the jetting process itself as well as of the fabricated devices. To study the influence of ink-jet printing on the conjugated polymer, photoluminescence quantum yield measurements on printed solutions were performed and analysed. Furthermore printed LECs and PLEDs have been fabricated by means of inkjet printing in surface cell geometry using regular conjugated polymers as well as semiconducting polymer nanospheres (SPNs) dispersed in water that were fabricated by a miniemulsion process. The performance of such printed devices is compared in detail with devices fabricated by spin coating.
9:00 PM - M7.5
Improvement Of Device Lifetime For Polymer Light-Emitting Diodes Using MoO3 Hole Injection Layer.
Takafumi Hamano 1 , Ryuichi Yatsunami 1 , Kei Sakanoue 1
1 , Panasonic Communications Co., Ltd., Fukuoka Japan
Show AbstractIn current polymer light-emitting diodes (PLEDs), hole injection layer, such as poly(3,4-ethylenedioxythiophene) (PEDT), is necessary to realize better hole injection from ITO substrate to emitter layer. Here, we show the initial characteristics and device lifetime using MoO3 hole injection layer instead of standard PEDT. The device structures are ITO/MoO3/Electron blocking layer (EBL) /Emitter layer (EML) /Cathode and ITO/PEDT/EBL/EML/Cathode for comparison. Polymer emitter material is applied in these experiments. The MoO3 device shows better I-V characteristics, for example, current density at 8V is around 1,000mA/cm2, that is two times larger than PEDT device. The EL efficiency is 1.2cd/A under 1,000mA/cm2 and slightly lower than PEDT device, however, no efficiency drop is observed at 2,000mA/cm2. The lifetime of MoO3 device shows five times longer than PEDT device, such as 100hours half lifetime at 10,000cd/cm2 indicating MoO3 has a better stability than PEDT. The MoO3 HIL looks promising for improving PLEDs device lifetime.
9:00 PM - M7.50
A Fully Aryl-Substituted Poly(Ladder-Type Pentaphenylene): A Remarkably Stable Blue-Light Emitting Polymer.
Stefan Sax 1 2 , Martin Gaal 1 2 , Emil List 1 2 , Josemon Jacob 3 , Andrew Grimsdale 3 4 , Klaus Muellen 3
1 Christian Doppler Laboratory Advanced Functional Material, Institute of Solid State Physics, Graz University of Technology, Graz Austria, 2 Christian Doppler Laboratory Advanced Functional Material, Institute of Nanostructured Materials and Photonics, JOANNEUM RESEARCH , Weiz Austria, 3 , Max-Planck-Institute for Polymer Research, Mainz Germany, 4 Bio21 Institute, University of Melbourne, Melbourne, Victoria, Australia
Show AbstractConjugated organic oligomers and polymers are the subject of considerable current interest as potential active materials in electronic applications such as organic light-emitting diodes (OLEDs), plastic lasers, photovoltaic devices (solar cells), and field-effect transistors. Two of the most important goals of research into organic materials for electronic applications are the obtaining of stable blue emission, which is particularly required for full colour displays, and the preparation of good electron accepting and transporting (n-type) materials. Most organic materials are p-type materials which accept and transport holes better than electrons, but n-type materials are urgently needed for use in n-type transistors, as the electron accepting components in photovoltaic devices, and to increase the efficiency of OLEDs by improving electron injection and transport. A new route to ladder-type pentaphenylenes has been developed in which both good hole accepting p-type and electron accepting n-type materials can be prepared from a common intermediate. This polymer bridges the gap in emission between polyfluorenes and fully ladder-type polyphenylenes. It will be shown that the presented new poly(ladder-type pentaphenylene) bearing aryl groups at all bridgehead positions produces remarkably color stable blue emission in light-emitting diodes, which is not subject to oxidative degradation induced defect formation. In particular a comparison of the fully aryl-substituted polymer with a partially aryl-substituted polymer reveals that the oxidative degradation induced defect formation can be almost completely eliminated upon complete aryl-substitution. This has been tested by accelerated thermal degradation experiments and under device operation.
9:00 PM - M7.52
Assembling Strategies For Nanostructured Polymer Light Emitting Devices.
Evelin Fisslthaler 1 2 , Thomas Piok 1 2 , Harald Plank 1 2 , Sigi Psutka 1 2 , Katharina Landfester 3 , Emil List 1 2
1 Christian Doppler Laboratory Advanced Functional Material, Institute of Solid State Physics, Graz University of Technology, Graz Austria, 2 Christian Doppler Laboratory Advanced Functional Material, Institute of Nanostructured Materials and Photonics, JOANNEUM RESEARCH, Weiz Austria, 3 Organic Chemistry III / Macromolecular Chemistry, University of Ulm, Ulm Germany
Show AbstractElectroluminescent polymers offer a lot of different opportunities for organic light emitting devices or other electronic applications. Unfortunately, resulting from the solution based application methods used for polymeric materials, one is usually limited to simple unstructured thin films for the emitting regions in of such devices.We investigated the possibility to realise polymer light emitting devices with semiconducting polymer nanospheres (SPNs) that were fabricated by a miniemulsion process. These nanobeads shall serve as nanosized emitting spots with a diameter of about 100 nm. This is a novel approach for light sources that can be structured in arbitrary dimensions, down to a nanoscopic scale. As a prerequisite we need to be capable of placing the SPNs in an appropriate arrangement. For this purpose we made use of a polymer multilayer configuration with a non-emitting polymeric host as a bedding layer for the nanospheres. Structuring these host layers by Soft Lithography in combination with Ink Jet Printing as deposition method for the SPNs leads to a procedure that yields chain –like assembled beads. The density of the alignment can be adjusted via the parameters of the fabrication process. The structural properties of the resulting system were investigated via atomic force microscopy and optical spectroscopy.
9:00 PM - M7.53
Broadband Coherent Anti-Stokes Raman Spectroscopy (CARS) For Characterizing Thin Films Of Semi-Conducting Polymers.
Marc Gurau 1 , Zachary Schultz 1 , Dean Delongchamp 2 , Joseph Kline 2 , Brandon Vogel 2 , Youngsuk Jung 2 , Eric Lin 2 , Lee Richter 1
1 Chemical Science and Technology Lab, National Institute of Standards and Technology, Gaithersburg, Maryland, United States, 2 Materials Science and Engineering Lab, National Institute of Standards and Technology, Gaithersburg, Maryland, United States
Show AbstractThe ongoing development of conjugated polymer electronic materials has shown that alterations of the structure of these materials and the means of processing can lead to large variations in the performance observed. Spectroscopic investigations of the vibrational structure of these systems provides a non-destructive means of monitoring such changes. Raman spectroscopy is a powerful tool for the characterization of polymers, however it typically lacks the sensitivity necessary for interrogating thin (<50 nm) films in the absence of surface enhancement. The work presented in this talk demonstrates the use of broadband coherent anti-stokes raman spectroscopy (CARS) to monitor thin films of polymeric materials. The sensitivity and low background realized in the CARS experiment allows for the observation of these thin films with good resolution resulting in more informative spectra than that obtained by conventional and even surface enhanced Raman spectroscopy. As in Raman spectroscopy, the CARS spectra of the C=C stretch region is very sensitive to aspects of film structure including conjugation length, oxidative doping and molecular orientation. Examples of CARS investigations of thin films of semi-conducting thiophene polymers will be presented.
9:00 PM - M7.54
The Medium-Depending Fluorescence of Conjugated Oligomers with a Quadrupolar Electronic Structure
Heiner Detert 0
0 Organic Chemistry, Universitaet Mainz, Mainz Germany
Show AbstractFluorescent conjugated pi-systems with monodisperse conjugation lengths are potent materials for optoelectronic applications since they can be prepared in high purity and their electrical and optical properties may be tuned over a wide range. Solubility and thermal properties are tuned via side chains und the rigid chromophore, additionally these may have a strong impact on electrical and optical characteristics. C2h-symmetrical Oligo(phenylenevinylene)s with a pronounced Donor-Acceptor-Donor or Donor-Acceptor-Donor character are in the focus of this paper. The quadrupolar electronic structure provokes a high sensitivity of the fluorescence towards the environment, making these chromophores interesting for non-linear optical applications. The impact of solvent polarity on the fluorescence spectra and efficiencies is correlated with the molecular structure.
9:00 PM - M7.56
Conjugated Polymer Directed Self-Organization of Nanoparticles for Micro-Structured Functional Thin Films
Nurmawati Muhammad Hanafiah 1 , Ajikumar Kumaran 2 , Suresh Valiyaveettil 1 2
1 Chemistry, National University Singapore, Singapore Singapore, 2 Singapore-MIT Alliance, National University of Singapore, Singapore, Singapore, Singapore
Show Abstract9:00 PM - M7.57
Alq3-Oligofluorene-Platinum Porphyrin Composites as Red Emitters for OLEDs.
Victor Montes 1 , Neeraj Agarwal 1 , Pavel Anzenbacher 1
1 Chemistry and Center for Photochemical Sciences, Bowling Green State University, Bowling Green, Ohio, United States
Show AbstractPreparation of high efficiency OLEDs of saturated red color has remained a challenge for more than a decade. Red light is usually obtained through introduction of dopants capable of harvesting energy from host electroluminescent materials. Co-deposition of these components is technically difficult, particularly in solution processing of blends, because dopants and host materials tend to aggregate, resulting in phase separation and decreased quantum efficiencies. Recently it has been demonstrated that direct incorporation of dopant moieties into the backbone of conjugated polymers reduces problems associated with phase separation, which results in an improved device performance compared to blend-based devices. We report the synthesis and study of solution-processible composite materials that comprise Al(III) quinolinolate (Alq3) centers as an energy donor and Pt(II)porphyrins as acceptors connected via covalent oligofluorene bridges of varying length (n=1-4 units). These materials were designed to facilitate energy transfer of both singlet and triplet excited states from quinolinolate/fluorene donor-bridges to the phosphorescent emissive centers and achieve saturated red color. The synthesis of the novel materials utilizes mainly Lindsey porphyrin condensation and platinum insertion, Pd0-catalyzed preparation of oligofluorene bridges via Suzuki-Miyaura cross-coupling reactions, and the attachment of quinolinolate ligands. The final materials were assembled through Al(III)-quinolinolate complexation. Optical properties of the compounds in fluid solution revealed typical UV-vis absorption spectra that combined features from these chromophores. Emission spectra were found to be highly dependent on the wavelength of excitation. Irradiation in the Q-band results only in red phosphorescence from the Pt(II) porhyrin (λmax = 665nm) in all cases, while excitation at 405 nm (Soret/π-π* quinolinolate band) generates mainly phosphorescence with a small contribution from Al(III) quinolinolate fluorescence (λmax ~ 540nm) when the number of fluorene units equals 4, which corresponds to Alq3-PtTPP distance ~ 35 Å. Excitation in the oligofluorene π-π* band revealed that the blue fluorene emission is highly quenched whereas strong phosphorescence from the porphyrin is observed as a result of efficient energy transfer. In solid state, the composite materials showed intense red phosphorescence when spun in thin films, which suggests a reduced degree of aggregation. OLEDs fabricated using these hybrid materials confirmed their electroluminescent nature and demonstrated their potential for fabrication of saturated red-emitting devices. In summary, we report the preparation of novel composite systems, their electro-optical properties and their electroluminescent behavior in OLEDs.
9:00 PM - M7.58
Profile Optimization of Wet-Printed Polymers for OLED Devices.
Hongzheng Jin 1 2 , James Sturm 1 2
1 Department of Electrical Engineering, Princeton University, Princeton, New Jersey, United States, 2 Princeton Institute for the Science and Technology of Materials (PRISM), Princeton University, Princeton, New Jersey, United States
Show AbstractInk-jet printing and other large-area wet printing [1] techniques are attractive for patterning solution-processible organic materials, such as for the integration of red, green and blue-emitting polymers for full-color OLED display manufacturing. In these methods, liquid droplets containing polymers in solution are printed and then the solvent is allowed to dry, leaving a patterned organic film on the target substrate. Due to the redistribution of the solute during the solvent evaporation process, the profile of organic materials deposited by such printing methods can be rather non-uniform, which will cause poor device performance, non-uniform luminance, etc [2,3,4]. Depending on the solvent, solute, drying time, and other parameters, the profile might be thickest in center, at edge, or multi-peaked. Thus, a method to improve the profile of organic layers by wet-printing techniques is important for full-color polymer LED displays.In this work, two approaches have been used to optimize the profile of printed polymers: co-solvents and surfactants. In the co-solvent approach, a second solvent with different evaporation rate and/or surface tension is added to the “ink” solution to be printed. Two solvents can affect the profile by altering the spatial distribution of the evaporation rate; by inducing a surface tension gradient; and by inducing fluid circulation within the droplet. In the surfactant approach, a small amount of surfactant is added to the "ink". Surfactants are found effective in flattening the profile, probably due to a lowered surface tension or a more homogeneous spatial distribution of evaporation flux.In this work, poly(9-vinylcarbazole) (PVK) is used as the light-emitting polymer to be printed, and chlorobenzene as the main solvent. Droplets were studied in holes in recessed windows, with the top surface made hydrophobic by surface-energy modification with 1H,1H,2H,2H-perfluorooctyl-trichlorosilane to locate the drop. In the co-solvent approach, 1,2-dichlorobenzene is added to the PVK in chlorobenzene solution at different concentrations, with best profile achieved at about 15-20% of dichlorobenzene. In the surfactant approach, two surfactants, Pluronic L62 (BASF) and Novec Flurosurfactant FC-4432 (3M), have been studied. Pluronic L62 is effective in flattening the profile at a concentration of about 0.05% (ratio to the solvent). However, in a spin-coated device with the same solution, we noticed a ~10X reduction in efficiency and ~70% increase in drive voltage. In contrast, the Novec FC-4432 at a concentration of ~ 0.005% had no negative effect on device characteristics, but is effective in flattening the profile.[1] H. Jin and J.C. Sturm, Mater. Res. Soc. Symp. Proc. 871E, I6.27 (2005)[2] C.F. Madigan, T.R. Hebner, and J.C. Sturm, Mater. Res. Soc. Symp. Proc. 624, V3.5 (2000)[3] T. Shimoda, K. Morii, S. Seki, and H. Kiguchi, MRS Bull., 28, 821 (2003)[4] B. de Gans and U.S. Schubert, Langmuir, 20, 7789 (2004)
9:00 PM - M7.59
Highly Efficient Top-Emissive Polymer Light-Emitting Diodes Utilizing Organic Oxide/Al/Ag Hybrid Semi-Transparent Cathode.
Tzung-Fang Guo 1 , Fuh-Shun Yang 1 , Zen-Jay Tsai 1 , Ten-Chin Wen 2 , Sung-Nien Hsieh 2 , Chia-Tin Chung 3 , Yaw-Shyan Fu 4
1 Institute of Electro-Optical Science and Engineering, National Cheng Kung University, Tainan Taiwan, 2 Department of Chemical Engineering, National Cheng Kung University, Tainan Taiwan, 3 , Chi Mei Optoelectronics Corporation, Tainan Taiwan, 4 Department of Environment and Energy, National University of Tainan, Tainan Taiwan
Show AbstractWe report the fabrication of efficient top-emissive polymer light-emitting diodes (PLEDs) by applying a semi-transparent cathode, composed of the hybrid organic oxide/Al complex as the injection buffer layer for electrons with a thin covering film of Ag. The anode is made of highly reflective AgO/Ag electrode coated on glass substrate. EL efficiencies for phenyl-substituted poly(para-phenylene vinylene) copolymer based top-emissive PLEDs with the semi-transparent hybrid organic oxide/Al/Ag cathode reach 7.64 cd/A (7401.5 cd/m2, EL emission centered at 568 nm), biased at 6.90 V (96.9 mA/cm2). We presumed that the formation of specific organic oxide/Al complex, facilitating the injection of electrons through the semi-transparent cathode and preventing the metal-induced quenching sites of luminescence in EL layer, is essentially crucial to the device performance. The interfacial reaction between the cathode with EL layer was analyzed by X-ray photoelectron spectroscopy. In addition, the micro-cavity effect the of EL spectra (reduce in width and shift of EL spectra) was investigated by changing the thickness of the Ag covering layer in the semi-transparent cathode.
9:00 PM - M7.6
Interface Effects in Polymeric MIS Devices.
Oscar Fernandez 1 , David Taylor 1
1 School of Informatics, University of Wales, Bangor United Kingdom
Show AbstractOrganic, bottom-gate metal-insulator-semiconductor field-effect transistors (MISFETs) were fabricated using poly-(3-hexylthiophene) (P3HT) as the semiconductor and polysilsequioxane (PSQ) or polyimide (PI) as the insulator. The insulator and semiconductor were both spin-coated onto ITO-coated glass slides. A circular gold electrode and a concentric gold ring, spaced ~ 100 μm away, were directly evaporated on top of the semiconductor through a shadow mask thus enabling capacitor and transistor measurements to be performed in the same device. Low-frequency admittance spectroscopy and capacitance-voltage (C-V) and conductance-voltage (G-V) measurements have been undertaken on devices undergoing different temperature anneal and bias stress conditions. Features displayed by the admittance spectra, and the C-V and G-V plots confirm that such effects arise from the interaction of majority hole carriers with localised states at the semiconductor-insulator interface [1,2]. Photocapacitance measurements are being undertaken to investigate the interface trapping states for minority electrons. References 1.I. Torres, D.M. Taylor and E. Itoh, Appl. Phys. Lett., 81, 314 (2005).2.I. Torres and D.M. Taylor, J. Appl. Phys. (paper in press).
9:00 PM - M7.60
Device Characteristics On Organic Light-Emitting Devices Based On Mixed Electron Transporting Layers.
H. H. Fong 1 , W.C.H. Choy 2
1 Materials Science and Engineering, Cornell University, Ithaca, New York, United States, 2 Electrical and electronic engineering, The University of Hong Kong, Hong Kong Hong Kong
Show Abstract9:00 PM - M7.61
Optimization of Pre- And Post-Production Conditioning of Bulk Heterojunction Solar Cells.
Kanzan Inoue 1 2 , Kamil Mielczarek 1 2 , Carlos Wiechers 3 , Ross Ulbricht 1 2 , Miaoxin Zhou 1 4 , Sergey Lee 1 , John Ferraris 1 4 , Anvar Zakhidov 1 2
1 NanoTech Institute, University of Texas at Dallas, Richardson, Texas, United States, 2 Physics, University of Texas at Dallas, Richardson, Texas, United States, 3 Physics, University of Guanajuato, Guanajuato, Guanajuato, Mexico, 4 Chemestry, University of Texas at Dallas, Richardson, Texas, United States
Show AbstractPreproduction parameters and postproduction heat-treatments have been extensively studied for high molecular weight regio-regular poly-3-hexylthiophene (RR-P3HT) and fullerene derivative (PCBM) composite bulk heterojunction solar cells. The optimum conditions for photon absorption and exciton dissociation was obtained by changing the thickness of the P3HT/PCBM composite film and varying the PCBM concentration. The optimal photoactive layer thickness and PCBM concentration were found to be 120nm and 46% in the final film, respectively. These values are much higher than the optimal parameters determined for low molecular weight RR-P3HT and PCBM devices: 100nm and 35%. The best temperature for post-production heat-treatment was found to be 155oC, which is also significantly higher than 97oC for low molecular weight RR-P3HT and PCBM solar cells. These differences can be attributed to the behavior of RR-P3HT with varying molecular weight when the composite film is heated. The atomic force microscopy (AFM), differential scanning calorimetry (DSC), and UV/Vis Spectroscopy are performed to understand the difference of low and. high molecular weight polymers in the formation of a continuous interpenetrating network for efficient charge collection for bulk HJ solar cells. An efficiency of ~4% was obtained with the optimized pre- and post-production conditions under AM1.5 100mW/cm2 solar simulator.
9:00 PM - M7.62
Photo-Physical And Semi Empirical Studies Of Blue Light Emitting 1,4-Dibenzthiazolyl Benzene.
Bincy Jose 1 , Qureshi Mohammad 1 , Solomon Manoharan 1
1 Chemistry, Indian Institute of Technology Kanpur, Kanpur India
Show Abstract9:00 PM - M7.63
Enhanced Luminescence Of Conjugated Molecules On Metal Functionalized Layers Via Surface Plasmons.
Hong Ju Park 1 , Seok-Soon Kim 1 , Dong-Yu Kim 1
1 Heeger Center for Advanced Materials, Dept. of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju Korea (the Republic of)
Show AbstractSurface plasmons(SPs) are charge density oscillationsconfined to metallic nanoparticles and metallic nanostructures. These oscillations can occur at the interface between a metal and a dielectric medium. The collective response of electrons in surface of conductor to optical fields is known as plasmon resonances or surface plasmons. The surface plasmons are affected by the size and shape of metal nanoparticles and the dielectric function of the surrounding medium. Due to the quenching of emission by surface plasmons in the light emitting device, only few attempts have so far been made for the application to light emitting devices. Recently, it was reported that the presence of metal particles or metal thin film can enhance the light emission. This enhancement may be attributed by two factors. The first factor is local field enhancement of the incident electromagnetic field causing absorption. The second factor is increased radiative decay rates which can be accelerated in the vicinity of metal surfaces for molecules. But several problems exist for metal particles or metal thin film to apply for light emitting devices. Examples of these problems would be quenching of excitons, phase separations and failure of device. In our presentation, we demonstrate enhanced PL intensity of organic fluorophores by using Ag and Au thin film as metal functionalized layers on the ITO which exhibit surface plasmons in the visible spectral region.
9:00 PM - M7.64
Synthetic Design of Novel High Mobility Oligomeric Semiconductors with Excellent Environmental Stability.
Jason Locklin 1 , Andrew Sung 1 , Mang-Mang Ling 1 , Zhenan Bao 1
1 Chemical Engineering, Stanford University, Stanford, California, United States
Show AbstractA series of co-oligomers have been synthesized that are based on the phenylene-fluorene and phenylene-thiophene framework using Stille and Suzuki coupling. The phenylene-fluorene derivatives exhibit charge carrier mobilities greater than 0.2 cm2V-1s-1 and have excellent air stability due to their low energy HOMO level. With the phenylene-thiophene derivatives, various alkyl and alkoxy endgroups were substituted at the periphery in order to study the effect of bulky substituents on the molecular ordering and electronic properties of the oligomers in thin film transistor devices. The thin film crystallinity and morphology was investigated using AFM and X-ray Diffraction. These materials are highly soluble in organic solvents and solution processing using the bottom contact device configuration is demonstrated.
9:00 PM - M7.65
Single-Crystal Organic Nanowires of TCNQ-Cu: Non-Volatile Memory Devices on Plastic
Kai Xiao 1 , Jing Tao 2 , Zhengwei Pan 1 , Ilia Ivanov 1 , Alexander Puretzky 1 , Zuqin Liu 1 , Stephen Pennycook 2 , David Geohegan 1 2
1 Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 2 Condensed Matter Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractSingle-crystal, one-dimensional semiconductor nanostructures are critical building blocks for nanoscale optical and electronic devices. However, most nanowires reported to date are based on inorganic materials. Few synthesis experiments and devices have been reported for nanowires of organic semiconductors, despite the wide use of these materials in organic thin-film optoelectronic devices such as transistors, OLEDs and photovoltaic cells. Although nanostructures of conductive polymers such as polythiophene and polyaniline have been synthesized, so far no reports have demonstrated the controllable synthesis of single-crystal, one-dimensional organic semiconductor nanostructures. Here we report a simple method for the synthesis of single-crystal nanowires from organic semiconductor materials. TCNQ-Cu is an organic charge-transfer complex with unique electrical properties which has applications in both optical and electrical recording media. In this report, single-crystal organic semiconductor TCNQ-Cu nanowires were controllably synthesized on different substrates such as Si, glass, copper foils and plastics with a simple, low-temperature CVD method. Transmission electron microscopy (TEM) and Selected Area Diffraction (SAD) were used to characterize the single-crystal structure and growth direction. Non-volatile memory devices were demonstrated from the TCNQ-Cu nanowires which were grown directly on the plastic substrate. Research on Functional Nanomaterials at the Center for Nanophase Materials Sciences is supported by the U. S. Department of Energy, Division of Materials Science, Basic Energy Sciences.
9:00 PM - M7.66
Hydrodynamic Focusing-Based Nanoreactor for Preparation of Morphology-Controlled and Mono-Dispersed Conducting Polymer Nanoparticles
Kala Coti 2 1 , Yanju Wang 3 , Matthias Selke 3 , Weixing Lu 1 , Hsian-Rong Tseng 1
2 Chemistry and Biochemistry, UCLA, Los Angeles, California, United States, 1 pharmacology, UCLA, Los Angeles, California, United States, 3 Chemistry and Biochemistry, California State University, Los Angeles, California, United States
Show AbstractHerein we present the development of a new open, dynamic system based on microfulidic hydrodynamic focusing to prepare mono-disperse inorganic and organic nanoparticles. This microfluidic platform addresses one of the biggest challenges faced by the current synthetic methodologies to control poly-dispersity; the difficulty to control the local physical environment, like mass transfer due to poor mixing, in macroscopic reactors. We have developed a hydrodynamic focusing-based nanoreator to achieve ultra-fast mixing leading to great homogeneity in the physical reaction conditions, resulting in mono-dispersed nanoparticles. As a proof-of-concept study, mono-dispersed conducting polymer nanoparticles with tunable dimensionalities ranging from 20 to 200 nm were produced by performing hydrodynamic mixing in the devices. The diversity of this newly developed platform is also demonstrated by preparing inorganic nanoparticles (i.e. Au and CdS) of various dimensions.
9:00 PM - M7.7
Degradation of Small-Molecule Organic Light Emitting Devices.
Jason Slinker 1 , Samuel Flores-Torres 2 , Daniel Bernards 1 , Leonard Soltzberg 3 , Ji-Seon Kim 4 , Velda Goldberg 3 , Michael Kaplan 3 , Richard Friend 4 , Hector Abruna 2 , George Malliaras 1
1 Materials Science and Engineering, Cornell University, Ithaca, New York, United States, 2 Chemistry and Chemical Biology, Cornell University, Ithaca, New York, United States, 3 Departments of Physics and Chemistry, Simmons College, Boston, Massachusetts, United States, 4 Cavendish Laboratory, University of Cambridge, Cambridge United Kingdom
Show AbstractIonic transition metal complexes (iTMCs), such as ruthenium (II) tris-bipyridine [Ru(bpy)3]2+, are receiving increased attention as these compounds have produced high efficiency organic light emitting devices with air-stable cathodes. Since iTMC-based devices can be fabricated using air-stable electrodes, their lifetime is ultimately limited by the degradation of the metal complex layer. This feature makes iTMC devices an ideal system for the study of degradation processes in organic semiconductors. Furthermore, there is hope that if one is able to understand degradation pathways and design complexes that are inherently more stable, device degradation can be minimized. This will enable the fabrication of electroluminescent devices with minimal encapsulation requirements, with important technological implications.Along these lines, we have used matrix assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry to identify a quenching species that is formed in [Ru(bpy)3]2+ electroluminescent devices during device operation. We identify this quenching species to be the oxo-bridged dimer [(bpy)2(H2O)RuORu(OH2)(bpy)2]4+ by correlation with mass spectra of the synthesized compound and show this dimer to be an effective quencher of device luminescence. This demonstrates that MALDI-TOF can be an effective tool for identification of quenching species in small-molecule organic electroluminescent devices. In addition, we have recently used Raman spectroscopy to confirm dimer formation and to study the in-situ formation of dimeric compounds during the operation of [Ru(bpy)3]2+ light emitting devices. Raman spectroscopy provides a non-invasive means of studying degradation of the metal complex layer.
9:00 PM - M7.8
Electrical Characterization of Polyfluorene-based Metal-insulator-semiconductor Diodes.
M. Yun 2 , M. Arif 1 , S. Gangopadhyay 2 , S. Guha 1
2 Electrical and Computer Engineering, University of Missouri, Columbia, Missouri, United States, 1 Physics, University of Missouri, Columbia, Missouri, United States
Show AbstractPolyfluorenes (PFs) have emerged as a promising family of blue polymer light-emitting diodes (PLED) owing to their high electroluminescence quantum yield. Despite the rapid progress in PF-based PLEDs these devices have lower charge carrier mobility compared to evaporated organic molecules. Additional issues such as interface states can considerably affect the electrical properties of such devices. Metal-insulator-semiconductor (MIS) diodes are the two-terminal analogues of thin film transistors sharing the same basic layer structure. Capacitance-voltage (C-V) and capacitance-frequency (C-f) measurements in these devices yield information about doping, mobile charges, and trapping processes in these devices.In this work we have investigated MIS structures based on PFs. Thin films of Poly[9,9-bis(2-ethylhexyl)fluorene-2,7-diyl] (PF2/6) were spin-coated on SiO2/Si substrates. The Si layer was highly p-doped (10^18/cm^3). From our C-V measurement we infer the dielectric constant and the related dielectric loss of the polymeric layer. By comparing the high and low frequency C-V curves we determine the flat-band voltage shift and interface state density throughout the insulating layer. The hysteresis between forward and reverse bias sweeps as a function of frequency yields information on the type of injected carriers in the insulating film. We have further carried out temperature-dependent C-f measurements to obtain localized charge concentration and carrier mobility perpendicular to the insulating layer. This work is supported by the National Science Foundation under grant no. ECS-0523656.
9:00 PM - M7.9
Supramolecular Self-assembly of a Complex 2D Donor-acceptor Network.
Luc Piot 1 , Alexandr Marchenko 1 , Yohann Nicolas 2 , Jean Roncali 2 , Denis Fichou 1
1 LRC Nanostructures et Semi-Conducteurs Organiques (CNRS-CEA-UPMC), SPCSI/DRECAM, CEA-Saclay, Gif sur Yvette France, 2 Groupe Systemes Conjugues Lineaires, IMMO, UMR CNRS 6501, Universite d'Angers, Angers France
Show AbstractRecent work has presented 2D-oligothiophenes molecules as good candidates for the fabrication of electronic devices, especially plastic photovoltaic cells, taking advantage of their p-type semi-conductivity. In addition to their electronic properties, these molecules show an ability to form two-dimensional highly ordered nanoporous honeycomb layer on atomically flat surfaces. Here we investigated the so called Btt(T2H3)3 molecule, this recently synthesized star-shape molecule belongs to the C3h symmetry group and is composed by a benzotrithiophene core on which short-chain oligothiophenes are connected.The adsorption behaviour of the star-shape molecules was studied by mean of Scanning Tunnelling Microscopy at the liquid-solid interface on an highly ordered pyrolitic graphite surface (HOPG). Such a technique appears to be suitable for the investigation of large and fragile molecules compared to the UHV techniques of deposition and characterization. The star-shape oligothiophenes form rather complex structures on the surface. Two different packings (only one of which is stable) are present on the surface both having the remarkable property to form nanopores. This oligomer layer was used as a 2D host system in order to steer the adsorption of C60 molecules on the surface. This approach is of particular interest because it provides a way to produce well defined nanostructure in a parallel fashion. Moreover, the resulting composite material combines two different sorts of molecules, which are known to be good donor (oligothiophene) and acceptor (C60) of electrons. This is promising for further photoconduction experiments.
Symposium Organizers
Zhenan Bao Stanford University
Anna B. Chwang Universal Display Corporation
Lynn Loo The University of Texas-Austin
Rachel A. Segalman University of California-Berkeley
M8: Organic Integrated Circuits, Memories and Lasers
Session Chairs
Thursday AM, April 20, 2006
Room 3001 (Moscone West)
9:30 AM - **M8.1
Organic Transistors and their Application in Integrated Circuits and Active-matrix Displays.
Gerwin Gelinck 1 2 , Erik Van Veenendaal 1 2
1 , Polymer Vision, Eindhoven Netherlands, 2 , Philips Research Laboratories, Eindhoven Netherlands
Show AbstractOrganic thin-film transistors have emerged as a promising technology for low-cost, large-area microelectronic applications. Improvements in materials and processing have resulted in the demonstration of relatively complex logic circuitry with ~ 2000 transistors and large-size active-matrix displays. With an increase in integration scale and operation speed it is becoming increasingly necessary to develop tools for the realistic simulation and optimization of more advanced and complex circuits. In this presentation, the current-voltage and capacitance-voltage characteristics of an organic transistor will be analyzed and a TFT model will be discussed that can be incorporated into circuit simulation programs. As an example of how this model can be used to understand and optimize circuit performance, we evaluate the link between transistor behaviour and inverter performance with reference to some basic digital building blocks. The insights thus obtained are used to design circuitry that exploits the characteristics of organic electronics to the fullest.Throughout the presentation some of the higher-order integration aspects are discussed that are associated with typical applications such as RF identification tags and active matrix displays. Achievements reported here include (non)-volatile memory elements, high-frequency rectifiers and rollable displays.
10:00 AM - M8.2
High-Frequency Rectification for Organic Radio Frequency Identification Tag.
Soeren Steudel 1 2 , Stijn De Vusser 1 2 , Kris Myny 1 , Maarten Debucquoy 1 2 , Jan Genoe 1 , Paul Heremans 1
1 Polymer & Molecular Electronics, IMEC, Leuven Belgium, 2 Electrical Engineering, Katholieke Universiteit Leuven, Leuven Belgium
Show Abstract10:15 AM - M8.3
Flexible Polymer Thin-Film Transistor Device Structures And Processes For 13.56 Mhz RF Rectifier Circuits.
Siddharth Mohapatra 1 , Robert Rotzoll 1 , Patrick Jenkins 1 , Viorel Olariu 1 , Michelle Grigas 1 , Robert Wenz 1 , Klaus Dimmler 1 , Ananth Dodabalapur 1 2
1 , OrganicID, Austin, Texas, United States, 2 MER, University of Texas, Austin, Texas, United States
Show AbstractPolymeric thin-film transistors (TFTs) have been proposed for several applications including displays, electronic paper, chemical sensors, and radio-frequency identification (RFID) tags. One important technical hurdle that has to be overcome for using organic transistors in RFID tags is for these devices to operate at RF frequencies (typically 13.56 MHz) in the front end. It was long thought that conjugated polymer transistors are too slow for this. In this presentation we will show that polymer transistor based full-wave rectifier circuits utilizing a polythiophene, a p-channel semiconductor, can operate at this frequency with a useful efficiency. In order to achieve such high-frequency operation, we make use of the non-quasi static (NQS) state of the transistors. Bottom gate and top gate structures are explored and a comparison is made between the observed electrical properties. These circuits are fabricated on PEN (polyethylenenapthalate) substrates using a spin on thermally curable dielectric. Gate, source and drain contacts are defined photolithographically using evaporated gold as the metal. Field-effect mobilities in the range of 0.02 and 0.2 cm2/Vs that equal or exceed the highest reported among the ones employing similar geometries on plastic substrates are easily obtained in these systems. In order for NQS based rectification to take place the carrier velocity in a 2 micrometer channel length device needs to be more than 2x104 cm/s. This would correspond to a mobility of about 0.1 cm2/V-s at a field of 10^5 V/cm. The rectifiers were based on a 4-transistor full-wave design. A coil (transformer secondary) drives the AC inputs in differential mode. The lower diode connected transistors in a manner similar to the half-wave rectifier define the DC voltage level of the two AC input signals. The two upper transistors are connected as switches and are used to move current from the AC inputs to the positive DC output. The capacitor is used to hold the peak voltage level supplied by the coil through the switch-connected devices. The voltage rectification efficiency is relatively high being as much as 30% at 13.56 MHz. This represents the fastest polymer transistor circuit of any kind demonstrated to date.
10:30 AM - **M8.4
Low-voltage, Low-power Organic Circuits.
Hagen Klauk 1
1 , Max Planck Institute for Solid State Research, Stuttgart Germany
Show AbstractSince n-channel organic semiconductors often suffer from low mobility or poor stability in air, organic circuits typically make exclusive use of p-channel transistors. The problem with circuits designed with only one type of transistor is the power dissipation due to static currents. In addition, organic transistors often have a relatively small gate dielectric capacitance, usually less than 100 nF/cm2. As a result, voltages greater than 10 or 20 V are typically required to induce sufficient charge in the channel to operate the transistors. For many circuit applications smaller operating voltages are desirable, either to relax power supply requirements, to extend battery life, or to make large area organic electronics compatible with silicon CMOS circuitry. To reduce the operating voltage of organic transistors, self-assembled monolayer and multilayer gate dielectrics with a thickness of a few nanometers and capacitance as large as 700 nF/cm2 have recently been reported [1,2]. These thin dielectrics allow organic transistors to operate with voltages of a few volts. To realize low-voltage, low-power organic circuits we are developing a complementary circuit technology that uses p-channel and n-channel organic transistors with monolayer gate dielectrics. These circuits can be operated with voltages as low as 1.5 V and have static currents of less than 1 nA and a static power dissipation of less than 1 nW per logic gate.[1] M. Halik et al., Nature, vol. 431, p. 963, 2004[2] M. H. Yoon et al., PNAS, vol. 102, p. 4678, 2005
11:30 AM - **M8.5
Reversible Electrically-Programmable Organic Memory from Self-Assembled Oligomer Monolayers.
James Sturm 1 , Troy Graves-Abe 2
1 Princeton Institute for the Science and Technology of Materials (PRISM), Dept. of Electrical Engineering, Princeton University, Princeton, New Jersey, United States, 2 Electrical Engineering, Princeton University, Princeton, New Jersey, United States
Show AbstractThe recent trend towards portable consumer electronics is driving a great interest in technologies for non-volatile memory which are cheap, fast, and low-power. This space is currently dominated by silicon “flash” memory, in which electrons stored on polysilicon layers surrounded by oxide are used to change the threshold voltage of a MOSFET. However, the future scaling of flash will be limited by minimum oxide thicknesses (needed to store electrons on floating gates), and the programming process of driving electrons through these oxides is not fast. Thus there is great interest in non-crystalline (low cost) material structures whose electrical resistance can be electrically programmed. Several types of organic memories have been proposed, the most desirable of which are “read/write” – i.e. the resistance change can be electrically reversed. Further, in practical applications, individual bits are arranged in an array, and the capacitances of the row and column lines can limit the readout as well as the programming process. Overcoming this limit requires current densities in excess of 105 A/cm2. Because organic devices are often used at current densities of milliamperes or amperes per square centimeter, high current densities will require very thin organic layers. In this paper, we will discuss a new organic memory structure based on a two-terminal structure of self-assembled monolayers of the alkane-based oligomer 11-mercaptoundecanoic acid (MUA), with typically five to ten layers (1.6 nm each) in a device structure. As fabricated, devices have a high resistance, which can then be switched low by voltage pulses in the 3-5 V range. Voltage pulses in excess of 6 V restore the high resistance state, and the state of the device can be “read” non-destructively by applying very low (< 1V) pulses to measure the resistance. In the on-state, current densities of up to 106 A/cm2 have been observed in the low resistance state. The devices have been cycled for over 104 times with little degradation, and data has been stored in excess of three months. Switching speeds in both directions can be 100 ns or less. We will focus on the mechanism behind this device, and present considerable evidence that the mechanism is not electronic (e.g. charge trapping), but rather the motion of metal atoms to form conducting paths for electronic hopping through the MUA.
12:00 PM - M8.6
Switching and Filamentary Conduction in Non-Volatile Organic Memories.
Michael Coelle 1 , Michael Buechel 1 , Dago de Leeuw 1
1 , Philips Research, Eindhoven Netherlands
Show AbstractNon-volatile organic memories are presently being investigated as low-end replacement of standard NAND Flash circuitry. The devices consist of a thin semi-conducting organic film sandwiched between two electrodes. Resistance switching has been reported for a wide variety of semiconductors such as penthacene, anthracene, Cu TCNQ, blends of organic materials and doping with nanoparticles. It is noteworthy that typically one of the electrodes is aluminum. In some of the reported devices reproducible switching is only observed when the aluminum electrode is deliberately oxidized or unintentionally passivated with native oxide. The devices switch reversibly between a high resistance off-state and one or more low resistance on-states at fields much lower than the breakdown fields. The current modulation at low bias is typically several orders of magnitude. Nanosecond switching times have been reported. In the on-state, the transport is symmetric for forward and reverse bias and transport has a negligible small thermal activation energy. Data retention times and programming cycle endurance are both excellent. Shelf-life data are not yet available but from similar devices like organic light emitting diodes it can be assumed that they are environmentally stable. The nature of the switching mechanism is still unknown. For heterogeneous systems field induced charge transfer leading to building up of space charges that inhibits injection has been suggested to explain switching combined with negative differential resistances. As an alternative charge tunneling and polarization of the trapped charge has been proposed. In homogenous organic systems switching has been ascribed to several different mechanism including bulk charge transfer, charge trapping, electro-reduction and conformational changes. Finally, formation of conducting filaments and influence of aluminum oxide has also been suggested.For practical utilization in electronic devices the properties of the organic memory must be adjustable. Memory circuit design imposes specific windows for both on- and off resistance. For instance, selection transistors cannot deliver the current to switch large arrays of metallic conductors. Moreover, the resistances of the cell should scale with the lateral dimensions. Hence, it is of paramount importance to know whether the switching is a bulk property or due to formation of filaments. To address this question we investigated resistance switching in electron-only, hole-only and bipolar polymeric light emitting diodes. Our findings suggest that switching is related to the nature of the electrode. By using a heat sensitive camera we show that the conduction in the on-state is due to filaments through the bulk of the semiconductor. A phenomenological interpretation of the switching is presented and the consequences for scaling are discussed.
12:15 PM - **M8.7
Conjugated Polymers for Memory Applications.
Luisa Bozano 1 , Sally Swanson 1 , Richard DiPietro 1 , Kerem Unal 1 , Jane Frommer 1 , J. Scott 1 , Scott Sills 2 , Matthias Beinhoff 3 , Kenneth Carter 4
1 Almaden Research Center, IBM, San Jose, California, United States, 2 CPIMA, Stanford, Palo Alto, California, United States, 3 Sigma-Aldrich, Sigma-Aldrich , Switzerland Switzerland, 4 Department of Polymer Science and Engineering, Univeristy of Massachusetts, Amherst, Massachusetts, United States
Show AbstractMemory elements are fabricated using a wide variety of conjugated and not conjugated organic materials. Small molecules Alq3, NPB and pentacene were used for the layered structure, while triarylamine (HTPA), polyvinylcarbazole (PVK), P3HT (poly-3-hexyl thiophene), and poly-biphenyl (BPF9) where used in blended structure. The organic/metal films were then sandwiched between metal contact and electrical measurements were performed. The DC electrical response of the memory element shows N-shaped current voltage (IV) characteristic and a hysteretic behavior at low voltages. This behavior is reproducible for different choices of materials. Switching performance of the memory elements like ON/OFF current ratio, switching voltages and times, retention and cycling endurance, were determined by DC measurement. The resulting data are presented in the context of a model in which charge transport occurs both by conduction in the polymer host and tunneling between nanoparticles.
12:45 PM - M8.8
Optically Pumped Lasers Based on High Mobility Polymers
Ruidong Xia 1 , Boon Yap 1 , George Heliotis 1 , Youngkyoo Kim 1 , Jingsong Huang 1 , Paul Stavrinou 1 , Donal Bradley 1
1 Ultrafast Photonics Collaboration, Experimental Solid State Physics Group, The Blackett Laboratory, , Imperial College London , London United Kingdom
Show AbstractConjugated polymers are promising solid-state gain media and optically pumped lasers based thereon have been demonstrated across the full visible spectrum. Electrically pumped lasers are yet to be realised, an important issue being the typically low mobility and frequently unipolar charge transport found in organic semiconductors. Efforts are consequently being made to both improve mobilities and lower lasing thresholds in order to make progress towards an electrically pumped laser diode. Here, we report recent results from our programme concerning optically pumped surface-emitting lasers with high slope efficiencies. We use a range of fluorene-based copolymers and their blends as gain media, focussing on polymers that demonstrate high charge carrier mobilities. One- and two-dimensional grating structures were electron-beam written into a resist and, after developing, the patterns were transferred into the underlying fused silica substrates via ion-beam etching. These substrates were used to fabricate distributed feedback lasers by spin-coating a thin polymer layer on top of the grating pattern. For one-dimensional gratings, the highest laser slope efficiency was 8% and the lowest lasing threshold was 5 nJ per 10 ns pulse (390 nm, 10 Hz). The output characteristics of our lasers will be reported in detail, including the influence of the specific grating structure, the blend composition and the polymer film thickness. We thank The Dow Chemical Company, The Sumitomo Chemical Company and Merck Chemicals for the polymers, and the UK Centre for Integrated Photonics for the grating structures that we have used in our study. We gratefully acknowledge financial support from the UK Engineering and Physical Sciences Research Council (Ultrafast Photonics Collaboration GR/R55078 and PRINCE GR/S86631) and The Sumitomo Chemical Company.
M9: Organic Optoelectronic Materials and Devices
Session Chairs
Thursday PM, April 20, 2006
Room 3001 (Moscone West)
2:30 PM - **M9.1
Competing with Incandescence: Methods for Acheiving Efficient Solid State Lighting Using OLEDs.
Stephen Forrest 1
1 Electrical Engineering and Computer Science and Physics, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractWe will discuss several strategies for achieving very high efficiency white light emission at high brightnesses for the next generation of efficient solid state lighting sources. Key to our approach is the use of electrophosphorescence as a means for converting all electrical into optical energy. We show that the highest luminance efficiencies can be obtained by a combination of flourescence and phosphorescence in a unique OLED structure. Furthermore, the highest brightnesses are achieved (without a significant loss in power efficiency) by stacking several flourescent/phosphorescent elements in a single OLED structure (called a SOLED), with each emitting element in the stack separated by a transparent charge generation layer. Prospects for OLEDs as the next practical generation of interior illumination sources will be reviewed.
3:00 PM - M9.2
Resonantly-enhanced Fluorescence in Small Molecular Weight Organic Light Emitting Devices.
Michael Segal 1 , M. Singh 1 , K. Rivoire 2 , M. Baldo 1
1 EECS, MIT, Cambridge, Massachusetts, United States, 2 Physics, MIT, Cambridge, Massachusetts, United States
Show AbstractThe electroluminescent efficiency of fluorescent small-molecular weight materials is typically limited to 25% of that of phosphorescent materials. We report a general technique for resonantly enhancing singlet exciton formation, in the process breaking the previous 25% limit. Resonant exciton formation creates relaxed excitons directly from charge separated pairs, allowing us to exploit the strong exchange splitting between triplet and singlet excitons and tune the singlet ratio with voltage. We have achieved > 25% singlet excitons in the small-molecular weight fluorophore aluminum-tris-quinolate (Alq3), doped with a low concentration of the phosphorescent dye 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine platinum(II) (PtOEP) to monitor the triplet population. In addition to its application in high efficiency organic light emitting devices (OLEDs), this novel process allows direct spectroscopy of the exciton formation process at the center of OLED operation, and it may be used as a probe of the energy structure of occupied and unoccupied states in organic semiconductors.
3:15 PM - M9.3
The Effects of Device Structure on Negative Capacitance in Small Molecule OLEDs.
Liam Pingree 1 , Matthew Russell 2 , Tobin Marks 2 , Mark Hersam 1
1 Material Science, Northwestern University, Evanston, Illinois, United States, 2 Chemistry, Northwestern University, Evanston, Illinois, United States
Show AbstractAs Organic Light Emitting Diodes (OLEDs) enter the consumer market, their frequency dependent charge transport characteristics are becoming more important. When these devices are rapidly cycled on and off, as in display applications, any inductive nature in the transport characteristics will lead to non-optimal device performance. While this inductive-type transport, or negative capacitance (NC), was reported in the literature only recently [1,2], similar characteristics have been seen in the electroluminescence transients of OLEDs for some time [3]. In our initial work [1], it was demonstrated that dangling bonds, surface dipoles, and defects at the anode/hole transport layer (HTL) interface create a series of charge traps that dominate the low-frequency response, thus masking the inductive nature of OLEDs. Once this interface was modified through the addition of a thin adhesion promoting layer, charge trapping did not occur, and the underlying behavior could be monitored using impedance spectroscopy. An equivalent circuit for hetero-layered OLEDs was developed, consisting of a parallel resistor and capacitor in series with a parallel resistor and inductor that models the charge transport through the electron transport layer (ETL) and HTL, respectively. In this work we examine the robustness of this model to modifications in the OLED structure, while focusing on the low-frequency behavior, where NC dominates transport. These modifications include various cathode and anode interface preparations, device evolution or “burn-in” effects, and organic layer thickness variations. The onset of NC is correlated with the appearance of space charge limited transport, without the presence of traps. The apparent magnitude of NC increases with cathode/ETL modification via the inclusion of a thin layer of LiF due to a shift in the percentage contribution of the HTL to the total potential drop across the device from 2.5% to 6%. Similarly, NC increases by doubling the ETL thickness though an increase in the percentage of the potential drop across the HTL from 2 % to 9.5%. NC is found in devices without the addition of an interfacial layer at the anode when proper anode surface preparation procedures are followed, however, this effect decays over time. NC appears unaffected by HTL layer thickness and “burn-in,” and these variations only slightly affect the device performance. The affects of these modifications to the OLED device structure are accounted for by our model.[1] L.S.C. Pingree et al. Appl. Phys. Lett. 86, 073509 (2005).[2] F.A. Castro et al. Appl. Phys. Lett. 87, 013505 (2005). [3] V.R. Nikitenko et al. J. Appl. Phys. 84, 2334 (1998).
3:30 PM - **M9.4
Advances in Nonvolatile Multilevel Conductance and Novel Infrared Light-Emitting Organic Materials and Thin Films
Ghassan Jabbour 1 , Mike Lauters 1 , Jian Li 1 , Evan williams 1 , kirsi haavisto 1 , dror sarid 1 , Brendan Mccarthy 1
1 Chemical and Materials Engineering and Flexible Display Center, University of Arizona, Tempe, Arizona, United States
Show AbstractWe will present our progress in the area of organic thin-film structures, including organic light-emitting diodes, as it applies to the demonstration of multiple nonvolatile conductance states at low read voltages. These devices also show retention time of states of several weeks, and more than 20,000 write-read-rewrite-read cycles with minimal degradation. We will also discuss our recent work on novel infrared light-emitting organic materials and their potential device applications. In this venue, we will describe a novel approach of developing iridium based complexes for near infra-red OLED and sensing applications. Preliminary results of OLED made with these materials will be shown.
4:30 PM - **M9.5
Conversion of Electrical Energy into Light and Solar Energy into Electricity Using Organic Materials and Nanostructures
Zakya Kafafi 1
1 Code 5615, NRL, Washington, District of Columbia, United States
Show Abstract5:00 PM - M9.6
Solution-processed Photosensitive Organic Thin Films for Solar Cell Applications.
Pavel Lazarev 1 , Andrew Kazanski 2 1 , Alexander Kozhanov 2 1 , Andrew Manko 1 , Alexey Nokel 1 , Elena Sidorenko 1 , Anatoly Solodov 1 , Vladimir Sulimov 3 1
1 , Kontrakt Ltd., Moscow Russian Federation, 2 , Moscow State University, Moscow Russian Federation, 3 , Research Computing Center of Moscow State University, Moscow Russian Federation
Show Abstract5:15 PM - M9.7
Excited State Dynamics And Photovoltaic Device Performance In Polymer:Fullerene Blends.
Stephanie Chasteen 1 , Veronica Sholin 1 , H.-H. Hoerhold 2 , Sue Carter 1
1 Physics Dept., UC Santa Cruz, Santa Cruz, California, United States, 2 , University of Jena, Jena Germany
Show AbstractWe have investigated the relationship between excited state dynamics and device performance in polymer-fullerene blends. We have blended the hole-transporting polymer M3EH-PPV with the electron-accepting fullerene derivative, PCBM. Photovoltaic devices are characterized and time-resolved photoluminescence measurements reveal information about the underlying morphology. Devices with 80 wt% of PCBM in the blend are found to have higher currents and fill-factor (Jsc = 13 mA/cm2, FF = 37% ) than blends with 20 wt% PCBM (Jsc = 4.8 mA/cm2, FF = 29%) under white light illumination. Despite poorer device performance, the time-decay of 20 wt% blends is more rapid than that of 80 wt% blends due to (a) better dispersion of the fullerene, and (b) the absence of a long PCBM decay component. This is due to the presence of large domains of PCBM in 80 wt% blends, which presumably provide a continuous percolation pathway for the transportation of mobile electrons, resulting in high currents and fill-factor. The ideal blend ratio for charge separation does not coincide with the ideal ratio for charge transport, due to blend morphology. Morphological changes in the blended film relative to the pristine polymer are also evidenced by a strong blueshift in the steady-state fluorescence. A new spectral peak of unknown origin is observed at 2.5 eV when the films are excited at low energy.
5:30 PM - **M9.8
Exciton Diffusion and Resonance Energy Transfer in Organic Photovoltaic Cells.
Michael McGehee 1 , Shawn Scully 1 , Melissa Summers 1 , Yuxiang Liu 1 , Carine Edder 3 , Jean Frechet 2 3
1 Materials Science and Engineering, Stanford University, Stanford, California, United States, 3 , The Molecular Foundry, Berkeley, California, United States, 2 , University of California at Berkeley, Berkeley, California, United States
Show AbstractAlmost all organic photovoltaic cells are based on either planar or bulk heterojunctions of two semiconductors. After light is absorbed, excitons must get to the interface between the two semiconductors to dissociate by electron transfer. In some cases, such as in dye-sensitized cells or polymer-fullerene bulk heterojunctions with very high fullerene concentrations, excitons are formed right at the interface and exciton transport is therefore not a limiting factor on the performance of the cells. In many other cases, such as in polymer-nanowire or polymer-titania cells, excitons need to travel at least five nanometers, if not more. For this reason exciton diffusion is a very important process to understand and optimize. We have carefully measured the exciton diffusion length in several polymers and found that the values are less than reported in the literature. Common sources of error in diffusion length measurements are neglecting interdiffusion between the donor and acceptor, interference effects and resonance energy transfer. Since the diffusion length in most polymers is 6 nm or less, we have explored ways to enhance exciton transport. One is to use resonance energy transfer from a donor to an acceptor with a slightly smaller energy gap. We will show an example where the effective diffusion length and consequently the efficiency of the photovoltaic cell is enhanced by a factor of three. We will also show that resonance energy transfer occurs in many previously studied donor-acceptor blends, including polymer-fullerene blends with low fullerene concentrations. Finally we will discuss how high the effective diffusion length could be if the donor-acceptor pair were optimized for resonance energy transfer.
M10: Poster Session: Organic Transistors and Arrays
Session Chairs
Friday AM, April 21, 2006
Salons 8-15 (Marriott)
9:00 PM - M10.1
Blends of Semiconductor Polymer and Small Molecular Crystals for Improved-performance Thin-film Transistors.
D. Russell 1 , C. Newsome 1 , S. Li 1 , T. Kugler 1 , T. Shimoda 2
1 , Cambridge Research Laboratory of Epson, Cambridge United Kingdom, 2 Technology Platform Research Centre, Seiko- Epson Corporation, Fujimi Japan
Show AbstractRecent advances have shown that small-molecular single crystals and evaporated organic molecules can compete with amorphous silicon in terms of performance. Although the performance of such materials is impressive, the complex processing is not suited to low-cost fabrication as is required for a disruptive technology. Solution-processed semiconductor polymers offer a low production cost through the combination with printing techniques. However, amorphous polymers typically exhibit mobilities in the order of 10-3cm2/Vs. It is desirable to combine the performance of small molecules with the ease of process of polymers. Although films of semiconductor small-molecular crystals have been grown from solution, positioning and orientation of the crystals is not trivial. For ease of processing, a semiconductor polymer binder is employed in this work to bridge the areas between the crystals, thus facilitating electrical contact between the crystals and the metal electrodes, and to provide a continuous film.This process has been demonstrated using a poly(3-hexylthiophene) (P3HT) binder to connect crystals of dihexyl-quarterthiophene (DH4T), to form the active layer of a thin film transistor. The field-effect mobility was enhanced by a factor of 10 over a TFT made with P3HT alone. Varying the DH4T concentration shows the improvement in mobility is due to the formation of crystals within the blend film, which occur at a critical concentration of 29% DH4T. Application of percolation theory reveals that transport in blend devices is limited by the mobility of the P3HT and by the crystal packing.
9:00 PM - M10.11
P(VDF/TrFE) Based Ferroelectric Organic Field Effect Transistors for Non Volatile Memories.
Klaus Mueller 1 , Karsten Henkel 1 , Ioanna Paloumpa 1 , Dieter Schmeisser 1
1 Applied Physics, BTU Cottbus, Cottbus Germany
Show AbstractThe ferroelectric polymer poly(vinylidene fluoride trifluoroethylene) (P(VDF-TrFE)) is used as insulating material for organic field effect transistors (OFET) and metal-insulator-semiconductor (MIS)-structures. For the MIS-structures, we find the typical hysteresis in the CV characteristic upon increasing the voltage scan window. A writing process with adequate electric fields causes shifts in the flatband voltage which are stable over days. Based on these results, we fabricate OFET with regioregular poly(3-hexlythiophene) (P3HT) as organic semiconductor. The transistors are constructed in bottom gate architecture with thin layers of spincoated P(VDF-TrFE) as gate insulation (100nm). The drain-source current of the OFET is reversible affected by the polarized gate, which gives opportunities for building up an organic nonvolatile memory element. We present characteristic features like the hysteretic drain current response or the data retention. Furthermore, we present measurements with Kelvin probe force microscopy, a method which gives informations on the lateral distributions of the surface potential. If we use rather thick layers of P(VDF-TrFE) (about 2µm), we find no significant hysteresis, a prerequisite for transistor operation. Compared to similar transistors (same geometric factors), but only with polymethylmetacrylate (PMMA) as dielectric layer, we observe a smaller operation voltage as well as a better transconductance. These enhanced gate effects are attributed to the higher dielectric constant DK of P(VDF-TrFE) (DK PVDF=12, DK PMMA=3.12).
9:00 PM - M10.12
Towards Stable Pentacene Derivatives for OTFT Applications.
Subramanian Vaidyanathan 1 , Sen Liu 1 , Olexsander Sydorenko 1
1 , Lucent Technologies, Murray Hill, New Jersey, United States
Show Abstract9:00 PM - M10.13
Kinetics of Charge Traps in Organic Field-effect Transistors.
Tse Nga Ng 1 2 , Michael Chabinyc 2 , John Marohn 1
1 Chemistry and Chemical Biology, Cornell University, Ithaca, New York, United States, 2 , Palo Alto Research Center, Palo Alto, California, United States
Show AbstractDevice stability is a major concern for organic field-effect transistors, and former studies on bias stress, in which the device current decreases with operation time, has shown that there are at least two distribution of charge traps. Electric force microscopy is used to locally image these charge traps. To help understand the nature of charge trapping, the release kinetics is also examined and correlated to current-voltage measurements. The trap distribution with solution-processed polymer (F8T2) is more homogeneous compared to vacuum-deposited polymer (pentacene). Different treatment of the polymer-dielectric interface does not change the trap distribution.
9:00 PM - M10.14
Bi-stable State for WORM Application Based on Carbazole-containing Polymer.
Teo Eric 1 , Qi-Dan Ling 2 , Yan Song 1 , Yoke Ping Tan 1 , Siew Lay Lim 2 , En-Tang Kang 2 , Siu Hung, Daniel Chan 1 , Chunxiang Zhu 1
1 SNDL, Department of Electrical and Computer Engineering, National University of Singapore, 10 Kent Ridge Crescent, 119260, Singapore, 2 Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, 119260, Singapore
Show AbstractRecently, several studies have been done by various groups to understand the memory effects behind organic materials, which include understanding in terms of conformation changes, conjugation modification and oxidation-reduction process. In this paper, a WORM (write-once read-many times) memory device using a new polymer material 2-(9H-carbazol-9-yl)ethyl methacrylate (PCz) containing carbazole donor group has been demonstrated. The device uses a MIM (metal-insulator-metal) structure with ITO coated glass as bottom electrode, the synthesized polymer material PCz as the active layer and Al as the top electrode. The toluene solution of PCz was spin-coated on the ITO, followed by solvent removal in a vacuum chamber. Finally, Al was thermally evaporated through shadow mask onto the PCz film.The memory effect of PCz was observed in the I-V characteristic of the MIM structure. The as-fabricated device is found to be in its OFF state, and can be programmed to ON state which is not reversible. The WORM device exhibits a high ON/OFF current ratio of up to 1E6, and shows a good retention time for both the ON and OFF states which can be sustained within a 24 h timeframe, and extrapolated to sustain for another 10 years. The effect of continuous read pulse on the ON and OFF states was evaluated and no resistance degradation is observed for read cycles up to 1E7 times. By comparing the electrical characteristics of PCz and PVK as well as their optimized geometry simulation corresponding to their minimized energy states, the memory effect or bi-stable states of PCz can be attributed to the long linker between the carbazole groups and backbone present in PCz which play a part in the conduction mechanism.
9:00 PM - M10.15
Low-Voltage Pentacene Transistors and Inverters with Plasma-Enhanced Atomic Layer Deposited Gate Dielectrics.
Jae Bon Koo 1 , Jung Wook Lim 1 , Seong Hyun Kim 1 , Sun Jin Yun 1 , Chan Hoe Ku 1 2 , Jung Hun Lee 1 2 , Sang Chul Lim 1 , Taehyung Zyung 1
1 Future Technology Research Division, Electronics and Telecommunications Research Institute(ETRI), Daejeon Korea (the Republic of), 2 Informatin Display Department, Kyung Hee University, Seoul Korea (the Republic of)
Show AbstractWe report on the performance of pentacene thin-film transistors and inverters with plasma-enhanced atomic layer deposited (PEALD) 150 nm thick Al2O3 and 120 nm thick ZrO2 dielectric films. Al2O3 and ZrO2 by PEALD with excellent electrical properties such as low leakage current and high dielectric constant, even grown at a temperature lower than 150C, are considered as promising candidates for gate dielectric materials of organic thin-film transistors (OTFTs) in flexible displays and electronics using a plastic substrate. The OTFTs with Al2O3 show depletion mode characteristics but those with ZrO2 show enhancement mode characteristics. In case of zero drive load logic inverter, which is composed of both depletion mode load transistor and enhancement mode driver transistor, full swing voltage transfer characteristics (VTCs) have been demonstrated at input voltages ranging from 0 to -3V at VDD of -3V. The hysteresis observed in VTC of inverter depends on the scan range of input voltage for the driver transistor, regardless of VDD applied to load transistor. The methods for control of threshold voltage (Vth) through simply changing of hexamethyldisilazane surface treatment conditions at each position and using dual gate structure are also introduced for making two different mode transistors at the same substrate and eventually full swing pentacene inverter.
9:00 PM - M10.17
Organic Transistor Sensors Based on Specific Semiconductor-Analyte Chemical Interactions.
Jordi Mata-Fink 1 , Andrew Sung 1 , Jason Locklin 1 , Colin Reese 1 , Zhenan Bao 1
1 Chemical Engineering, Stanford University, Stanford, California, United States
Show AbstractThe performance of organic thin film transistors (OTFTs) is dependent on the environmental conditions under which they are operated. It has been shown that OTFT parameters such as drain current and mobility decrease in the presence of water vapor or other stimuli. This functional response to surroundings opens the door to using organic thin film transistors as chemical sensors. Their potential applications include the detection of harmful chemicals for homeland or workplace security and medical diagnostics. In this work, a series of high mobility phenylene-thiophene oligomers with different chemical substituents are reported that have specific chemical interactions with different vapor analytes. The mechanism of analyte adsorption with respect to side-chain polarity and terminal reactive group is investigated.
9:00 PM - M10.18
Understanding of Anisotropy of Field-effect Mobility in Single-crystal Transistors for the Design of High-performance Organic Semiconductors for Flexible Electronics.
Colin Reese 1 , Wook-Jin Chung 1 , Mang-mang Ling 1 , Abhijit Mallik 1 , Toshihiro Okamoto 1 , Zhenan Bao 1
1 Chemical Engineering, Stanford University, Stanford, California, United States
Show AbstractThe promise of organic semiconductors for use in new and existing electronic devices has born the exploration of the underlying transport mechanisms in such materials. Organic molecular crystals (OMCs) offer a unique way to investigate charge transport as a function of molecular properties and arrangement. In this work, novel field-effect transistor architectures have been employed to probe the anisotropy of key performance metrics single crystals of tetracene and halogenated tetracene derivatives. Poly(dimethylsiloxane) (PDMS) has been employed as a dielectric layer to facilitate conformal contact to the fragile single crystal, with multidirectional probes placed to allow multiple measurements with the crystal remaining in place. This configuration has several advantages: contact to the crystal is good due to the wetting properties of PDMS, the crystal need not be repositioned between measurements, and no damage is incurred by direct deposition onto the OMC. Measurements of tetracene crystals display a marked anisotropy in the field-effect mobility, with maxima and minima corresponding to the short and long axes of the crystal structure, respectively.
9:00 PM - M10.19
Interface Engineering and Double Insulated-gate Structure for Organic Transistors.
Mang-mang (Mike) Ling 1 , Zhenan Bao 1
1 Chemical Engineering, Stanford University, Stanford, California, United States
Show AbstractOrganic thin film transistors (OTFT) are essential in various applications including large-area electronics, light-emitting diodes, smart cards, and sensors (1,2,3). Better device performance and optimal device structures are required. We present results on improving device performance by interface engineering as well as a double insulated-gate device structure.First of all, we show enhancement of device performance through interfacial engineering, that is, insertion of an organic buffer layer between metal electrodes and organic semiconductor active layer. Higher charge carrier mobility and smaller threshold voltage were obtained this way. Possible mechanisms are discussed.Secondly, interface between metal electrodes and organic semiconductor plays an important role in device performance. Therefore, a lamination technique was applied to device fabrication in order to improve charge carrier injection from metal electrodes into organic layer to avoid damage of organic semiconductor layer during vacuum deposition of metal electrodes. N-type OTFTs with higher mobility was made. Thirdly, we investigated device performance of both the top and bottom surface of a same organic film under various environmental conditions. We found that mobilities associated with the top surface of semiconductor is very close to that of the smooth bottom surface. 2D-Grazing Incidence X-Ray Diffraction (2D GIXD) study also provides useful information about the degree of crystallinity and molecular orientation through various thickness of the film. Correlation between molecular packing and charge carrier mobility was discussed.1) Kelley, T.; Boardman, L. D.; Dunbar, T. D.; Muyres, D. V.; Pellerite, M. J.; Smith, T. P. J. Phys. Chem. B 2003, 107, 58772) Dimitrakopoulos, C. D.; Malefant, P. R. L. Adv. Mater. 2002, 14, 99. 3) Katz, H. E.; Bao, Z. J. Phys. Chem. B 2000, 104, 671.
9:00 PM - M10.2
Ambipolar Charge Transport in Organic Field-Effect Transistors Based on Low Band Gap Semiconductors.
Thomas Anthopoulos 1 , Sepas Setayesh 1 , Eugenio Cantatore 1 , Edsger Smits 2 1 , Paul Blom 2 , Dago de Leeuw 1
1 Philips Research Laboratories Eindhoven, Philips, Eindhoven Netherlands, 2 Molecular Electronics, Materials Science Centre Plus , University of Groningen, Groningen Netherlands
Show AbstractAmbipolar charge transport (i.e. transport of holes and electrons) is a generic property of many small molecules organics and polymeric semiconductors [1]-[3]. Recently such materials have attracted considerable attention due to their potential utilization in complementary-like digital circuits [1] [4] [5]. Unfortunately, single ambipolar organic materials are air sensitive and devices based on them degrade rapidly upon exposure to ambient air [2] [3] [5]. Here we report on air and light stable organic ambipolar transistors based on organic near infrared absorbing dyes. Due to their low energy gap devices based on these materials exhibit nearly ideal ambipolar transport characteristics even when gold source and drain electrodes are employed. Device operation is successfully described using a newly developed ambipolar transport model based on the variable-range-hopping theory [6]. To demonstrate the suitability of our ambipolar organic transistors for practical applications we demonstrate integrated complementary-like logic circuits with excellent environmental stability and very high yield. REFERENCES [1]E. J. Meijer, et al. Nat. Mater. 2, 678 (2003). [2] T. D. Anthopoulos, et al. Adv. Mater. 16, 2174 (2004). [3] L.-L. Chua, et al. Nature 434, 194 (2005). [4] T. D. Anthopoulos, et al. Appl. Phys. Lett. 85, 4205 (2004). [5] T. D. Anthopoulos, et al. J. Appl. Phys. 98, 054503 (2005). [6] E. Smits, et al. (unpublished).
9:00 PM - M10.20
High Performance Organic Thin Film Transistors with Transition-metal-oxide/metal Stacking Electrodes.
Sheng-han Li 1 , Chih-wei Chu 1 , Chieh-wei Chen 2 , Vishal Shrotriya 1 , Yang Yang 1
1 Department of Materials and Engineering, University of California Los Angeles, Los Angeles, California, United States, 2 Graduate Institute of Electro-optical Engineering, National Taiwan University, Taipei Taiwan
Show AbstractWe demonstrate stacking source-drain (S-D) electrodes for organic thin film transistors (OTFT). This bilayer structure consists of a transition metal oxide (MoO3, WO3, or V2O5) layer and a metal layer. The transition metal oxide layer, directly contacting the organic semiconductor, serves as a charge-injection layer. The upper-coated metal layer is responsible for the conduction of charge carriers. We found that the metal oxide layer coupled between pentacene and metal layers played an considerable role in improving the field-effect transistor characteristics of OTFTs. High field-effect mobility of 0.4 cm2/V-s and on/off current ratios of 104 were obtained in the pentacene based TFTs using the stacking S-D electrodes at a gate bias of -40 V. There are two possible reasons for this significant improvement. One reason is the reduction of the injection barrier at the contact interface; the other is the prevention of metal diffusion into the organic layer and/or unfavorable chemical reaction between the organic layer and the metal electrode. Therefore, using this stacking electrode is an effective way to obtain high performance of OTFTs, making the device suitable for commercial products.
9:00 PM - M10.21
All-Laser Patterned Polymer MISFET
Francisco Gil-Martinez 1 , David Taylor 1 , Nadeem Rizvi 1 , Simon Ogier 2
1 School of Informatics, University of Wales, , Bangor United Kingdom, 2 LC Manchester Technical Centre, Merck Chemicals Ltd, Manchester M9 8ZS United Kingdom
Show AbstractLaser micromachining of high resolution features offers an interesting alternative approach to the standard optical photolithographic patterning of polymeric devices. Laser machining uses no chemicals, is less time consuming and is suitable for mass production. Excimer lasers operating in the UV are particularly well-suited to the machining and the photobleaching of polymers and are preferred over other types of laser. In this contribution we report the all-laser patterning of polymer MISFETs combined with standard spin-coating and evaporation processes. Good definition and micron-scale resolution are achieved in the patterning of source and drain electrodes formed from both metals and conducting polymers. In the case of conducting polymers such as PANI or PEDOT:PSS the results are better than those achieved using the lift-off technique. Furthermore, parasitic lateral currents are avoided since the precise patterning of the semiconductor confines current flow to the channel. Using the laser for gate alignment with the channel minimises significantly the overlap with the source and drain electrodes. In a comparison with photolithography-produced transistors, improvements are observed in the transistor characteristics.
9:00 PM - M10.22
Electrical Characteristics of Pentecene Organic Thin Film Transistor with Various Electroplated Metal Gate Electrodes on Polyimide.
J. G. Lee 1 , Y. G. Seol 1 , N.-E. Lee 1
1 School of Advanced Materials Science, University, Suwon, Kyunggi-do, Korea (the Republic of)
Show Abstract9:00 PM - M10.23
A Polymer Flash Memory Device
David Taylor 1 , Stephen William 1 , Paul Sayers 1
1 School of Informatics, University of Wales, Bangor United Kingdom
Show Abstract9:00 PM - M10.24
Hopping Transport and Voltage Induced Metal-Insulator Transition in High Mobility Polythiophene Field-Effect Transistors.
Anoop Dhoot 1 2 , Guangming Wang 1 2 , Dan Moses 1 2 , Alan Heeger 1 2
1 Center for Polymers and Organic Solids, University of California, Santa Barbara, California, United States, 2 Mitsubishi Chemical Center for Advanced Materials, University of California, Santa Barbara, California, United States
Show AbstractBoth temperature and carrier density influence the carrier transport in polymer field-effect transistors (FETs). We have studied the carrier transport in regio-regular poly(3-hexylthiophene) FETs by careful four-probe measurement of the steady-state channel conductance between room temperature and 4.2 K. At high gate voltages, we find a strong variation in conductance with source-drain voltage that becomes weakly temperature dependent at the lowest temperatures. The field- and temperature-dependent behaviour of the transport in these field-effect structures can be well described by carrier hopping with Coulomb correlations. At high gate voltage (constant carrier density) and at low temperatures, Zabrodskii plots (dlnρ/dlnT) demonstrate that the source-drain voltage induces the insulator-to-metal transition.
9:00 PM - M10.25
Pentacene-based OTFTs with Self-aligned Self-assembled Monolayer.
Cheng-Wei Chou 2 , Hsiao Wen Zan 1 , Kuo-Hsi Yen 2 , Yu-Min Chiou 3 , Ting-Yuan Tu 1
2 National Chiao Tung University, Institute of Electro-Optical Engineering, HsinChu Taiwan, 1 National Chiao Tung University , Department of Photonics and Display Institute, HsinChu Taiwan, 3 National Chiao Tung University, Degree Program of Electrical Engineering and Computer Science, HsinChu Taiwan
Show Abstract9:00 PM - M10.26
Application of AlN Film for Organic Thin Film Transistors
KuoHsi Yen 1 , Hsiao-Wen Zan 2 1 , Pu-kuan Liu 1 , Chien-Hsun Chen 3 , Jennchang Hwang 3
1 The Institute of Electro-optical Engineering, NATIONAL CHIAO TUNG UNIVERSITY (NCTU), Hsinchu Taiwan, 2 Display Institute, NATIONAL CHIAO TUNG UNIVERSITY (NCTU), Hsinchu Taiwan, 3 Department of Materials Science and Engineering, National Tsing Hua University (NTHU), Hsinchu Taiwan
Show AbstractWe used the AlN film as a new application for pentacene-based organic thin-film transistors (OTFTs). We fabricated OTFTs with low temperature AlN film as the gate insulator. In our experiment, the AlN film was deposited in the RF-ICP system on Si(100) substrates in a radio frequency inductively coupled plasma (RF/ICP) system. In the RF/ICP system, a 2-inch Al target (purity 99.999%) of the RF gun was immerged in a 4-inch inductive coil which can be controlled by an independent RF coil power. The system was firstly pumped down to a base pressure less than 2×10-6 torr. Then a mixed gas of nitrogen and argon was introduced into the chamber through the mass flow controllers at a N2/(N2+Ar) ratio of 75 %. AlN films were deposited at the pressure of 2.5 mTorr and the substrate temperature of 150 degrr C. The RF gun power varied from 50 to 200W. With the radio frequency-induced couple plasma sputtering (RF-ICP), the low temperature AlN film was grown on Si wafer with roughness about 0.5 (nm). Then the bottom contact pentacene-based organic TFT with bi-layer metals (Pd/Ti) as source and drain electrodes were fabricated. The highest on and off current ratio is more than 6 and filed effect mobility about 1×10-3 (cm2/V-s) can be achieved. In Addition to the electrical properties, we also observe the surface morphology of the pentacene film on the AlN. The growth characteristics and dynamics of pentacene film had been widely studied on either SiO2 or other metal oxides, however, there is no related researches about the pentacene film on the 3-5 compounds. The pentacene with grains size of 200 (nm) were distributed uniformly on the AlN. The grains size were smaller than that on the SiO2. In the early stage (sub-monolayer) of the pentacene growth, the nucleation density on AlN was smaller than that on SiO2. The film coverage phenomenon is also different on the two substrates. In order to enhance the pentacen film properties, we use the spin on polymer to improve the AlN surface. The roughness of the AlN film significantly reduced to 0.27nm. The device output characteristic was also improved. All the relative researches are currently under our studying. Better device characteristics can be achieved with further study on suitable surface treatment
9:00 PM - M10.27
Organic Thin Film Transistor with High k Gate Insulators Formed by Plasma Enhanced Atomic Layer Deposition.
Jin-Seong Park 1 , Taeho Kim 2 , Pan-kwi Park 3 , Min Chul Suh 1 , Chung Kun Song 2 , Yeon-Gon Mo 1 , Ho-Kyoon Chung 1
1 CRD center, Samsung SDI, Yongin-si, Kyunggi-do, Korea (the Republic of), 2 School of Electronics & computer Engineering, Dong-A University, Busan Korea (the Republic of), 3 department of Materials science and engineering, KAIST, Daejeon Korea (the Republic of)
Show AbstractThe high k gate inorganic insulator films (Al2O3, HfO2 etc) deposited by plasma enhanced atomic layer deposition (PEALD) have been adopted as a gate insulator for the organic thin film transistor (OTFT) fabricated with pentacene. All process temperature is low (below 200oC) enough to build OTFTs on flexible substrate. The thin gate insulators (below 100nm) on silicon wafer and poly-Arylate (PAR) substrate are formed by PEALD to improve device properties (operation voltage and sub-threshold swing etc). The organic transistors show saturation curves at low driving voltage (~ 5V), good sub-threshold voltage swing (~0.2V/dec) and Ion/off ratio (> 10^4) despite of no surface treatment on gate insulator.
9:00 PM - M10.28
The Effect of 3-mercapto-5-nitro-benzimidazole (MNB) and Poly (methyl methacrylate) (PMMA) Treatment Sequence on a Bottom Contact Organic Thin Film Transistor.
Jin-Seong Park 1 , Min Chul Suh 1 , Taek Ahn 1 , Jonghan Jeong 1 , Suyoung Kim 1 , Yeon-Gon Mo 1 , Ho-Kyoon Chung 1
1 CRD center, Samsung SDI, Yongin-si Korea (the Republic of)
Show Abstract9:00 PM - M10.29
A WORM-Type Memory Device with Rectifying Effect Based on a Conjugated Copolymer of PF6Eu on Si Substrate.
Yoke Tan 1 , Q.D. Ling 2 , E.Y.H. Teo 1 , Y. Song 1 , S.L. Lim 2 , P.G.Q. Lo 3 , E.T. Kang 2 , D.S.H. Chan 1 , C.X. Zhu 1
1 Electrical and Computer Engineering, National University of Singapore, Singapore Singapore, 2 Chemical and Biomolecular Engineering, National University of Singapore, Singapore Singapore, 3 , Institute of Microelectronics, Singapore Singapore
Show Abstract9:00 PM - M10.30
Oxidation Stability of Pentacene Thin Films for OTFT application
Hoichang Yang 1 , Tae Joo Shin 2 , Mang-Mang Ling 3 , Zhenan Bao 3
1 Nanocenter, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 National Synchrotron Light Source, Brookhaven National Laboratory, Upton, New York, United States, 3 Chemical Engineering, Stanford University, Stanford, California, United States
Show AbstractOrganic thin film transistors (OTFT) have been considered to be useful in liquid crystal display, organic light-emitting diodes, smart cards, gas sensors, and other fields. Evaporated pentacene thin films in OTFTs have been reported to show great performance with charge mobility (~ 3 cm2/Vs), and on/off ratio (> 106). Effects of crystalline morphology and structure in pentacene films on charge mobility in OTFTs have been studied through controlling dielectric surfaces and deposition conditions. Crystalline orientation and grain morphology of the first pentacene monolayer strongly depend on pentacene-substrate interactions, which can be controlled by modifying the dielectric surface with a self-assembled monolayer (SAM). Recently, we found that there is a direct correlation between the crystalline morphology of the initial sub-monolayer (sub-ML) of a pentacene film and the mobility of the corresponding thick film. Specifically, ultrathin pentacene films (less than 2 ML < 3.2 nm) on silicon oxide or other inert substrates showed a pseudo-centered rectangular structure with a herringbone packing and molecules tilted along the b-axis by ~ 4o with edge-on, referred as the “thin film phase”, different from “bulk phase” with a triclinic unit cell structure. Most works on OTFTs have been focused on improvement of the charge mobility. However, the stability of OTFTs is another challenging issue which should be kept in mind. It has been reported that humidity and light can affect the charge mobility properties of pentacene thin films in many different ways. Qiu et al. reported that the charge mobility of pentacene thin films in OTFT devices was decreased by 30 % after storing them in atmosphere for 500 h. Maliakal et al. found a drastic decrease (~ 80 %) in charge mobility of 60 nm thick pentacene films after UV irradiation in a presence of air for 20 min. However, correlation between the charge mobility and the change of crystalline structure in pentacene films in an atmospheric condition has not been much studied. Here, we report H2O and UV oxidation effects on crystalline structure and charge-carrier ability of pentacene thin films exposed in an atmospheric condition using two-dimensional grazing-incidence X-ray diffraction (2D GIXD), atomic force microscopy (AFM), and TFT measurement. Films were prepared by vacuum-depositing pentacene at a rate of 0.5 Å/s and 25 oC on SiO2/Si substrates, which had been pretreated with O2-plasma and different types of SAMs. The mobility of top contact OTFTs was measured using 60-nm-thick pentacene films. 2D GIXD and AFM were employed to observe the crystalline morphology and structure of multilayered pentacene films with different thicknesses, ranging from sub-ML to 60 nm.
9:00 PM - M10.31
PAN-based Gate Dielectric for OTFTs
Hui-Lin Hsu 1 , Ya-Lien Li 1 , Wei-Chang Yang 1 , Zing-Way Pei 2 , Po-Yuan Lo 2 , Tri-Rung Yew 1
1 Materials Science and Engineering, National Tsing-Hua University, Hsinchu Taiwan, 2 Electronic Research and Service Organization, Industrial Technology Research Institute, Hsinchu Taiwan
Show AbstractPAN (Poly AcryloNitrile)-based, a soluble polymer, was investigated for the application in organic thin film transistors (OTFTs) as a gate dielectric material. Superior low leakage current of 0.7 pA at 10 V for 50nm PAN-based dielectric material (e.g., current density = 0.1n A/cm2), which is comparable to the value of 1 pA at 10 V for that of 100nm thermal oxide (e.g., current density = 0.4n A/cm2), has been demonstrated by the I-V characteristics measured from the MOS (metal-oxide-semiconductor) structure. In additions, electrical characteristics of PAN-based as the dielectrics for OTFTs has been demonstrated including carrier mobility, on/off current ratio, and threshold voltage. Furthermore, results show that the process of PAN-based dielectric materials was compatible with all soluble organic electrode such as PEDOT:PSS-based (Poly(3,4-EthyleneDiOxyThiophene): Poly(StyreneSulfonate))-based materials and organic semiconductor like (Poly(3-HexylThiophene)) P3HT materials on flexible Polyimide substrate from device fabrications.
9:00 PM - M10.32
Pentacene Thin-film Transistors Fabricated with a Simple Solution Process.
Yutaka Natsume 1 2 , Takashi Minakata 1 2
1 , Asahi-Kasei corporation, Fuji, Shizuoka, Japan, 2 , Optoelectronic Industry and Technology Development Association, Tsukuba, Ibaraki, Japan
Show AbstractWe have developed the direct formation method for pentacene thin-films with solution process. This method doesn’t need particular precursor molecules. The solution-processed pentacene thin-films had a molecular orientation similar to that of single crystal and high crystallinity determined by in-plane X-ray diffraction analysis. The surface morphology of the solution-processed thin-films observed with atomic force microscopy showed flat plate-like structures different from vacuum-deposited pentacene thin-films. Field effect transistors (FETs) fabricated with solution process exhibited good performances with the hole mobility of 0.8 cm2/Vs and the on/off ratio of exceeding 105. The threshold voltages of transfer characteristics ranged from 0 to 10V where a steep subthreshold swing was also confirmed. Furthermore we estimated the trap density in the channel from the gap between threshold voltage and turn-on voltage based on the analytical model proposed by G. Horowitz and P. Delannoy. They described the current-voltage characteristics as the equilibrium between trapped and free carriers. It revealed that the solution-processed thin-films contained the trap density in the order of 1017cm-3, which was one or two orders lower than that of vacuum-deposited pentacene thin-films. Therefore, we concluded that this relatively low trap density of the solution-processed thin film corresponded to its high crystallinity.We have studied the stability of the FET characteristics accompanied with storage in air for several months. The transfer characteristics for solution-processed thin-film FETs didn’t change so large during storage for 77 days, i.e. the mobility was almost same and the threshold voltage shift was within 5V. In contrast, the vacuum-deposited thin-film FETs indicated slightly decreasing of the mobility and large threshold voltage shift of 10 to 20V during storage for 44days. This large threshold voltage shift might be attributed to its granular surface morphology that enables oxygen and moisture to react with pentacene molecules easily. Further studies on influence of impurities in the solution-processed films to the FET characteristics and preparation conditions to grow high quality thin films will be presented.
9:00 PM - M10.33
Unidirectionally Alligned Conducting Polymers For Charge Storage Devices.
Gursel Sonmez 1 , Muge Acik 1
1 , Sabanci University, Istanbul Turkey
Show Abstract9:00 PM - M10.34
Copper Phthalocyanine Single-crystal Field-effect Transistors Stable Above 100°C.
Koichi Yamada 1 , Jun Takeya 1 2 3 , Kunji Shigeto 2 4 , Kazuhito Tsukagoshi 2 4 , Yoshinobu Aoyagi 2 5 , Yoshihiro Iwasa 3 6
1 , CRIEPI, Tokyo Japan, 2 , RIKEN, Wako Japan, 3 , IMR, Tohoku University, Sendai Japan, 4 , PRESTO, Kawaguchi Japan, 5 , Tokyo Institute of Technology, Tokyo Japan, 6 , CREST, Kawaguchi Japan
Show AbstractOrganic field effect transistors (OFETs) have been receiving considerable attention for decades due to their simple fabrication processes. Though a wide variety of OFETs have been studied so far, one of the remaining subjects would be their stability at high temperatures; since devices with lower mobility suffer from larger Joule heating in principle, it is desirable even for room-temperature use to guarantee higher-temperature tolerance. Heat resistance of organic materials, however, is rather poor in general, so that performance of the OFETs may be chemically unstable at high temperatures. Though Sekitani et al. showed that pentacene thin-film transistors could operate above 100°C in inert atmosphere [1], the high-temperature stability has been rarely reported for other OFETs.Among various semiconductor materials reported for OFETs, copper phthalocyanine (CuPc) holds excellent chemical stabilities up to 500°C, as well as high FET mobility [2,3]. In this presentation, we report field effect of the CuPc single-crystal transistors at various temperatures from 175 K to 430 K. The results demonstrate the stable operation even at 430 K with typically twice as large mobility as that at room temperature.In order to avoid extrinsic influences of grain boundaries and parasitic contacts, and to extract the intrinsic material properties, we prepared single-crystal devices and employed the four-probe method for the conductivity measurements. The CuPc single crystals are grown by horizontal physical vapor transport; the crystals are needle-shaped as they grow much more rapidly to the direction of the one-dimensional stack of the CuPc molecules. The FET structure is composed along the long direction by depositing a parylene gate dielectric film [4]. The room-temperature values of hole mobility range between 0.1 and 0.6 cm2V–1s–1 for our devices.As temperature is varied from 175 K to room temperature, the mobility goes up monotonically with activation-like temperature dependence, indicating hopping-type transport along the direction of the molecular stacks. The activation energy is estimated to be ~ 55 meV from the slope of log(μ) – 1/T plot. Experiments up to 430 K shows that the tendency extends at higher temperatures; μ is enhanced by factor of two when we warm up some of our average samples of room-temperature mobility ~ 0.20 cm2V–1s–1. When the samples are cooled down to room temperature, the original value of m is recovered, indicating the stability of the device performance up to at least ~ 430 K. The result, combined with the high-mobility operation above room temperature, demonstrate potential advantage of the CuPc FETs in terms of high-temperature tolerance. [1] T. Sekitani et al., Appl. Phys. Lett. 85, 3902 (2004).[2] R. Zeis et al., Appl. Phys. Lett. 86, 022103 (2005).[3] R. W. I. de Boer et al., Appl. Phys. Lett. 86, 262109 (2005).[4] V. Podzorov et al., Appl. Phys. Lett. 82, 1739 (2003).
9:00 PM - M10.35
Hall Effect Measurements in Organic Single-Crystal FETs
Vitaly Podzorov 1 , Etienne Menard 2 3 , John Rogers 2 , Michael Gershenson 1
1 Physics Department, Rutgers University, Piscataway, New Jersey, United States, 2 Department of Materials Science and Engineering, University of Illinois, Urbana Champaign, Illinois, United States, 3 LCR Semi-Conducteurs organiques, CEA-Saclay, Gif-sur-Yvette France
Show Abstract We have observed classical Hall effect in the single-crystal rubrene OFETs [1]. The mobility determined from the Hall measurements (mu_H) represents intrinsic, i.e. trap independent mobility of the charge carriers. At high temperatures, Hall mobility coincides with the longitudinal mobility determined from the standard FET measurements.In the investigated temperature range T = 170-300 K, mu_H monotonically increases with decreasing T, while the longitudinal mobility first increases at high T (intrinsic regime) and then decreases at low T (non-intrinsic regime), consistent with the previous observations [2, 3]. In the intrinsic regime, the density of mobile field-induced charge carriers extracted from the Hall measurements, n_H, coincides with the density n calculated using the gate-channel capacitance, and becomes smaller than n in the trap-dominated regime. The Hall data strongly support a band-like nature of the charge carrier transport in this system.1. To appear in Phys. Rev. Lett; available at http://arxiv.org/abs/cond-mat/0508006 2. V. C. Sundar, et al., Science 303, 1644 (2004); 3. V. Podzorov et al., Phys. Rev. Lett. 93, 086602 (2004);
9:00 PM - M10.37
A Study of Sandwiched CuPc/CoPc Heterostructure OTFTs.
Jianbin Xu 1 , Jianbin XU 1 , Jia Gao 1 , Ming ZHU 1
1 Department of Electronic Engineering and Materials Science and Technology Research Center, The Chinese University of Hong Kong, Hong Kong China
Show AbstractSandwiched CuPc/CoPc heterostructure OTFTs were found to have a surprisingly high mobility of 0.11 cm2/Vs, much higher than the mobility of 0.04 cm2/Vs for single-layer CuPc OTFTs, and 0.01 cm2/Vs for CoPc OTFTs [1]. However, the mechanism behind this phenomenon remains elusive. In this work, we report on characterizations of CuPc/CoPc heterostructure by capacitance-voltage spectroscopy (C-V) and ultraviolet photoemission spectroscopy (UPS). C-V measurement shows that there is a thin layer of charges at the interface of the CuPc/CoPc. UPS results confirm that CuPc and CoPc have different HOMO positions so that charges can accumulate at the interface of CuPc/CoPc. It is also found that the mobility of the CuPc/CoPc heterosture OTFTs can be influenced by the thickness of the CuPc layer or the CoPc layer, especially the CuPc layer. Based on these experimental results, we propose a model to analyze how the interface charges contribute to the high mobility of sandwiched CuPc/CoPc heterostructure OTFTs. [1] J. Zhang, J. Wang, H. B. Wang and D. H. Yan, Appl. Phys. Lett., 84, 142 (2004).
9:00 PM - M10.38
The Effects of Surface Characteristics of Polymer Gate Dielectrics of Pentacene Transistors Using O2 Plasma Treatment.
Kwonwoo Shin 1 , Chanwoo Yang 1 , Sang Yang 1 , Hayoung Jeon 1 , Chan Park 1
1 Chemical engineering, Pohang university of science and technology, Pohang, Gyungbuk, Korea (the Republic of)
Show Abstract The effects of surface characteristics of polymer gate dielectrics on pentacene field effect transistors were investigated with O2 plasma treatment and aging behaviors of plasma treated gate dielectrics. The O2 plasma treatment introduced polar functional groups, such as –OH, -COOH, and increased gate dielectric roughness of the poly (methylmethacrylate) gate dielectrics. The amount of polar groups and surface roughness of gate dielectrics were controlled by modulating plasma treating condition. And, the aging behavior that the introduced polar functional groups diffuse into gate dielectric layer from its surface could show the effect of polar functional groups of gate dielectric surface.The mobility measurement and atomic force microscopy showed the plasma treated gate dielectrics induced lower carrier mobility of transistors and small grain size of pentacene active layer. And, temperature dependent mobility measurement revealed the plasma treated gate dielectrics resulted in higher activation energy for carrier transport and decreased the intrinsic carrier mobility. Two weeks of aging time restored the chemical state of plasma treated gate dielectrics to almost their original values. Atomic force microscopy showed the surface morphologies of gate dielectrics was not change during aging process. The pentacene transistors with aged gate dielectrics showed that the carrier mobility decreased with increasing plasma treating time. However, if comparing with transistors with unaged gate dielectrics, they showed much increased carrier mobility. The temperature dependent mobility measurement did not show the increase of activation energy with plasma treating time. These results revealed the trap states of pentacene transistors are related with polar functional groups rather than surface roughness. The X-ray diffraction analysis showed the more plasma treating time resulted in decreasing the ordering of crystal structure of pentacene both in-plane and out of plane direction, and decreased the domain size of pentacene. However, the aging behaviors did not improve the ordering of pentacene. These results mean the etched surface by plasma disturbs the ordering of pentacene rather than polar functional groups do.
9:00 PM - M10.39
Top-contact Pentacene Thin Film Transistors with Submicron-length Channel.
Kunji Shigeto 1 , Iwao Yagi 1 , Kazuhito Tsukagoshi 1 2 , Yoshinobu Aoyagi 1
1 , RIKEN, Wako, Saitama, Japan, 2 PRESTO, JST, Kawaguchi, Saitama, Japan
Show AbstractWe have realized alignment-free top-contact (TC) formation for an organic thin film transistor (OTFT) with submicron-length channel [1]. We drastically improved the operation of the short channel TC-OTFT by controlling both the interface between the channel and the gate insulator, and the interface between the channel and the contact.The fabrication method of the OTFT requires a prefabricated insulating structure. The structure has two cavities with undercut edges, and they are connecting under a suspended bridge. Oblique evaporation of organic semiconductor (Pentacene) and normal evaporation of metal electrodes (Au) onto the prefabricated insulating structure are continuously performed in vacuum. The area under the suspended bridge is the channel of the TC-OTFT. In this experiment, the prefabricated structures were prepared by electron beam lithography.Although the TC-OTFT with short channel length of 0.4 µm showed typical output and transfer characteristics, the OTFT without any treatment showed low field-effect mobility of 0.029 cm2/Vs and unstable operation for gate voltage cycles. We succeeded in stable operation by covering the prefabricated insulating structure with a thin poly-para-xylylene (diX-C) layer of 10 nm. And the mobility was improved to 0.1 cm2/Vs by doping the inside of the pentacene layer under the contact area with TCNQ. The channel length (L) was changed from 0.18 µm to 0.41 µm. The field-effect mobility decreased when the channel length became shorter than 0.3 µm. This originates in the fact that off-current exponentially increases with the decrease of L below 0.3 µm while on-current is almost constant.References1) I. Yagi, K. Shigeto, K. Tsukagoshi, and Y. Aoyagi, Jpn. J. Appl. Phys. 44 (2005) L479.
9:00 PM - M10.4
Organic Semiconductor Diodes as Magnetic Field Sensors.
Govindarajan Veeraraghavan 1 , Omer Mermer 2 , Thomas Francis 3 , Yugang Sheng 2 , Tho Nguyen 2 , Markus Wohlgenannt 2
1 Department of Electrical and Computer Engineering, University of Iowa, Iowa city, Iowa, United States, 2 Department of Physics and Astronomy, University of Iowa, Iowa city, Iowa, United States, 3 , OMR Sensors Inc, Dubuque, Iowa, United States
Show AbstractOrganic semiconductors are being researched upon since the past decade for use in making devices like organic light emitting diodes (OLEDs), transistors and photovoltaic devices. Our recent discovery of large magnetoresistance (MR) effect at room temperature, in organic pi-conjugated semiconductors, has indicated a huge potential for these materials in use as magnetic sensors. The effect was dubbed as organic magnetoresistance effect (OMAR) and is observed in both macromolecular polymers and small molecular devices. The effect is as high as 10% for small magnetic fields of B = 10 mT at room temperature. This MR effect is therefore amongst the largest of any bulk material. The magnetoresistive devices have a sandwich structure similar to an OLED device. A thin layer of polymer or small molecule is deposited between two non-magnetic electrodes. The most surprising feature is the presence of such a large MR effect in devices made with non-magnetic electrodes. To the best of our knowledge, the discovered MR effect is not adequately described by any of the mechanisms known to date.We characterize the effect in polymer and small molecular sandwich devices. The dependence of the OMAR effect on voltage, polymer or small molecule film thickness, temperature, electrode materials, and (unintentional) impurity concentration is discussed. The device properties and preliminary results regarding timing response of the devices are discussed. OMAR devices made from polyfluorene (poly (9,9-dioctylfluorenyl-2,7-diyl)) polymer and small molecule tris-(8-hydroxyquinoline) aluminum (Alq3) show the most promise among the materials we tested. We show the functional dependence of OMAR on the magnetic field among different materials and show that it is approximately universal in all the materials we studied.The OMAR devices can be manufactured cheaply on flexible substrates, and can be transparent. Our devices therefore hold promise for applications where large numbers of MR devices are required, such as magnetic random-access-memory (MRAM) and applications related to OLED display screens such as touch screens where the position of a magnetic stylus is detected (patent pending). Our devices do not require ferromagnetic electrode materials resulting in flexibility in material choice not available for other MR devices. Interested readers should further look at our publications and a demonstration video at our website http://ostc.physics.uiowa.edu/~wrg/.
9:00 PM - M10.40
Visualization of Charge Injection in an Organic Thin-film Transistor using Kelvin Probe Force Microscopy.
Susumu Ikeda 1 , Toshihiro Shimada 2 , Manabu Kiguchi 3 , Koichiro Saiki 1 2
1 Department of Complexity Science and Engineering, Graduate School of Frontier Sciences, The University of Tokyo, Kashiwa Japan, 2 Department of Chemistry, Graduate School of Science, The University of Tokyo, Tokyo Japan, 3 Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo Japan
Show AbstractIn order to investigate the working mechanism of organic thin-film transistors (OTFTs), in particular, charge injection mechanism at the semiconductor/insulator interface, we carried out cross-sectional potential imaging of a working OTFT using Kelvin probe force microscopy (KFM). The measured sample was a Cu(II)-phthalocyanine (CuPc) TFT. The device had a bottom-contact structure consisting of a highly doped n-Si(100) substrate with a thermally oxidized film (300 nm in thickness), Au/Cr source and drain electrodes (30 nm), and a CuPc film (400 nm). The source and drain electrodes with channel length of 5 μm were prepared by photolithography and a CuPc film was deposited by molecular beam deposition under a pressure of 8×10-7 Pa. The CuPc-TFT thus prepared was cleaved for cross-sectional KFM study in air. In the cross-sectional potential maps, various changes were recognized depending on the gate voltage (VG) and drain voltage (VD). In the potential map of VD=-2 V and VG=0 V, the negative potential region appeared only near the drain electrode in the CuPc film. In the map of VD=-2 V and VG=-5 V, that is, when the negative voltage was applied to the gate, the negative potential region around the drain electrode extended to the source side. When the positive voltage (+5 V) was applied to the gate, on the other hand, the negative potential region near the drain electrode was suppressed and did not extend to the source direction. These changes of potential distribution in the organic film are consistent with the previous reports on the surface potential distribution of working OTFTs (not cleaved) measured by KFM and contact-mode AFM potentiometry. The most intriguing feature by our cross-sectional imaging was recognized at the CuPc/SiO2 interface when the negative voltage was applied to the gate. In the diagrams of VG=-5 V, a potential hill appeared along the CuPc/SiO2 interface, though no feature was seen when VG=0 V. Under the condition of VD=0 V and VG=-5 V, the size of the hill was almost the same near the source and drain electrodes, while the size of the hill near the drain electrode decreased by applying negative voltage to the drain (-2 V). Assuming that the size of the hill corresponds to the induced charge, the decrease of the size from source to drain probably resulted from the decrease of effective potential (-5 V → -3 V), because the induced charge at the interface is proportional to C(VG-V), where V is a local potential at the interface. Therefore, the appearance of the potential hill at CuPc/SiO2 interface strongly suggests the direct observation of the charge injection by field effect. This first success of visualization of the charge injection provides much information with respect to the working mechanism of OTFTs. We are now trying the cross-sectional investigation using other conjugated organic materials.
9:00 PM - M10.41
Polarity Control in Organic Single-crystal Field-effect Transistors.
Taishi Takenobu 1 2 , Tetsuo Takahashi 1 , Jun Takeya 1 3 4 , Yoshihiro Iwasa 1 2
1 IMR, Tohoku University, Sendai Japan, 2 CREST, JST, Kawaguchi Japan, 3 , CRIEPI, Komae Japan, 4 , RIKEN, Wako Japan
Show AbstractOrganic semiconductors have been a subject of active research for over a decade now, with applications emerging in light-emitting displays and printable electronic circuits. Despite a substantial body of work aimed at understanding charge transport in these materials, a well-developed, microscopic description is still lacking. The study of organic single crystal is an important approach, because grain boundaries are eliminated and the concentration of charge traps is minimized. A particularly promising material is rubrene (a tetraphenyl derivative of tetracene, C42H28), for which the reported field-effect mobility of single crystals reaches 20 cm2/Vs.[1]Recently, ambipolar organic field-effect transistors (OFETs) are attracting increasing attention for their possible use in complementary-like inverters. In contrast to film OFETs, in which ambipolar device characteristics have been observed, unipolar transport is typically observed in single-crystal OFETs, i.e., one type of charge carrier is transported preferably so that the transistor operates as either a p- or n-channel device. Very recently, Chua et al. reported n-channel FET conduction in various conjugated polymers using a hydroxyl-free gate dielectric.[2] They reported that n-type behaviors have previously been elusive due to the trapping of electrons at the semiconductor-dielectric interface by hydroxyl groups, which are present in the form of silanols in the case of the commonly used SiO2 dielectric. Upon careful consideration, we therefore attempted to apply a hydroxyl-free gate dielectric to rubrene single crystals, using various metal electrodes.We used a bottom-gate/top-contact FET device configuration, in which polymethylmethacrylate (PMMA) provides the buffer gate dielectric interface to the rubrene single crystals. On these substrates, thin rubrene crystals with a thickness of approximately 1 μm were laminated using a technique described in Takeya et al.[3] Finally, source and drain contacts were fabricated by thermally evaporated gold, magnesium and painted silver paste, which have various work functions. We observed ambipolar behavior for all devices by using a PMMA buffer gate dielectric, except for gold electrodes. In addition, we observed a similar ambipolar behavior in rubrene OFETs using an anther type of hydroxyl-free gate dielectric, a derivative of divinyltetramethylsioxane-bis(benzocyclobutene),[2] even though rubrene OFETs without the buffer layer are always unipolar transistors. Crucial to ambipolar transistors based on the rubrene single crystals is the buffer gate dielectric that reduces the electron trap densities. Optimization of processing conditions and matching with the metal work function may lead to increases in the mobilities.References:[1] E. Menard et al.,Adv. Mater. 16, 2097 (2004).[2] L.-L Chua et al.,Nature 434, 194 (2005).[3] J. Takeya et al.,J. Appl. Phys. 94, 5800 (2003); J. Takeya et al.,Appl. Phys. Lett. 85, 5078 (2004).
9:00 PM - M10.42
Fabrication of Organic Thin Film Transistor Based on the Ink-Jet Printed Electrodes of Nano Silver Particles
Dongjo Kim 1 , Sunho Jeong 1 , Sul Lee 1 , Bong-Kyun Park 1 , Jooho Moon 1
1 School of Advanced Materials Engineering, Yonsei University, Seoul Korea (the Republic of)
Show AbstractInk-jet printing of functional materials is of interest in a variety of fields including displays, electronics, optics and sensors. Especially, ink-jet printing of metal nano-particles is an attractive alternative to photolithography for direct patterning conductive metal lines owing to low-cost, low-waste, and simple process. In this work, we have developed a conductive ink which contains silver nano-particles from which the source/drain electrodes for organic thin film transistor (TFT) were directly patterned by ink-jet printing. Nano-sized silver particles having 10~20nm diameter was used for the direct metal printing. Silver conductive ink was printed on the heavily doped n-type silicon wafer with 300-nm thick thermal SiO2 layer as a substrate. To achieve a high line resolution and smooth conductive path, the printing conditions such as the inter-drop distance, stage moving velocity, and temperature of pre-heated substrates were optimized. After the heat-treatment at temperature of about 150 ~ 200 oC for 30 min in air, the printed silver electrodes exhibit metal-like appearance and high conductivity. To complete the fabrication of the coplanar-type TFTs, a solution-processable semiconducting materials were deposited by spin-coating between the ink-jet printed electrodes of ~ 50 μm channel width. Interfaces between the semiconducting - either dielectric layer or electrode layer play a critical role to determine the TFT performances. The contact resistances with the semiconducting layer and work function of the printed electrode were measured as a function of surface treatment conditions. Various self-assembled monolayer molecules (SAM) were also utilized to modify the dielectric layer. The output and transfer characterization for the OTFT based on the printed electrodes were obtained and the influence of the surface treatments on the device performance was investigated.
9:00 PM - M10.43
Hall Effect Measurements Using Pentacene Thin-film Transistors on Plastic Films.
Tsuyoshi Sekitani 1 , Yasushi Takamatsu 1 , Shintaro Nakano 1 , Takayasu Sakurai 2 , Takao Someya 1
1 QPEC, Univ. of Tokyo, Bunkyo-ku, Tokyo, Japan, 2 CCR, Univ. of Tokyo, Meguro-ku, Tokyo, Japan
Show AbstractWe have successfully performed Hall measurements in van der Pauw geometry using high-quality polycrystalline pentacene thin-film transistors (TFTs) manufactured on plastic base films with polyimide gate dielectric layers. This is the first demonstration to the best of our knowledge of Hall measurements using organic TFTs manufactured on plastic films, which are important for practical applications, although Hall effects have been reported using field-effect transistors with organic single crystals. We have unambiguously detected a change in the Hall voltages (VH), which linearly increased with the magnetic field (B) and reached VH=450 μV at B=9 T and a gate voltage bias (VGS) of – 60 V. From the evaluations, it was found that the maximum sheet carrier concentration (Ns) was 1.1 × 10^13 cm^-2 and the Hall mobility (μH) was 0.1 cm2/Vs. Furthermore, the field-effect mobility (μFE) strongly depends on Ns, which is controlled by VGS. μFE of this device also depends on the voltage bias and it is typically 1 cm2/Vs at VGS=VDS= –100 V. The on/off ratio is 10^6. It was very difficult to perform Hall effect measurements using organic TFTs due to significant changes in the performance of the TFTs when DC biases are applied. Therefore, we suppressed the degradations in the DC bias stress by a novel post-annealing technique, which enabled us to precisely detect, for the first time, even small changes in VH.High-performance pentacene TFTs were fabricated by a vacuum evaporation process. First, the gate electrode was formed by the thermal evaporation of a 5-nm-Cr and a 50-nm-Au layer through a shadow mask on a 75-μm-thick flexible polyimide film. Then, a 500-nm-thick polyimide gate dielectric layer was prepared by spin coating and a 50-nm-thick pentacene layer was deposited via thermal evaporation. Finally, a 60-nm-thick Au layer was evaporated to form four electrodes. The total size of the sample is 300 × 300 μm2. For reference, pentacene TFTs with a channel length of 100 μm and width of 1 mm were also fabricated. In this case, μFE was 1 cm2/Vs and the on/off current ratio exceeded 10^6.The transistor properties were measured using a precision semiconductor-parameter analyzer, while the Hall effect measurements were performed using a lock-in amplifier system, whose operation frequency and voltage were 1~10 Hz and 5 V, respectively. This allowed the measurement of VH for values less than 100 μV. Magnetic fields up to 9 T were applied using a physical property measurement system in a helium gas environment at 300 K. When VGS of –60 V was applied to the TFTs and the magnetic field was swept from 0 to 9 T, VH linearly increased from 0 to 450 μV. Ns was evaluated to be 1.1 × 10^13 cm^-2. Furthermore, when VGS was changed from –20 to –60 V with a magnetic field of 9 T, VH increased linearly; further, Ns increased linearly from 3.9 × 10^12 to 1.1 × 10^13 cm^-2. The authors thank MEXT IT program, TOKUTEI (15073204) and CREST, JST for financial supports.
9:00 PM - M10.44
Perfluorocyclobutane (Pfcb) Based Crosslinked Polymers For Gate Dielectrics For Organic Thin Film Transistors (Otfts).
Jieun Ghim 1 , Seok-Soon Kim 1 , Bogyu Lim 1 , Doojin Vak 1 , Yong-Young Noh 1 , Tae-Wook Kim 1 , Takhee Lee 1 , Dong-Yu Kim 1
1 Heeger Center for Advanced Materials (HCAM), Dept. of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju Korea (the Republic of)
Show AbstractOver the past 10 years, there have been remarkable advances in the development of organic thin film transistors (OTFTs). The performance of OTFTs is improved via the development of new organic semiconductors or gate dielectrics, and the investigation of charge transport mechanism. To develop high performance gate dielectric materials, the requirements must be satisfied are high dielectric constant, good heat and chemical resistance, pinhole free thin film formability with high breakdown, and compatibility with organic semiconductors in interfacial properties.1 Recently low-k polymers have been studied for gate dielectrics for OTFTs due to low carrier density of state within organic semiconductors by low electronic polarization of insulators, which reduces hysteresis and threshold voltage in the charge transfer characteristrics.2 Perfluorocyclobutane (PFCB) containing polymers possess many advantages such as their ease of processing, and their excellent thermal and mechanical properties.3 Monomers of PFCB poly aryl ethers can be synthesized from di- or tri-hydroxyl aromatic groups in two or three steps and can be polymerized in the neat or in solution without the need for a catalyst or initiator by 2Π + 2Π step-growth cyclopolymerization.3 Thermally crosslinked PFCB dielectrics could be excellent for solution process in OTFTs. In addition, low surface energy and low polarizability of PFCB polymers are good for the morphology of semiconductor polymers and low hysterisis in the I-V characteristics for gate dielectrics, respectively. Hydroxyl-free PFCB polymeric dielectrics would have potential for the application to ambipolar OTFTs as well.4 In this presentation, synthesis and insulating properties of new naphthalene and Si based PFCB aryl ether polymers will be presented and the effect of fluorine containing dielectrics on electronic properties for OTFTs will be discussed.1. J. Veres, O. Simon, G. Lloyd Chem. Mater. 2004, 16, 4543.2. J. Veres, S. D. Ogier, S. W. Leeming, D. C. Cupertino, S. Mohialdin-Khaffaf Adv. Funct. Mater. 2003, 13, 199.3. (a) J. Ghim, H.-S. Shim, B. G. Shin, J.-H. Park, J.-T. Hwang, C. Chun, S.-H. Oh, J.-J. Kim, D.-Y. Kim Macromolecules 2005, 38(20), 8278.(b) J. Ghim, D.-S. Lee, B. G. Shin, D. Vak, D. K. Yi, M.-J. Kim, H.-S. Shim, J.-J. Kim, D.-Y. Kim Macromolecules 2004, 37(15), 5724.4. L.-L. Chua, J. Zaumseil, J.-F. Chang, E. C.-W. Ou, P. K.-H. Ho, H. Sirringhaus, R. H. Friend, Nature 2005, 434(7030), 194.
9:00 PM - M10.46
Thickness Dependent Phenomena in Organic Field Effect Transistors Investigated by in-situ Electrical Measurement.
Koichiro Saiki 1 , Tetsuhiko Miyadera 1 , Manabu Nakayama 1
1 Department of Complexity Science and Engineering, The university of Tokyo, Kashiwa Japan
Show AbstractAlthough organic thin-film transistors (OTFTs) currently attract a great deal of attention as the electronic devices of next generation, the low carrier mobility is a significant problem for their practical use. In order to improve their performance, it is important to clarify the working mechanism of the OTFTs, especially, the carrier injection process by electric fields. We have devised a measurement system, in which the FET characteristics of organic thin films could be investigated during film growth. Organic substances were evaporated onto the substrate and the FET characteristics together with complex impedance were measured in situ as a function of film thickness. This method has several advantages described in the following. First, the intrinsic property of OFET could be measured without suffering from the influence of impurities. Second, the thickness dependence of FET properties could be correctly evaluated. The electrical properties of organic thin films are likely to show discrepancies in the data so that it is rather difficult to distinguish the thickness dependent phenomena from the specimen dependent ones. In situ measurement of transport phenomena during film growth, on the other hand, could extract the effect of film thickness by using the same specimen. Third, effect of gas adsorption could be investigated in a controlled manner. Making use of the above-mentioned merits, we have carried out several types of experiments to get insight into the conduction mechanism of OFETs, the typical examples of which are shown in the following.1.Evaluation of depletion and accumulation thicknesses in organic thin film transistors [1]The threshold thickness dth of electrical conduction for pentacene and sexithiophene films depends strongly on gate voltage. The depletion layer thickness was obtained from dth. In addition, the accumulation layer thickness could be estimated to be 1-2 molecular layers from the current-thickness curve measured at various gate voltages.2.Estimation of trap site densityTransient source-drain current (ISD) induced by sudden change of gate voltage was measured for C60 FET as a function of film thickness. Integration of transition current provides information on the number of trapped carriers, which increases abruptly around the C60 thickness of 3 nm and saturates to a constant value. This strongly correlates with the morphological change of C60 film from which the origin of trap sites was discussed.3.Resistance of grain interior and grain boundaryComplex impedance of the FET strucure was measured and only the component of the organic channel could be extracted. Analyzing the frequncy dependence of impedance, the ratio of resistance in the grain to the total channel resistance could be evaluated with a help of an analytical model. [1] M. Kiguchi, M. Nakayama, T. Shimada, and K. Saiki: Phys. Rev. B71 (2005) 035332, Jpn. J. Appl. Phys. 42 (2003) L1408
9:00 PM - M10.47
Light Emission From an Ambipolar Semiconducting Polymer Field-Effect Transist
James Swensen 1 , Cesare Soci 1 , Alan Heeger 1
1 Center for Polymers and Organic Solids, University of California, Santa Barbara, Santa Barbara, California, United States
Show AbstractWe report on ambipolar light emitting field effect transistors (LEFETs) which are fabricated with two different metals for the top-contact source and drain electrodes; a low work function metal defining the channel for the source electrode and a high work function metal defining the channel for the drain electrode (W ~2000 microns and L ~8 microns). The angled evaporation technique, which was developed to obtain this two-color electrode geometry, will be described. In order to enable electron transport in the devices, a thin film of polypropylene-co-1-butene was used to passivate the SiNx gate dielectric. Transport data show ambipolar behavior. Recombination of electrons and holes results in a narrow zone of light emission within the channel. The brightness and the location of the emission zone are controlled by the gate bias. Maximum efficiency occurs at the current crossover point where the electron and hole currents are approximately equal. Our current research is focused on improving the LEFET operation by investigating different dielectric materials which could replace the polypropylene-co-1-butene/ SiNx gate dielectric bilayer.
9:00 PM - M10.48
UV and Visible Radiation Detection Using Luminescent Polymers.
Rodrigo Bianchi 1 , Dayana Bazani 1 , Adnei de Andrade 1
1 Departamento de Engenharia de Sistemas Eletronicos - LME, Escola Politecnica da Universidade de Sao Paulo, Sao Paulo, SP, Brazil
Show Abstract9:00 PM - M10.49
Electrical Characterization Of In-Situ Polymerized Polyaniline Thin Films.
Rodrigo Bianchi 1 , Silmar Travain 2 , Jose Giacometti 3
1 Departamento de Engenharia de Sistemas Eletronicos - LME, Escola Politecnica da Universidade de Sao Paulo, Sao Paulo, SP, Brazil, 2 Instituto de Fisica de Sao Carlos, Universidade de Sao Paulo, Sao Carlos, SP, Brazil, 3 Faculdade de Ciencias e Tecnologia, Unesp, Presidente Prudente, SP, Brazil
Show Abstract9:00 PM - M10.5
Polymer Gate Dielectrics for High Performance Organic Field-Effect Transistors.
Faruk Yildirim 1 , Ronald Meixner 2 , Wolfgang Bauhofer 1 , Holger Goebel 2 , Wolfgang Krautschneider 3
1 Department of Materials for Electrical Engineering and Optics, Hamburg University of Technology, Hamburg Germany, 2 Department of Electronics, Helmut-Schmidt-University, University of the Federal Armed Forces, Hamburg Germany, 3 Department of Microelectronics, Hamburg University of Technology, Hamburg Germany
Show Abstract9:00 PM - M10.50
Electrical Properties Of Pentacene-Based Field-Effect Transistor With A Dielectric Layer Formed By Obliquely Evaporated Silicon Dioxide.
Jae-Il Jung 1 , Hak-Rin Kim 1 , You-Jin Lee 1 , Jae-Hoon Kim 1 , Lee-Mi Do 2
1 Department of Electronics and Computer Engineering, Hanyang University, Seoul Korea (the Republic of), 2 Basic Research Laboratory, Electronic and Telecommunications Research Institute, Daejeon Korea (the Republic of)
Show AbstractRecently, organic thin film transistors (OTFTs) have drawn much attention for flexible electronic devices such as smart cards, flexible displays, and several types of low cost and low-end electronics. Since the electrical performances of OTFTs are highly affected by the molecular ordering or crystalline orientation of the organic semiconductors, there are several efforts to modify an insulator surface which is in contact with the evaporated pentacene molecules. In general, highly oriented polymeric layers by mechanical rubbing or irradiation of polarized UV light are used for the purpose. But, the electrical performances of OTFTs on the organic insulators are not satisfactory yet, comparing with those of on the inorganic insulators. In this work, we investigated anisotropic conduction effects of pentacene-based OTFTs on the inorganic insulators using obliquely evaporated silicon dioxide substrate. For comparison, isotropic silicon dioxide surface was prepared by thermally grown oxidization with about 1400 Å thick on the heavily doped silicon wafer. The effects of the anisotropic inorganic interface were investigated by obliquely evaporating silicon dioxide with e-beam method on the thermally oxidized surface. The thickness of the additional dielectric layer was about 1000 Å. On the inorganic gate dielectric layer, a pentacene layer was deposited by 300 Å thick with a deposition rate of 0.4 Å/s under the base pressure of 1*10-6 Torr. As source and drain contacts, gold was deposited on top of the pentacene film, where the channel length and width were 100 μm and 1 mm, respectively. Two types of samples were prepared, where the channel direction was parallel or perpendicular to the oblique incidence of the e-beam evaporation. The mobility of the pentacene film in parallel one was higher than that in perpendicular one. The induced mobility anisotropy was about 8 when the inclination angle of the e-beam evaporation was 40o. As the inclination angle was increased, the mobility became even smaller than that on the thermally grown isotropic interface. It was probably due to the increased surface roughness, which resulted in decreased grain sizes.
9:00 PM - M10.51
A New Top Gate Pentacene Organic Thin Film Transistor Employing Vapor Deposited Polyimide As Gate Dielectric.
Chang-Wook Han 1 , Chang-Yeon Kim 1 , Min-Koo Han 1 , Dong-Hyun Lee 2 , Gun-Woo Hyung 2 , Sang-Woo Pyo 3 , Young Kwan Kim 2
1 School of Electrical Engineering, Seoul National University, Seoul Korea (the Republic of), 2 Department of Information Display, Hongik University, Seoul Korea (the Republic of), 3 Department of Electrical Information & Control Engineering, Hongik University, Seoul Korea (the Republic of)
Show AbstractMost of pentacene organic thin film transistor (TFT) reported employed a bottom gate structure. Bottom gate structure with top contact is rather difficult to fabricate fine-sized transistor due to shadow mask method. In bottom gate structure employing bottom contact, it is difficult to obtain high performance due to high contact resistance between source drain metals and pentacene. A transistor having fine channel length can be accomplished from a top gate structure with high performance compared with a transistor of the bottom gate. Gate dielectric of top gate structure should be chosen not to damage the pentacene by organic solvent or heat treatment for polymerization.The purpose of our work is to report the characteristics of top gate pentacene TFT fabricated employing vapor deposited polyimide as gate dielectric. Pentacene was evaporated at room temperature on Au as source drain electrodes and the thickness of pentacene is 60nm which is deposited at the rate of 0.3Å/s. Polyimide with the thickness of 375nm as gate dielectric was deposited on pentacene film from co-evaporation of hexafluoroisopropylidene-2,2’-bis [phethalic acid anhydride] (6FDA) monomer and 4,4’-oxydianiline (ODA) monomer. And thermal annealing at 150°C was performed for 1hour in vacuum. Pentacene covered by co-evaporated monomers does not be degraded or vaporized at the temperature of 150°C because analyzing from thermogravimetric analyzers (TGA) proved it is safe until around 300°C. Finally, Au was patterned as gate electrode on polyimide film. Fourier transform infrared (FT-IR) spectroscope was used for characterization of polyimide film. Peaks were observed at 1379 cm-1 (stretching vibration of C-N), 1720 cm-1 (asymmetric stretching vibration of C=O), which indicate that the obtained layer is polyimidized. A breakdown voltage of polyimide film employing vapor deposition was measured by metal-insulator-metal (MIM) structure and it was found to be 0.9MV/cm. The transfer characteristic curve was measured when a gate voltage was 0 and -40V when the drain voltage Vds set at -30V. By using the value of W/L = 25 and Cox = 6.6 nFcm-1, we have obtained a value of 0.01 cm2/Vs as a saturation mobility, about 103 as an on-off ratio (Ion/off), -7.5V as a threshold voltage and 9V/decade as a sub-threshold slope. The output characteristic of top gate pentacene TFT showed a typical p-type characteristic and a good saturation behavior.We have fabricated top gate pentacene TFT successfully. Our experimental results show that top gate pentacene TFT has a possibility when vapor deposited polyimide film was chosen as gate dielectric. The pentacene TFT of top gate structure will be able to be far better than that of bottom gate because the surface of pentacene having larger grains is used as a channel of TFT.
9:00 PM - M10.52
Solution-Processed Electrodes for Organic Field Effect Transistors
Stefan Gamerith 1 2 , Andreas Klug 1 2 , Ullrich Scherf 3 , Emil List 1 2
1 Christian Doppler Laboratory Advanced Functional Material, Institute of Solid State Physics, Graz University of Technology, Graz Austria, 2 Christian Doppler Laboratory Advanced Functional Material, Institute of Nanostructured Materials and Photonics, JOANNEUM RESEARCH, WEIZ Austria, 3 Makromolekulare Chemie, Bergische Universiaet Wuppertal, Wuppertal Germany
Show AbstractOne way to ultra-low cost organic electronic devices will be the route via all-solution based processes using direct patterning techniques such as e.g. inkjet-printing.A critical technological issue when combining several solution-processable materials within an electronic device is the compatibility of the materials with respect to the used solvents and to the dispersing agents. Basically there are two conceptual approaches to tackle this problem. The first one is simply to use material combinations where the solvent of the material applied on top of already deposited device components does not attack the underlying material for reasons of solubility. With organic field effect transistors this approach opposes strong limitations on the materials which can be used since different solvent systems are required for electrodes, dielectric and active material with any two of the three systems being compatible with the corresponding third system. The other important concept for stacked deposition of different materials from the fluid phase, which also works with similar fluid systems, is to convert the applied material with an appropriate curing procedure in order to prevent any attack during the preparation of the following layers. In case of particle dispersions such a curing procedure might simply be a drying or baking step in order to create larger agglomerations of the solid material which are not easily re-dispersed in the carrier phase while other concepts for polymers or monomers are based on cross-linking particular molecular units by means of appropriate chemistry thus creating a solid-state network. In the presented work inkjet printed source and drain electrodes for organic field effect transistors have been used in combination with the semiconducting polymer poly(3-hexylthiophene) (P3HT). Both of the concepts mentioned above regarding the material compatibility issues have been explored with commercially available electrode materials. This was done by i) combining the P3HT active layer with water based PEDOT:PSS or poly(aniline sulfonic acid) (PANI) electrodes on one hand and ii) using bottom contact organo-metallic-based silver electrodes from toluene solution, which were transferred into metallic silver by a baking step, and using metal electrodes fabricated from a silver-copper nanoparticle based dispersion on the other hand. The different concepts were compared regarding device performance as well as fabrication related issues. With PEDOT:PSS and the new approach using the silver-copper nanoparticle based dispersion, device performance comparable to that of devices with vacuum evaporated gold electrodes could be achieved.
9:00 PM - M10.53
PVA Gate Dielectrics in Organic Field Effect Transistors - Improvement of Device Characteristics by Material Purification
Stefan Brandstaetter 1 2 , Stefan Gamerith 1 2 , Emil List 1 2
1 Christian Doppler Laboratory Advanced Functional Material, Institute of Solid State Physics, Graz University of Technology, Graz Austria, 2 Christian Doppler Laboratory Advanced Functional Material, Institute of Nanostructured Materials and Photonics, JOANNEUM RESEARCH, Weiz Austria
Show AbstractAll-solution based processes are considered a straightforward way to ultra-low cost organic electronic devices. With organic field effect transistors (OFET) the all-solution processed approach, however, requires the compatibility of three different materials for electrodes, the dielectric and the active layer concerning solubility issues.In this respect Poly(vinyl alcohol) (PVA) has frequently been used as dielectric in OFETs. The material is soluble in water and insoluble in most organic solvents and thus can be combined with virtually any solvent-based organic semiconductor. The fact that PVA can be cross linked even further increases the range of principally compatible materials. However, devices based on PVA often show very poor characteristics with output curves lacking the expected current saturation and large hysteresis. In the presented work we investigate the characteristics of top-contact organic field effect transistors with an ITO gate, PVA dielectric, a poly(3-hexylthiophene) (P3HT) active layer and vacuum evaporated gold source-drain electrodes where two different PVAs, a fully hydrolyzed PVA from Sigma Aldrich and a partially hydrolyzed PVA from Mowiol have been used. Devices prepared from the two PVAs as delivered show very poor characteristics, similar to most reports in literature. However, purification of the material by means of dialysis, leads to a significant improvement of the device performance i.e. to a pronounced appearance of expected current saturation in the output characteristics as well as to a strong reduction of the hysteresis-effects. One of the main reasons for the observed improvements is found in the reduction of the residual salt content in the dielectric as the PVA is dialyzed. This is also confirmed via FTIR measurements of as- delivered and purified PVA where the change of the spectra by the dialysis can be attributed to a reduction of sodium-acetate, which is a known by-product of PVA synthesis.
9:00 PM - M10.54
Double Exponential Trap Distribution in Organic Thin-film Transistors
Richard Yang 1 2 , Amos Sharoni 3 , Jeongwon Park 1 , Corneliu Colesniuc 3 , Ivan Schuller 1 , Andrew Kummel 2
1 Material Science and Engineering, UC San Diego, La Jolla, California, United States, 2 Chemistry and Biochemistry, UC San Diego, La Jolla, California, United States, 3 Physics, UC San Diego, La Jolla, California, United States
Show AbstractCharge trapping effects on subthreshold characteristics and gate-voltage dependent mobility are investigated based on CuPc OTFTs. Bottom-contact devices are fabricated on highly doped n+ silicon substrates, with 100 nm thermal SiO2 as the gate dielectric. 50 nm thick gold source and drain electrodes with 5 micron channel length are deposited by e-beam evaporation and lift-off lithography. CuPc films between 15 – 75 nm are deposited by organic molecular beam epitaxy (OMBE) at room temperature; the CuPc acts as the channel layer. The inverse subthreshold slopes (S) are found to increase linearly with the channel layer film thickness, indicating bulk traps have significant contribution to the slope S. The trap distribution energy Et has been determined by space-charge-limited current spectroscopy in the reverse mode (hole transport to a grounded source). The current-voltage curves are fitted with exponential distributions of trap energies. The trap distribution energy decreases as the film thickness increases. This is consistent with the bulk films being more ordered and having a narrower trap energy distibution than the interfacial CuPc monolayers. A double exponential trap distribution model has been proposed to account for the effect of film thickness on the trap energy distribution. The same model of more order bulk CuPc and less order interfacial CuPc monolayers also explains the gate voltage dependent mobility in organic transistors. The field-effect mobility increases with gate voltage as a result of sequential filling of interfacial and bulk traps. At higher gate voltage, the traps states near the interface are filled up. It takes less activation energy to fill up bulk traps subsequently. Therefore, at high gate bias more free carriers contribute to the field-effect mobility and the measured mobility is higher.
9:00 PM - M10.55
Chemically Robust Platform for Solid-state Conducting Polymer Sensor
Amanda Holt 1 , Jane Bearinger 2 , Sue Carter 1
1 Physics, UC Santa Cruz, Santa Cruz, California, United States, 2 , Lawrence Livermore National Laboratory, Livermore, California, United States
Show Abstract Conducting polymers are unique materials for use in the development of chemical and biological sensors because of their ability to function as both the sensing element and the signal transducer. Utilizing both optical and electrical signals, these materials are being explored as sensors in solution as well as in thin-film form via spin-casting, drop-casting, electrochemical polymerization and layer-by-layer deposition methods. In order to produce a more chemically robust thin film, for use as a “solid-state” sensor, we succeeded in grafting polythiophene, poly 3-methyl thiophene and poly 3-hexyl thiophene to substrates of glass, quartz, silicon, indium tin oxide and silver. This was accomplished by first grafting a thiophene monomer to the surface by silanization of a metal or insulator oxide layer with an isocyanate functionalized silane, followed by reaction with an amine functionalized thiophene monomer. The polymer films were then chemically grown via surface oxidative polymerization using ferric chloride in acetonitrile or chloroform. X-ray photoelectron spectroscopy studies indicated that the altered surface chemistries that accompany each chemical reaction were accurately understood. Film thicknesses range from 20 to 100 nm and depending on the substrate type exhibit varying degrees of surface roughness, an aspect that is important when anticipating the sensitivity of the sensor device. The reaction times and solvents were varied in order to optimize the desired film properties. The robust polythiophene growth, unaltered by sonication and tape peeling tests, was uniform across the substrate and could be directed by selectively silanizing the area of interest. The absorption and photoluminescence properties of the thin films compared well with literature on spun-cast polythiophene films, as did the electrical resistivities of the doped and undoped material. Studies are currently being conducted to analyze the films as both optical and electrical chemical and biological sensors.
9:00 PM - M10.56
P to N-Type Transitions in Polyaniline Hybrid Chemosensors
Aisha Bishop 1 , Elisabetta Comini 2 , Pelagia-Irene Gouma 1
1 Materials Science and Engineering, SUNY Stony Brook, Stony Brook, New York, United States, 2 Dept. of Chemistry & Physics, INFM Unita de ricerca di Brescia, Brescia Italy
Show AbstractAn active sensing matrix consisting of polyaniline (PANI) and poly-vinyl-pyrrolidone(PVP) was prepared via the electrospinning method. Several advantages associated with this process include a potential enhancement of gas- polymer surface interactions (associated with the porous, high surface to volume ratio of the prepared non-woven matrix) over traditional thin films, and a novel bi-polymer system that exhibits improved gas selectivity. The change in concentration of PANI: PVP yields high sensor sensitivity at low PANI concentrations and a low sensor sensitivity at high PANI concentrations. Furthermore sensitivity studies of the PANI-PVP matrices to NO2 and NH3 reveal a p to n-type transition in sensing response as a function of changing PANI: PVP ratio. The kinetics of these reactions and the effect of PANI concentration on sensing response to NO2 and NH3 will be discussed.
9:00 PM - M10.57
High Speed Pentacene Thin Film Photodetector.
Jianbo Gao 1 , Frank A. Hegmann 1 , Ray F. Egerton 2
1 Department of Physics, University of Alberta, Edmonton , Alberta, Canada, 2 Department of Physics, University of Alberta, Edmonton , Alberta, Canada
Show Abstract9:00 PM - M10.58
High Performance Organic Complementary Ring Oscillators and D Flip-Flops based on Novel n-type Organic Semiconductors
Byungwook Yoo 1 , Taeho Jung 1 , Ashwin Madgavkar 1 , Debarshi Basu 1 , Ananth Dodabalapur 1 , Brooks Jones 2 , Antonio Facchetti 2 , Michael Wasielewski 2 , Tobin Marks 2
1 Electrical and Computer Engineering, The University of Texas at Austin, Austin, Texas, United States, 2 Chemistry, Northwestern University, Evanston, Illinois, United States
Show AbstractOrganic field effect transistors (OFETs) have attracted considerable attention for low cost applications such as radio-frequency identification tags, display drivers, and sensors. A number of approaches to fabricate the complementary metal-oxide-semiconductor (CMOS) circuits based on the organic transistors have been reported due to the advantages of the lower power dissipation and better noise margin compared with simple p-channel FET circuits. Air-stable n-channel FETs are crucial to the development of organic CMOS circuits. The results of electrical characteristics of 4 μm channel length bottom-contact organic field-effect transistors with the PDI-8CN2 (N,N’-bis(n-octyl)-dicyanoperylene-3,4:9,10-bis(dicarboximide)) are described. The mobility, threshold voltage, sub-threshold swing, and Ion/Ioff ratio (VDS = 40 V, VG = 0 ~ 40 V) are 0.14 cm2/Vs, 1.6 V, 2.0 V/decade, and 1.2 × 103, respectively. The effect of electrode/dielectric surface treatment on the response of these devices is also examined, with a combination of 1-octadecanethiol (Au electrode functionalization) and hexamethyldisilazane (SiO2 dielectric functionalization) affording maximum PDI-8CN2 mobility. Organic complementary five-stage ring oscillators and D flip-flops were fabricated using pentacene and PDI-8CN2. An oscillation frequency of 34 kHz was obtained with channel lengths of 7.5 μm and a propagation delay per stage was 3 μs at a five-stage ring oscillators. D flip-flops operate at a frequency of 5 kHz and a rising pulse delay of ~14 μs. Future work is needed to increase the speed and fabricate multistage CMOS shift registers.
9:00 PM - M10.59
Low Voltage Operation Of Organic CMOS Inverter Circuit With Double-Gate Structure.
Kazuki Hizu 1 , Tsuyoshi Sekitani 1 , Youko Shimada 3 , Joe Otsuki 3 , Makoto Takamiya 2 , Takayasu Sakurai 2 , Takao Someya 1
1 QPEC, University of Tokyo, Bunkyo-ku, Tokyo, Japan, 3 Materials Chemistry, Nihon University, Chiyoda-ku, Tokyo, Japan, 2 CCR, University of Tokyo, Meguro-ku, Tokyo, Japan
Show AbstractWe have successfully reduced the operation voltage (Vdd) of organic CMOS inverter circuits on plastic films from 40 V to 5 V using the novel double-gate structure. Organic CMOS inverter circuits were constructed by high-quality pentacene and fluoroalkyl NTCDI field-effect transistors (FETs) on plastic films. These FETs were fabricated with double-gate structures, where each top and bottom gate electrode can apply voltage biases to channel layers independently. The threshold voltage (Vth) of each organic FET has changed systematically with application of voltage bias to top gate; When voltage bias from –10 V to 30 V was applied to top gate of p-type FETs, Vth changed from -0.4 V to –20 V, while Vth of n-type FETs changed from -10V to 10V with swept top gate voltage from 0 V to –40 V. Although organic CMOS inverter circuits function at Vdd larger than 40 V without application of voltage biases to both top gates, the minimum Vdd drastically decreased down to 5 V using optimized top gate biases. The p-type and n-type organic FETs were fabricated by vacuum evaporation system. First, a 5-nm-thick Cr and a 50-nm-thick Au layer were deposited as gate electrodes on 75-μm-thick polyimide substrate in a vacuum evaporation with resistive heating. Then, polyimide is spin-coated as a 500-nm-thick bottom gate insulator. A pentacene layer was evaporated to form a p-type channel layer with thickness of 50 nm. A fluoroalkyl NTCDI layer [1] was also deposited to form a n-type channel layer with thickness of 60 nm. A 50-nm-thick Au layer was deposited as source-drain electrodes. Finally, a 500-nm-thick parylene layer was coated as a top gate insulator, and a 50-nm-thick Au layer was deposited as top gate electrodes. Channel width of p-type FETs was 120 μm and that of n-type FETs was 500 μm. Channel length of both FETs was 50 μm. Pentacene FETs showed mobility of 0.23 cm2/Vs in the saturation regime, while fluoroalkyl NTCDI FETs showed mobility of 0.08 cm2/Vs. Although the maximum on/off current ratios of both FETs exceeded 10^6, the on/off current ratios defined as off current at VGS=0 V were at most 10^1 because of atmospheric air degradations.We used a precision semiconductor parameter analyzer. When voltage bias was applied to top gate of p-type FETs from –10 V to 30 V, Vth changed from -0.4 V to –20 V, and on/off current ratio changed from 10^1 to 10^6 defined off current at VGS=0V. Similarly, when voltage bias was applied to top gate of n-type FETs from 0 V to –40 V, Vth changed from –10 V to 10 V, and on/off current ratio changed from 10^1 to 10^5. Consequently, organic CMOS inverter with top gate biases can function at Vdd down to 5 V by applying –30 V to top gate of n-type FET and 10V to that of p-type FET. Such an application of double-gate structure will lead to low-power-consumption driving technology for organic circuits. [1] Katz et al. Nature 404(2000) 478.The authors thank MEXT IT program, TOKUTEI (15073204) and CREST, JST for financial supports.
9:00 PM - M10.6
Hall Effect of Organic Single-crystal Field-effect Transistors.
Jun Takeya 1 2 3 , Kazuhito Tsukagoshi 3 4 , Koichi Yamada 1 , Yoshinobu Aoyagi 3 6 , Taishi Takenobu 2 5 , Yoshihiro Iwasa 2 5
1 , CRIEPI, Komae Japan, 2 , IMR, Tohoku University, Sendai Japan, 3 , RIKEN, Wako Japan, 4 , PRESTO, Kawaguchi Japan, 6 , Tokyo Institute of Technology, Tokyo Japan, 5 , CREST, Kawaguchi Japan
Show Abstract As technological progress pushes organic field-effect transistors (OFETs) towards the market, demand for elucidating character of the field-induced charge is getting urgent, desiring a fundamental guide to elevate mobility as well as other device performance factors. Although the reported mobility reaches 20 cm2/Vs for single-crystal devices [1], it is not yet known whether surface charge carriers can be approximated as a gas of spatially extended particles or are basically localized at trap centers and occasionally hop to their neighbouring sites. Here, we report the detection of the Hall effect, which provides a measure of the spatial extension of the surface charge, using appropriately shaped rubrene (C42H28) single-crystal field-effect devices. We crystallized rubrene molecules to a thin platelet with a thickness of approximately 1 μm by physical vapor transport. To fabricate an FET, the thin rubrene crystal is laminated on a SiO2 / doped Si substrate with gold electrodes patterned for both transverse and longitudinal voltage detections [2]. As the transverse signal for the Hall effect measurement is only 0.1% of the source-drain voltage applied in the longitudinal direction, it is essential to regulate the current and symmetrically position the voltage-probing electrodes at the opposite sides. For this purpose, we trimmed the laminated rubrene crystal into a Hall-bar shape using a scanning laser-etching equipment [3]. Such a dry-etching process is preferable not to damage the device channel.It turned out that inverse Hall coefficient, having a positive sign, is close to the amount of electric-field induced charge upon the hole accumulation. The result corresponds to the free-electron model where the charge amount is equal to the inverse Hall coefficient 1/RH. If hopping transport is dominant on the other hand, 1/RH would be much smaller (typically one order less), because magnetic field does not provide a transverse electromotive force for a single tunnelling (hopping) process, as was demonstrated for amorphous silicon samples. Therefore, the electromagnetic character of the charge field-induced at the surface of the rubrene crystal is not of hopping carriers but resembles that of a two-dimensional hole-gas system.[1] E. Menard et al., Adv. Mater. (Weinheim, Ger.) 16, 2097 (2004).[2] J. Takeya et al., J. Appl. Phys. 94, 5800 (2003); Appl. Phys. Lett. 85, 5078 (2004); Mater. Res. Soc. Symp. Proc. 871E, I7.1, (2005).[3] I. Yagi et al., Appl. Phys. Lett. 84, 813, (2004).
9:00 PM - M10.61
Poly(3-octylthiophene) Fibers and Non-woven Mats Prepared by Electrospinning.
Layza Arizmendi 1 , Rodolfo Cruz-Silva 2 , Jorge Romero-Garcia 1 , Eduardo Arias-Marin 1 , Ivanna Moggio 1
1 Departamento de Materiales Avanzados, Centro de Investigacion en Quimica Aplicada, Saltillo, Coahuila, Mexico, 2 Laboratorio de Polimeros, Centro de Investigacion en Ingenieria y Ciencias Aplicadas, Cuernavaca, Morelos, Mexico
Show AbstractElectrospinning is a relatively simple process useful to produce polymeric fibers with submicron diameter and up to several microns length. In this work, fibers and non-woven mats of regiorandom poly(3-octylthiophene) [P30T] were prepared by electrospinning. P3OT was synthesized in chloroform solution by chemical oxidation using iron chloride(III) as oxidizing agent. Coaxial and side-by-side electrospinning configurations were used, in both cases, polyvinylpyrrolidone solutions were used as processing aid due to the low viscosity of P3OT solutions. The electrospinning experiments were carried out varying several parameters such as voltage, solution concentration, and tip-to-collector distance. Toluene and dichloromethane were used as solvent for P3OT and ethanol for polyvinylpyrrolidone. Side-by-side electrospinning configuration yielded fibers with cylindrical section and diameter ranging from 130 to 400 nm, whereas the coaxial set-up resulted in a combination of cylindrical fibers with diameter ranging from 150 nm to 400 nm, and ribbon-shaped fibers with dimension up to 800 nm width. ITO coated glasses were used as collector to obtain non-woven mats, whose properties were evaluated by UV-Vis and photoluminescence spectroscopy. Non-wovens morphology and fiber diameter distribution were studied by scanning electron and atomic force microscopy. In addition, laser scanning confocal fluorescence microscopy indicates the formation of P3OT fibrillar structures. Results of the evaluation of the P3OT non-wovens as biosensors will be presented.
9:00 PM - M10.62
Analysis Of The HMDS Surface Treatment Effect On Pentacene-Based Otfts With Four-Probe Structure.
Hsiao Wen Zan 1 , Shih-Chin Kao 1 , Ting-Hsuan Cheng 1
1 Department of Photonics, Display Institute, HsinChu Taiwan
Show AbstractIn this experiment, the HMDS surface treatment effect on pentacene growth condition, OTFT output characteristics are carefully studied. Four-probe structures are designed to characterize the contact resistance and the channel film properties separately. Our results reveal that the HMDS treatment enhances the growth of pentacene layer and therefore reduce the film resistance while the devices are turned on. However, it also serves as a dipole layer below the pentacene layer that shifts the turn-on voltage and also changes the carrier injection barrier. By comparing the contact resistance variation versus the gate bias, it is found that the contact resistance drops dramatically at the turn-on voltage. The HMDS treatment increases the carrier injection barrier and therefore the turn-on voltage shifts to be more negative. However, it is observed that once the device is turned on, the contact resistance for devices with HMDS treatment becomes exactly the same as that of control samples. This implies that under large gate bias, the dipole layer no longer influences the carrier injection behavior. It is proposed that the HMDS layer loses its dipole character or even changes its dipole direction under large electric field. This gives rise to the greatly improved subthreshold swing for devices with HMDS treatment. Since the retardation of turn-on due to the dipole layer suddenly loses its effect under certain electric field and therefore carrier density increases dramatically and the devices exhibit better subthreshold swing. The device fabrication procedure in this experiment is as follows. Firstly, a heavily-doped Si wafer is served as the gate. After depositing 200-nm-thick thermal oxide, 100-nm-thick Pd metal with Ti adhesion layer (10 Å) was sputtered and patterned as Source/Drain contact. Then the wafer was put into the oven with HMDS steam for 20mins at 150 degree celcius. Finally, the pentacene layer was deposited in thermal evaporation chamber via shadow mask patterning. The substrate was heated to 65 degree celcius during the deposition at the pressure of 1×10-6 torr. The thickness of pentacene film is 1000 Å and the deposition rate is 0.5 Å/sec, which were monitored by the quartz crystal oscillator. Control samples without HMDS treatment were fabricated in the same run. Further analysis on 4-probes devices with top contact structure and on devices with other surface treatment will be proceeded in the following few months. More comprehensive understanding will be discussed.
9:00 PM - M10.63
Characteristics of Pentacene Organic Thin-film Transistors with Different Polymer Gate Insulator.
Jung Min Kim 1 , J. H. Yoon 2 3 , Hyun-Jung Her 1 , C. J. Kang 3 , D. Jeon 2 , Yong-Sang Kim 1
1 Electrical Engineering and Nano-Bio Research Center, Myongji University, Yongin, Gyeonggi-Do, Korea (the Republic of), 2 Physics Education and Nano Systems, Seoul National University, Seoul Korea (the Republic of), 3 Physics and Nano-Bio Research Center, Myongji University, Yongin, Gyeonggi-Do, Korea (the Republic of)
Show AbstractThe organic materials have attracted much attention these last few years for their unique semiconducting properties. Organic thin-film transistors (OTFTs) have the key advantage of relatively simple and low temperature processing, low cost and mechanical flexibility that could be useful in many applications, such as smart cards, RFID tags and flexibly displays, etc. Several different organic semiconductors have been investigated for application of TFTs. Among the variety of organic semiconductors, pentacene-based OTFTs have been the best results at the electrical performance. In this reason, we have fabricated OTFTs in which the conventional inorganic active layer is replaced by pentacene organic semiconductors. However, the electrical conducting mechanisms for these organic semiconductors are not completely understood but could be sensitive function of the morphology and grain structure of the organic semiconductors. Thus, OTFTs are important means for examining organic semiconductors conduction. Properties of an organic semiconductors thin film depend not only on the growth conditions but also on the substrate. We have studied how the number of nucleus, grain size and morphology of pentacene thin film varied depending on the kind of polymer substrates by the atomic force microscope (AFM). We have chosen HMDS (Hexamethyldisilazane), PMMA (Polymethylmethacrylate), PVA (Poly vinyl alcohol) and THB-151N (Propylene glycol monomethyl ether acetate, multifunctional acrylate) as the polymer substrate and deposited pentacene under different deposition rates and substrate temperatures. We also fabricated pentacene OTFTs using several kinds of polymer as a gate insulator with top-contact geometry, and compared with the electrical performance for polymer gate insulator by conventional I-V measurement. From these results, we have better understandings on the conducting mechanism of organic semiconductors and characterizes of pentacene OTFTs with different polymer gate insulator.
9:00 PM - M10.65
A Novel triblock copolymer: Synthesis, Characterization, and Micellization.
Hak Yong Kim 1 , K.C. Remant Bahadur 2 , Shanta Bhattarai 2 , Santosh Aryal 2 , Byoung Min Lee 2
1 Textile Engineering, Chonbuk National University, Chonju Korea (the Republic of), 2 Bionanosystem Engineering, Chonbuk National University, Chonju Korea (the Republic of)
Show Abstract9:00 PM - M10.67
Polymer Composite Film with Controlled Optical Activity by using Chirally-ordered Lipid Assemblies.
Makoto Takafuji 1 , Shiro Sawada 1 , Hideaki Tsuji 1 , Hirotaka Ihara 1
1 Department of Applied Chemistry and Biochemistry, Kumamoto University, Kumamoto Japan
Show Abstract9:00 PM - M10.68
The Nature of the Bimodal Growth of Pentacene: Bulk vs. Thin-Film
Alex Mayer 1 , George Malliaras 1 , Randall Headrick 3 , Kazimirov Alexander 2
1 Materials Science and Engineering, Cornell University, Ithaca, New York, United States, 3 Physics, University of Vermont, Burlington, Vermont, United States, 2 , Cornell High Energy Synchrotron Source, Ithaca, New York, United States
Show AbstractIt is known from specular x-ray diffraction studies that pentacene films are composed of two phases: the bulk crystal structure and a substrate-induced structure known as the “thin-film” phase. It is also known that the film initially adopts the “thin-film” phase when deposited on inert substrates up to some critical thickness. Upon reaching this thickness a small bulk phase signal appears in the specular x-ray pattern. What is unknown is the exact nature of this transition. That is whether part of the film relaxes to the bulk phase like in inorganics deposited on lattice-mismatched substrates or if the bulk phase nucleates isotropically on top of the “thin-film” phase or some scenario in between. We have performed real-time x-ray measurements of pentacene film growth throughout this transition to show that the two phases are distinct (the thin-film is not a strained version of the bulk) and that the thin-film phase continues to grow long after the critical thickness is reached.
9:00 PM - M10.69
Self Assembled Monolayer (SAM) Formation on Copper Surfaces by Perfluorosilanization.
M Enamul Hoque 1 , J.A. DeRose 2 , J.R. Thome 2 , P. Hoffmann 3 , H.J. Mathieu 1
1 LMCH, IMX, STI, EPFL, Lausanne, Vaud, Swaziland, 2 LTCM, ISE, EPFL, Lausanne, Vaud, Switzerland, 3 LOA, IOA, EPFL, Lausanne, Vaud, Switzerland
Show Abstract9:00 PM - M10.7
Star Shaped 2-D Phenylene-Thiophene-Based Organic Field-Effect Transistors.
Minglee Tang 1 2 , Ashok Maliakal 2
1 Chemistry, Stanford, Stanford, California, United States, 2 , Bell Laboratories, Lucent Technologies, Murray Hill, New Jersey, United States
Show AbstractThree novel two-dimensional conjugated phenylene-thiophene oligomers have been synthesized and characterized. These molecules have a tri-substituted phenyl core, but vary in the length and nature of the substitution. Vacuum-evaporated and solution-cast field-effect transistors have been made, with mobilities up to 0.027 cm2/Vs for the evaporated hexyl-substituted oligomer, and 2.06 x 10-6 cm2/Vs for the dropcast unsubstituted molecule on SiO2/Si. X-ray diffraction reveals that the devices are amorphous. These measurements demonstrate to the best of our knowledge the highest mobilities reported to date for star shaped organic semiconductors.
9:00 PM - M10.8
Organic Field-Effect Transistors From Self-Assembled Oligothiophene Monolayers.
Amanda Murphy 1 , Jean Frechet 1 , Clayton Mauldin 1 , Paul Chang 2 , Vivek Subramanian 2 , Jeyaprakash Samuel 3 , Robert Miller 3 , Dean DeLongchamp 4 , Eric Lin 4 , Daniel Fischer 4
1 Chemistry, UC Berkeley, Berkeley, California, United States, 2 Electrical Engineering and Computer Sciences, UC Berkeley, Berkeley, California, United States, 3 , IBM Almaden Research Center, San Jose, California, United States, 4 Polymers Division, National Institutes of Standards and Technology, Gaithersburg, Maryland, United States
Show AbstractWe report here the first OFET devices fabricated from self-assembled monolayers of end-functionalized thiophene oligomers. Monolayer films were made using solution or Langmuir-Blodgett deposition techniques. Characterization of the films by AFM and ellipsometry confirm the presence of a single monolayer. The molecular orientation with respect to the surface was evaluated using multi-angle NEXAFS spectroscopy. Finally, the performance and stability of these monolayers were investigated in both top and bottom contact field-effect transistors.
9:00 PM - M10.9
Determination Of Density Of States (DOS) In Polymeric Semiconductor Materials Using Photocurrent Spectra Of Diodes.
Yashowanta Mohapatra 1 2 , Girija Samal 2 , Shibabrata Nandi 1 , Samarendra Singh 1
1 Physics, Indian Institute of Technology-Kanpur, Kanpur, U.P, India, 2 Samtel Centre for Display Technology and Material Science Programme, Indian Institute of Technology-Kanpur, Kanpur, U.P, India
Show Abstract
Symposium Organizers
Zhenan Bao Stanford University
Anna B. Chwang Universal Display Corporation
Lynn Loo The University of Texas-Austin
Rachel A. Segalman University of California-Berkeley
M11: Molecular Level Studies of Electronic Properties
Session Chairs
Friday AM, April 21, 2006
Room 2006 (Moscone West)
9:30 AM - **M11.1
Single Molecule Spectro-Electrochemistry of Single Isolated Conjugated Polymers and Inorganic Semiconductor Nanoparticles
Paul Barbara 1
1 CNM, University of Texas, Austin, Texas, United States
Show AbstractElectrochemistry of single organic and inorganic nanoparticles were investigated by single molecule fluorescence spectroscopy on molecules lcoated within the double-layer of an electrochemical cell. Alternatively, these mateirials were also studied in an OLED like structure. The results lead to new insights on the oxidation/reduction potentials and mechanisms these materials.
10:00 AM - M11.2
Organic Field Effect Transistor Interfaces Probed In-situ During Operation by Sum Frequency Generation Vibrational Spectroscopy
Hongke Ye 1 , Jia Huang 3 , Howard Katz 3 , David Gracias 1 2
1 Chemical and Biomolecular Engineering, Johns Hopkins University, Baltimore, Maryland, United States, 3 Materials Science and Engineering, Johns Hopkins University, Baltimore, Maryland, United States, 2 Chemistry, Johns Hopkins University, Baltimore, Maryland, United States
Show AbstractIn order to gain a molecular level understanding of the electrical conduction in thin film organic semiconductors, we have integrated a four point electrical probe station with a non-linear optical sum frequency generation (SFG) spectroscopy system. SFG is a non-linear surface specific spectroscopic technique that allows non-destructive analysis of the surface of a sample in air. It has been convincingly demonstrated, both on the basis of theoretical as well as experimental studies, that SFG spectroscopy has monolayer sensitivity for materials that possess bulk inversion symmetry; the surface sensitivity is a result of the fact that this second order non-linear optical process vanishes under centrosymmetry. The surface vibrational spectrum was obtained by overlapping a tunable infra red (IR) and visible picosecond laser beam at the interface of the semiconductor. The spectra was obtained by tuning the IR beam over the relevant resonant vibrational modes and measuring the intensity of the reflected sum frequency (SF) beam as a function of IR frequency.Thin film transistors consisting of organic semiconductors 5,5’-bis(4-hexylphenyl)-2,2’-bithiophene (6PTTP6) and pentacene, were fabricated on silicon substrates with top contacts (gold evaporated through a shadow mask). SFG spectra at the semiconductor interfaces were obtained in two polarization combinations (in order to monitor molecular orientation changes) at the interfaces during operation. SFG spectra show strong correlations with the electrical characteristics measured. In particular, the SFG spectra of the interfaces changed with increasing gate voltage (Vg). For both 6pttp6 and pentacene the non-resonant SFG background correlated with the increase in conductivity (slope of the IV curve after turn on) measured of the semiconductor layer with increasing Vg. For 6pttp6 we observed from the SFG spectra that the relative peak intensity of methylene and phenyl vibrational modes increased with increasing gate voltage, while the peak intensity of methyl groups was relatively unchanged. The dependency of the methylene peak intensity on gate voltage correlated well with the dependence of the saturation current on gate voltage. These results point to the fact that charging of molecules and the field effects in OFETs can be probed in-situ using SFG and electrical testing, and we can gain a molecular level understanding of OFET interfaces from the results. We will also discuss control experiments on other semiconductors that do not have the ability to hold and transfer charge.
10:15 AM - M11.3
Molecular Energy Level Alignment At The HATNA/Metal Interface.
Fabrice Amy 1 , Wei Zhao 1 , Antoine Kahn 1 , Bilal Kaafarani 2 3 , Stephen Barlow 2 3 , Seth Marder 2 3
1 Electrical Engineering, Princeton University, Princeton, New Jersey, United States, 2 Center for Organic Photonics and Electronics (COPE) and School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia, United States, 3 Department of Chemistry, University of Arizona, Tucson, Arizona, United States
Show AbstractThe realization of cheap, lightweight, energy efficient organic electronics has triggered the need for molecular semiconductors with high charge carrier mobility that can be processed from solvent solutions. In this context, the discotic liquid crystals Hexaazatrinaphthylene (HATNA) and its derivatives are extremely promising. Indeed HATNA and its derivatives offers a unique combination of advantages such as high solubility, high supramolecular order leading to charge mobilities comparable or even greater to those of amorphous silicon and an exciton diffusion length larger than in conjugated polymers [1]. Bulk transport properties of HATNA are well documented [2] but the traditional bottleneck of the charge injection at the metal electrode as well as energy level offset at organic/organic interfaces still needs to be investigated. In this study we use ultra-violet and inverse photoemission to investigate molecular energy level alignment at metal/HATNA and organic/HATNA interfaces. We report on the pure HATNA as well as three electron transport reduced derivatives with ionization energies raging from 6.35 eV to 7.76 eV and electron affinity ranging from 2.76 eV to 4.59 eV. Electron injection barriers as low has 0.25 eV are measured and are supported by J-V measurements. We also use different metals (Au, Ag, Al) substrates to estimate the position of the charge neutrality level (CNL) of one of the derivatives. We found the CNL to be lying high in the gap which can explain the relatively low electron injection barriers we measured independently of the metal work function. We also use the concept of CNL alignment to predict the molecular energy level alignment at interface between HATNA and copper-phthalocyanine (CuPc) and confirm it with experimental results.[1] X. Crispin et al. J. Am. Chem. Soc. 126, 11899 (2004).[2] B.R. Kafaarani J. Am. Chem. Soc. In press (2005)
10:30 AM - **M11.4
Material and Interface Engineering for High Efficiency Light-emitting Materials and Devices
Alex Jen 1 2 , David Ginger 2 , Yuhua Niu 1 , Michelle Liu 1 , Julie Bardecker 1 , Jae-Won Ka 1 , Angus Yip 1 , Hong Ma 1 , Jingdong Luo 1 , Baoquan Chen 1 , Yen-Ju Cheng 1 , Jialong Zhao 2 , Andrea Munro 2 , Matthew Adams 2 1
1 Materials Science & Engineering, University of Washington, Seattle, Washington, United States, 2 Chemstry, University of Washington, Seattle, Washington, United States
Show AbstractOur effort in improving the performance of light-emitting materials and devices have been focused on two areas: 1) modify the interface between light-emitting polymers and electrodes for better charge injection, transport, and recombination, and 2) enhance the efficiency of polymer LEDs through the development of new conjugated materials with balanced charge-transporting properties. In this talk, we will review our recent progress using dipolar self-assemble monolayer (SAM) to form dense assembly on indium tin oxide (ITO) to tune the energy barrier between ITO and hole-transporting polymers. We systematically measure these barriers using the internal photoemission technique. We have also developed several in-situ polymerizable hole-transporting materials with desirable energy level to facilitate cascade hole-injection for quantum dot (QD)- and phosphorescent dopant-based LEDs. As a result of this integrated material development approach, very efficient and bright devices have been demonstrated for applications in displays and solid state lighting.
11:30 AM - **M11.5
Structure, Conformational Dynamics and Photophysics of Single Conjugated Polymer Chromophores.
Lewis Rothberg 1 3 2 , Zhenjia Wang 1
1 Chemistry, University of Rochester, Rochester, New York, United States, 3 Chemical Engineering, University of Rochester, Rochester, New York, United States, 2 Physics, University of Rochester, Rochester, New York, United States
Show AbstractConjugated polymers are possible active materials in light-emitting diodes (OLED), photovoltaics and thin-film transistors. We report studies of chromophores in single polymer chains of MEH-PPV and related polymers using single molecule fluorescence and Raman spectroscopy. Single chain studies confirm a model of the polymer with two types of chromophores. In fluorescence, we observe tightly packed configurations that behave as single chromophores exhibiting polarization correlation between excitation and emission, fluorescence blinking and sudden photobleaching. We also observe an extended conformation where polarization correlation is weak, there is no blinking and photobleaching is gradual. Single chromophore Raman spectroscopy on nanotextured silver surfaces confirms this picture and we observe behaviors suggesting a bimodal distribution of behaviors that can be associated with “loosely packed” and “tightly packed” configurations. We observe violent spectral and intensity fluctuations in the Raman spectra of single conjugated polymer chains and oligomers and we perform correlation analyses to try to associate these phenomena with specific molecular motions. We also see the signature of long-lived polaron photogeneration in the Raman spectrum and irreversible photochemistry of the chromophores. These data provide a useful picture of the relationships between microscopic conformations of the polymer and their optical properties.
12:00 PM - M11.6
Unraveling the Structural Properties of Polyfluorene Based Polymeric Semiconductors Using Single Molecule Spectroscopy.
Klaus Becker 0 , John Lupton 0 , Florian Schindler 0 , Manfred Walter 0 , Jochen Feldmann 0 , Ullrich Scherf 2
0 Physics and CeNS, Ludwig-Maximilians-University, Munich, Bavaria, Germany, 2 Chemistry, University of Wuppertal, Wuppertal Germany
Show AbstractConjugated polymers offer both a broad spectrum of device applications as well as raising many fundamental issues relating to the nature of excited state species in complex macromolecules. A driving force in their optical spectroscopy has been the desire to establish reliable relationships between chemical structure and conformation and their optical properties. By applying low temperature single molecule fluorescence spectroscopy, disorder broadening can be overcome and the optically active subunit on the polymer chain -the chromophore- is identified [1]. Conformational information on the chromophores can be obtained from polarized absorption and emission spectroscopy [2], but is also available from fluorescence signatures such as the spectral linewidth and temporal dynamics [3]. Chain folding is a prominent conformational effect, which can give rise to self-aggregation of the polymer and resultant spectral broadening [3]. Polyfluorene exhibits a particularly interesting conformational transition of the chain between a planar and a twisted structure [4]. This transition bares a number of spectral signatures such as a change in vibrational coupling strength. Most importantly, we find that the photophysical stability is strongly affected by the chain conformation [4]. Planarisation of the polymer backbone effectively inhibits photoquenching of the single polymer molecule. This study suggests that a mechanical force, exerted by the embedding polymer matrix in this case, is capable of changing both the electronic as well as the photochemical properties of a polymer molecule. Our results therefore provide important input into material design by demonstrating that structure-property relations in conjugated polymers derive from the chain morphology rather than from the chemical details of the conjugated backbone.An important exception to this rule is given by oxidative defects on the polymer backbone, which yield well-defined spectroscopic signatures in the single molecule emission, red shifted by over 100 nm from the backbone transition [5]. Here, we demonstrate with the aid of single molecule spectroscopy of well-defined fluorene:fluorenone copolymers that the green emission band frequently observed in polyphenylenes is of monomolecular origin. As a consequence of this fact we can spectrally identify a single oxidation step on a polymer molecule of high molecular weight.[1] F. Schindler et al., Angew. Chem. Int. Ed. 44, 1520 (2005).[2] J. G. Müller et al., Appl. Phys. Lett. 84, 1183 (2004).[3] F. Schindler et al., Proc. Natl. Acad. Sci. USA 101, 14695 (2004).[4] K. Becker et al., J. Am. Chem. Soc. 127, 7306 (2005).[5] K. Becker et al., Adv. Funct. Mater. (in press).
12:15 PM - M11.7
All Electronic Time of Flight Mobility Measurements in Pentacene Organic Field Effect Transistors.
Lawrence Dunn 1 2 , Debarshi Basu 1 , Liang Wang 1 , Ananth Dodabalapur 1
1 Microelectronics Research Center, University of Texas at Austin, Austin, Texas, United States, 2 Molecular Performance and Materials Cluster, Institute of Materials Research and Engineering, Singapore Singapore
Show AbstractOne of the most important figures of merit for an organic semiconductor is its mobility, which is the proportionality constant relating the velocity of charge carriers to the electric field in the material. In organic field-effect transistors (OFETs), the most common method of estimating mobility is to extract it from the transfer characteristics (e.g., Id-Vds and Id-Vdg curves). Time-of-flight measurements via optical excitation of charge carriers in the organic material are also used, but this technique measures the mobility of carriers perpendicular to their motion in the OFET geometry. Additionally, in some polycrystalline organic semiconductors, the optically excited time-of-flight measurements are rendered difficult due to charge trapping.In this work we present an all-electronic time of flight method for measuring the in-plane mobility of charge carriers using an applied voltage pulse to the source of the OFET with a rise time of < 60 ns. Pentacene transistors were fabricated with a channel width and length of 2000 μm and 7.5 μm, respectively. The device dielectric consisted of 1000 Å of SiO2 under 2000 Å of Si3N4. The device mobilities ranged from ~0.05-0.2 cm^2/Vs. The surfaces of both the gate insulator and metal contacts were treated to improve device performance. The gate of the OFET was grounded and the drain was connected to ground across a small load resistor. The transit time of the charge carriers from the source to the drain was measured by recording the voltage drop across the load resistor using a high-speed oscilloscope. These measurements were carried out between 300 K and 77 K. Our new method has also been used for mobility measurements in polythiophene transistors as well as n-channel OFETs. At higher temperatures, the transit time was typically on the order of ~100 ns, and increases to a few μs at lower temperatures. The mobilities extracted from the transit times at all temperatures are compared with the mobilities extracted from the DC transfer characteristics in the saturation regime. We observe significantly higher transit time mobilities at higher temperatures that merge with the DC mobilities at lower temperatures. This is significant because the transit time technique has the advantage of measuring the mobility of the fastest charge carriers, which at higher temperatures seems to occur before the charge carriers have had time to fall into deep traps, and also before the bias stress effect influences transport. By comparing the DC and transit time mobilities at varying temperatures we may gain an understanding of the time scale of the charge carriers’ interaction with traps in semiconducting material. We also note that in this new method, the role of electrical contacts in influencing the device characteristics and therefore the value of the extracted mobility is greatly reduced.
12:30 PM - **M11.8
The Push and Pull of Electrons in Variable Band Gap Conjugated Polyheterocycles.
John Reynolds 1
1 Chemistry, Univ. of Florida, Gainesville, Florida, United States
Show AbstractBy controlling the energy levels of the HOMO and LUMO states in conjugated polyheterocycles, along with the magnitude of the electronic band gap, optoelectronic properties can be optimized for specific applications. To this end, we have developed a family of soluble conjugated polymers with band gaps ranging from 3.2 eV – 1.5 eV and evaluated the utility of these polymers for device applications. Our use of 3,4-alkylenedioxythiophene (XDOT) donors, along with cyanovinylene and pyridopyrazine acceptors has led to the development of a representative class of soluble narrow gap (Eg = 1.5 to 1.8 eV) polymers; nicely absorbing across the solar spectrum. Regiosymmetric mid-gap (Eg = 2.0 to 2.5 eV) polymers are designed using thienylene and dialkoxyphenylene moieties to provide highly ordered semi-crystalline electron donor and hole carrying materials. The introduction of severe steric interactions decreases the extent of conjugation and yielding an elevated band gap. N-alkyl derivatized poly(3,4-alkylenedioxypyrroles) have band gaps greater than 3.0 eV, while retaining high HOMO levels due to the electron rich nature of the dioxypyrrole moiety. We will discuss fundamental optical and electrochemical data in order to establish the band structure of the newly synthesized polymers, along with presenting results from various device studies (photovoltaic and electrochromic) as platforms for materials property optimization.
M12: Active Layer Structures and Thin Film Studies
Session Chairs
Friday PM, April 21, 2006
Room 2006 (Moscone West)
2:30 PM - **M12.1
The Nanoscale Structure of Interfaces Between Semiconducting Polymers.
Richard Jones 1 , Anthony M. Higgins 1 , Simon Martin 1 , Mark Geoghegan 1 , Ilaria Grizzi 2 , Robert Cubitt 4 , Dalgleish Robert 3
1 Physics and Astronomy, University of Sheffield, Sheffield United Kingdom, 2 , Cambridge Display Technology Ltd, Cambridge United Kingdom, 4 , Institut Laue-Langevin, Grenoble France, 3 ISIS, Rutherford Appleton Laboratory, Chilton, Nr Didcot United Kingdom
Show AbstractInterfaces between different polymers are almost never atomically sharp, even when the polymers are strongly immiscible in the bulk. Instead, we expect them to have both an intrinsic diffuseness, resulting from mixing at the interface at the segment level, and roughness on various lengthscales resulting from a freezing in of thermally excited capillary waves. The width of interfaces between polymers may be measured with sub-nanometer resolution using neutron reflectivity.We have used neutron reflectivity to study the structure of interfaces between different semiconducting polymers and between a semiconducting polymer and a polymeric hole injection layer. We find that the interface is, as expected, diffuse on the nanoscale, and that the interfacial widths can vary very substantially as a function of processing conditions. We also find that the interfacial structure in a working light emitting diode can change after the device has been run. We discuss the implications of these findings in terms of designing processing routes to optimise device performance and lifetime.
3:00 PM - M12.2
Synchrotron X-ray Scattering Studies of Thin-films of Poly(thiophenes)
Michael Chabinyc 1 , Alberto Salleo 1 , Michael Toney 2 , R. Kline 3 , Iain McCulloch 4 , Heeney Martin 4 , Yiliang Wu 5 , Liu Ping 5 , Beng Ong 5
1 Electronic Materials Laboratory, PARC, Palo Alto, California, United States, 2 , Stanford Synchrotron Radiation Laboratory, Stanford, California, United States, 3 Polymers Division, National Institute of Standards and Technology, Gaithersburg, Maryland, United States, 4 , Merck Chemicals, Southampton United Kingdom, 5 , Xerox Research Centre of Canada, Mississuaga, Ontario, Canada
Show AbstractThe connection between the microstructure of thin films of semiconducting polymers and the field-effect mobility of charge carriers in these films is poorly understood. Processing conditions, such as thermal annealing, and interfacial chemistry are known to have a strong impact on field-effect mobilities of polymeric thin-film transistors (TFTs). We have used specular and grazing incidence synchrotron x-ray scattering to determine the structure and orientation of the crystalline domains in thin films of several high-performance poly(thiophene)-based semiconducting polymers including poly[5,5’-bis(3-dodecyl-2-thienyl)-2,2’-bithiophene] and poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophenes). Our studies show broadly that these materials contain domains whose extent and orientation can be controlled, in part, by thermal annealing. Thin films of PQT-12 were found to have similar structures on both bare and alkyltrichlorosilane-treated silicon dioxide despite these films having widely different field-effect mobilities (0.001 and 0.1 cm2/Vs respectively). The implications of these results for the analysis of new high performance semiconducting polymers will be discussed.
3:15 PM - M12.3
Nanocomposite Dendrimers for Application in Organic Electronic Devices
Sundarraj Sudhakar 1 , Mee Yoon Lo 1 , Alan Sellinger 1
1 , Institute of Materials Research and Engineering (IMRE), Singapore Singapore
Show AbstractNanocomposite dendrimers based on 3-dimensional cyclic phosphazene (CP) and silsesquioxane (SSQ) cores represent a new class of versatile materials for application in solution processable organic electronic devices. The dendrimers are prepared in high yield from readily available materials using Buchwald-Hartwig amination, Heck and Suzuki chemistry to decorate the periphery of the inorganic cores with conjugated dendrons having emissive, electron- and/or hole-transport properties. As the dendrimers are monodisperse, they can be purified to a high degree using chromatographic techniques. Furthermore, by varying the link of the dendron to the inorganic core from rigid to flexible, the morphology can be tuned from amorphous to semi-crystalline respectively. The dendrimers are soluble in common solvents, and form defect free thin films upon spin and/or dip coating. We will report on the synthesis, characterization, and preliminary device results using these nanocomposite dendrimers as active materials in organic electronic devices.
3:30 PM - **M12.4
Controlling Nanoscale Architecture in Optoelectronic Devices Using Polymer Brushes.
Wilhelm Huck 1
1 Chemistry, University of Cambridge, Cambridge United Kingdom
Show AbstractOver the last decade considerable progress has been made in the development of electronic devices based on charge transporting organic molecules.1 However, the low mobility of charges within these materials has limited the ultimate utility of these devices. In order to improve the characteristics of such devices further, it is essential to increase the ordering of the components of the organic layer to provide direct pathways for the movement of charge though the film.2 In this report we examine the use of polymer brushes, synthesized via a surface-initiated polymerization,3 as a method for alignment of hole-transporting polymer chains, out of the plane, in a thin film. Polymer brushes have previously been used to control surface properties such as adhesion, corrosion resistance and wettability,4 and recently as dielectric films in field-effect transistors.5 The vertically stretched nature of polymer brushes6 make them ideal candidates to incorporate as an electro-active component within organic semiconductor devices, potentially improving charge transport properties, providing clear pathways for charge transport in the direction normal to the substrate. Here we describe the synthesis of poly(triphenylamine acrylate) brushes from various surfaces, demonstrate the vertically stretched structure of these polymer chains, and show up to a 1000-fold increase in current density through the polymer brush film, as compared to a spin-coated film of the same polymer.Solution processing a second component with favorable enthalpic interactions can form a composite with mesoscale order and be exploited for heterojunction diodes. We find substantial uptake of CdSe nanocrystals (with diameter in the range 2.5-2.8 nm), and such composites show photovoltaic quantum efficiencies of up to 50%.
4:30 PM - M12.5
Structural Evolution of Hexa-peri-hexabenzocoronene Adlayers in Heteroepitaxy on n-Pentacontane Template Monolayer.
Luc Piot 1 , Alexandr Marchenko 1 , Jishan Wu 2 , Klaus Muellen 2 , Denis Fichou 1
1 LRC Nanostructures et Semi-Conducteurs Organiques (CNRS-CEA-UPMC), SPCSI/DRECAM, CEA-Saclay, Gif sur Yvette France, 2 , Max-Planck-Institute for Polymer Research , Mainz Germany
Show AbstractFriday, April 21Transfer Poster (M3.15) to Oral (M12.5)3:30 PMStructural Evolution of Hexa-peri-hexabenzocoronene Adlayers in Heteroepitaxy on n-Pentacontane Template Monolayer. Luc Piot
4:45 PM - M12.6
Short-channel Effects in Diodes and OFETs
David Taylor 1 , Francisco Gil-Martinez 1 , John Cambridge 1 , Simon Ogier 2
1 School of Informatics, University of Wales, Bangor, Bangor United Kingdom, 2 LC Manchester Technical Centre, Merck Chemicals Ltd, Manchester United Kingdom
Show AbstractFriday, April 21New Presentation Time and Paper NumberM12.9 to M12.6 3:45 PMShort-channel Effects in Diodes and OFETs. David Martin Taylor
5:00 PM - M12.7
Internal Phase Structures of Polyfluorene-based Blend Films and the Influence on Device Operations.
Yajun Xia 1 , Richard Friend 1
1 Dept. of Physics, University of Cambridge, Cambridge United Kingdom
Show AbstractFriday, April 21New Paper NumberM12.6 to M12.7Internal Phase Structures of Polyfluorene-based Blend Films and the Influence on Device Operations. Yajun Xia
5:15 PM - M12.8
Meso-Structure Formation for Enhanced Organic Photovoltaic Cells
Marisol Reyes-Reyes 1 , Kyungkon Kim 1 , Manoj Namboothiry 1 , David Carroll 1 , Roman Sandoval 2 , James Dewald 3 , Aditya Avadhanula 3 , Jamal Talla 3 , Seamus Curran 3
1 Physics, The Center for Nanotechnology and Molecular Materials , Wake Forest University, Winston Salem, North Carolina, United States, 2 , IPCYT Advanced Materials Department, San Luis Potosi Mexico, 3 Department of Physics, New Mexico State University, Las Cruces , New Mexico, United States
Show AbstractFriday, April 21New Paper NumberM12.7 to M12.8Meso-Structure Formation for Enhanced Organic Photovoltaic Cells. Manoj A.G. Namboothiry
5:30 PM - M12.9
Monolayer Thiophene Transistors and their Thiophene-Gold contacts.
Torsten Balster 1 , Tobias Muck 1 , Arne Hoppe 1 , Joerg Seekamp 1 , Veit Wagner 1
1 School of Engineering and Science, International University Bremen, Bremen Germany
Show AbstractFriday, April 21New Paper NumberM12.8 to M12.9Monolayer Thiophene Transistors and their Thiophene-Gold contacts. Veit Wagner