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Symposium O: Organic Thin-Film Electronics--Materials, Processes, and Applications

April 9 - 13, 2007

Chairs

Ana C. Arias Electronic Materials and Devices Laboratory Palo Alto Research Center 3333 Coyote Hill Rd. Palo Alto, CA 94304 650-812-4199		J. D. MacKenzie Add-Vision, Inc. Ste. 206 1500 Green Hills Rd. Scotts Valley, CA 95066 831-438-8192
Alberto Salleo Dept. of Materials Science Stanford University 239 McCullough Bldg. Stanford, CA 94305 650-725-1025		Nir Tessler Dept. of Electrical Engineering Technion-Israel Institute of Technology Haifa, 3200 Israel 972-4-829-4719

Symposium Support
Asahi KASEI Corporation
Merck Chemicals UK
Peptronics Ltd.


Proceedings to be published online
(see Proceedings Library at www.mrs.org/publications_library)
as volume 1003E
of the Materials Research Society
Symposium Proceedings Series.
This volume may be published in print format after the meeting.

---

* Invited paper

TUTORIAL

O: Organic Electronics-Materials Development, Device Physics, Processing and Applications
Monday April 9, 2007
1:30 PM - 5:00 PM
Room 2006 (Moscone West)

The tutorial is divided into three parts and covers materials development of polymeric semiconductors, device physics of organic transistors and device processing, as well as applications. Each module will be taught by a leader in the field. The tutorial will build knowledge from the ground up, starting with the molecular structure of the most relevant materials and continuing through the most exciting technological applications of organic semiconductors.

Materials Development Module---Iain McCulloch

The development of high-performing organic semiconductors, and the interrelationships between organic semiconductor molecular structure, thin-film morphology, and charge transport in field-effect transistors will be discussed. Particular emphasis will be placed on polymeric semiconductors, and the impact of backbone conformation and pi- electron delocalization and density on the frontier molecular orbital energy levels will be highlighted. The influence of the organic dielectric on device performance will be discussed. Thiophene and thienothiophene polymers will provide examples to illustrate molecular design principles, mechanisms of doping and degradation, and the effect of molecular weight and regioisomerisation on microstructure and charge transport.

Device Physics of Organic Transistors---Alberto Salleo

The basic working principles of field-effect transistors will be explained using relevant examples of device architecture from the recent literature. Non-ideal effects such as contact resistance and bias stress will be reviewed, with a particular emphasis on differences between organic and conventional semiconductors. Charge trapping and release, as well as electrical degradation, will be discussed in detail. Charge-transport models will be presented and their relationship to the molecular and microstructure will be explained.

Applications and Processing of Organic Thin-Film Semiconductors---J. Devin McKenzie

This module will be an overview of applications requirements for the performance of novel thin-film semiconductor devices and materials. These performance requirements will be placed in the context of the solution, print, and other novel processing approaches that have been principal motivating factors for research and development in these new classes of thin-film materials. Applications and process approaches including displays, RFID, photovoltaics, high-resolution printing, and coating will be discussed.

Instructors:
Iain McCulloch
Merck Chemicals Ltd.

J. Devin McKenzie
Add Vision Inc.

Alberto Salleo
Stanford University


SESSION O1: Synthesis of Materials I
Chair: Nir Tessler
Tuesday Morning, April 10, 2007
Room 2002 (Moscone West)

8:00 AM O1.1
Small-Molecule Design for Organic Electronics. John E Anthony, Chemistry, University of Kentucky, Lexington, Kentucky.

The ease with which small-molecule organic semiconductors can be functionalized allows a single chromophore framework to be tuned for use in a myriad of electronic applications. Further, substitution can dramatically increase the solubility of aromatic compounds, allowing device fabrication by solution processing. Such functionalization must take careful account of how substitution of the chromophore will change intermolecular interaction in thin films and crystals, as well as the desirability of such interactions in specific applications. Using 5-ringed acenes and heteroacenes as the chromophore, this report will describe the use of a straightforward functionalization approach to create organic materials tuned for use in organic transistors, organic solar cells and organic light emitting diodes. For transistors, extensive two-dimensional π-overlap was critical to obtaining high thin film mobility, yielding µ = 1.5 cm2 / Vs for solution-cast films of a simple substituted pentacene compound. Further tuning of the aromatic chromophore, coupled with the use of arenethiol treatments on gold electrodes to direct the crystallization of the organic semiconductor, have yielded solution-cast transistors with mobility greater than 4.0 cm2 / Vs, demonstrating the unique opportunities available to enhance the properties of organic semiconductor films grown from solution. In the case of bulk heterojunction organic solar cells, donors with strong π-stacking interactions crystallized too readily, causing complete segregation of donor and acceptor leading to poor photovoltaic performance. The addition of substituents that slightly disrupted π-stacking interactions yielded solar cells with power conversion efficiencies greater than 1%. Further disruption of π-stacking in these systems again led to poor photovoltaic performance. For light-emitting diodes, any close contacts between chromophore atoms in the solid state can lead to broadening of the emission spectrum and decreased device performance. In this case, functionalization that leads to complete isolation of the chromophore yielded the best results, with red-emission efficiency greater than 3%.


8:15 AM O1.2
Solvent Vapor Annealing Improves Device Characteristics of Transistors with Solution-Processable Triethylsilylethynyl Antradithiophene. Kimberly C. Dickey¹, John E. Anthony² and Lynn Loo¹; ¹Chemical Engineering, University of Texas at Austin, Austin, Texas; ²Chemistry, University of Kentucky, Lexington, Kentucky.

The development of organic semiconductors is driven by the promise of low-cost device applications. To fully realize cost-effective organic electronics, solution-processable materials need to be developed. While several solution-processable materials have been demonstrated, these materials often suffer from significantly reduced carrier mobilities due to defects and grain boundaries introduced during the deposition process. We have been studying triethylsilylethynyl anthradithiophene (TES ADT), a solution-processable, p-type organic semiconductor. Transistors fabricated with spun-cast TES ADT exhibit low carrier mobilities (0.002 cm²/V-s), low on-off current ratios and significant current-voltage hysterisis. Subjecting the fabricated transistors to dichloroethane solvent vapor annealing, however, yields average carrier mobilities of 0.2 cm²/V-s, high on-off current ratios (10^4-5), and significantly reduces the current-voltage hysterisis. This dramatic improvement in transistor performance is solvent choice dependent, and can be directly correlated with morphological transformations in the thin films. Specifically, the solvent vapor is able to partition into the organic semiconductor thin film during the annealing process to induce structural rearrangment. TES ADT crystallizes as a consequence. The improvement in device characiteristics appear to be directly correlated with the grain size within the thin films. The polarity of the solvent, on the other hand, has a dramatic impact on the threshold voltage. In general, polar solvents can induce the presence of a dipole barrier at the organic semiconductor-dielectric interface, thereby increasing the threshold voltage. Annealing with non-polar solvents, like hexanes, results in a threshold voltage that is close to zero.


8:30 AM O1.3
Theoretical studies of P3HT, PQT, and PBTTT John Northrup, Palo Alto Research Center, Palo Alto, California.

There is considerable interest in soluble polymer semiconductors such as P3HT [1], PQT [2] and PBTTT [3]. These materials have hole mobilities exceeding 0.1 cm2/Vs. As a consequence there is an impetus to determine the atomic and electronic structure of the crystalline lamellae exhibited by these materials. It remains a very challenging problem to determine the structure from experiment alone. Energy minimization calculations were therefore performed using the density functional theory to determine the atomic structure of these materials. A tilting of the plane containing the polymer backbone is found to be energetically favorable for all three materials. It is argued that this tilting is a general feature of this class of materials, and that this tilting has a significant affect on the electronic and optical properties. The impact of the deviation from ideal cofacial pi-stacking on the hole mobility will be discussed within a simple model of the scattering.[4] [1] Z. Bao, A. Dodabalopur, and A. J. Lovinger APL 69, 4108 (1996). [2] B. S. Ong, Y. Wu, P. Liu, and S. Gardner, J. Am. Chem. Soc. 126, 3378 (2004). [3] I. McCulloch et al. Nature Materials 5, 328 (2006). [4] R. A. Street, J. E. Northrup and A. Salleo, Phys. Rev. B 71, 165202 (2005).


8:45 AM O1.4
Comparative Study of Charge Transport in High-mobility Microcrystalline Conjugated Polymers Ni Zhao¹, Jui-Fen Chang¹, Marta Tello¹, Henning Sirringhaus¹, Iain McCulloch² and Martin Heeney²; ¹Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom; ²Merck Chemicals, Chilworth Science Park, Southampton, United Kingdom.

The performance of polymer field-effect transistors (FETs) crucially depends on the intermolecular interaction in the semiconductor polymer, as it dominates the charge conduction in the active layer. Recently a new class of semiconducting liquid-crystalline polymers, poly(2,5-bis(3-alkylthiophen-2-yl) thieno[3,2-b]thiophene) (PBTTT) [1], was reported with a considerably increased mobility (up to 0.6 cm²/Vs) as compared with the prototype microcrystalline conjugated polymer, poly(3-hexylthiophene) (P3HT). It is believed that such an increase of the mobility is due to the enhanced intermolecular interaction in the PBTTT thin films. However, microscopic charge transport studies that support the correlation between the microstructure and the mobility are still lacking. Here we present a comparative study on the nature of the charge transport species in the P3HT and PBTTT systems with the aim of understanding the difference of the charge carrier mobility from a molecular scale. It has been discovered recently that upon crystallizing from the melting phase a highly ordered film structure consisting of aligned long nanoribbons can be achieved in PBTTT films [2]. These nanoribbons are arrays of π-π stacked extended chain lamellae with a width that approximates to single molecular length. We focus our studies on this morphology because, unlike the chain folding arrangement in P3HT nanoribbons, the PBTTT molecules are able to form a system with fewer folding-induced intrachain defects and enhanced intermolecular interaction. By using charge modulation spectroscopy we observe that the charge transfer (CT) transition in PBTTT films with the nanoribbons exhibits the signature of strong electronic coupling as a result of interchain polaron delocalization and narrower band width as compared to P3HT films. We also perform Scanning Kelvin probe microscopy to directly measure the local field-effect mobility in the PBTTT FETs without being affected by the contact resistance. This allows us to obtain a clear correlation between the microstructure and the charge conduction in the semiconducting polymer films. [1] Iain McCulloch, et. al., Nature Materials 5, 328, (2006). [2] Dean M. DeLongchamp, et. al., 2006 MRS spring meeting, M4.5


9:00 AM *O1.5
The Development of pBTTT. Iain McCulloch¹, Clare Bailey¹, Martin Heeney¹, Maxim Shkunov¹, David Sparrowe¹, Steve Tierney¹, Michael Chabinye² and R. Joseph Kline³; ¹Merck Chemicals, Southampton, United Kingdom; ²Palo Alto Research Center, Palo Alto, California; ³National Institute of Standards and Technology, Gaithersburg, Maryland.

The further development of organic semiconductors to be utilised in field-effect transistors (OFETs) for incorporation into lightweight, flexible electronic devices, requires high performing, solution processable materials. Polymeric semiconductors offer an attractive combination in terms of appropriate solution rheology for printing processes, mechanical flexibility for rollable processing and applications, but their electrical performance has been significantly lower than amorphous silicon, limiting their application. In this work, we report the design, synthesis and characterisation of a range of liquid crystalline pi conjugated thieno[3,2-b]thiophene co-polymers (pBTTT) exhibiting high charge carrier mobilities. A detailed study of the polymer thin film morphology by both AFM and XRD reveals an extremely ordered and oriented microstructure. Field effect transistors were fabricated, and their performance and lifetimes will be discussed.


9:30 AM O1.6
Reliable Suzuki Chemistry For Functionalised Polythiophene Synthesis. Simon J. Higgins¹, Iain A. Liversedge¹, Iain McCulloch², Mark Giles² and Martin Heeney²; ¹Chemistry, University of Liverpool, Liverpool, United Kingdom; ²Merck Chemicals Ltd., Southampton, United Kingdom.

Regioregular polyalkylthiophenes and their derivatives have been widely-studied as polymeric organic semiconductors. Synthetic chemists strive to improve the already impressive field effect mobilies of these materials, while increasing their stability to ambient conditions and their purity. We recently described the application of functionalised polyalkylthiophene derivatives in biosensing (see M. Fouzi et al., Chem. Comm. 2004, 2314, and S.J. Higgins et al., Mater. Res. Soc. Symp. Proc. 871E, Warrendale, PA, 2005, I1.3). Additionally, functionalised polyalkylthiophene derivatives designed for improved intermolecular interaction (e.g. via H-bonding) might show improved transistor performance (higher field effect mobility). Therefore, in spite of the success of the conventional Grignard and organozinc-based routes to the parent polyalkylthiophenes, it is important to devise new chemistry, applicable to the synthesis of functionalised derivatives. We have therefore investigated Pd complexes of bulky, electron-rich phosphines as catalysts for the polymerisation of halothiophene boronate ester derivatives via Suzuki coupling. With the best catalysts, we can obtain >98% regioregular polyalkylthiophenes with useful molecular weights (MW 17000). Importantly, we have also found that it is possible to synthesise the key AB-type monomers by Ir-catalysed boronation of 2-halo-3-alkylthiophenes. This is significant because it means that we can synthesise the monomer without recourse to strong organolithium bases, making the route truly functional group-tolerant.


9:45 AM O1.7
Non-conjugated Polymer Hosts with High Triplet Energies Lan Deng¹, Biwu Ma² and Jean M. J. Frechet^1,2; ¹Department of Chemsitry, University of California, Berkeley, California; ²Material Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California.

Incorporating carbazole units into polymers to make high triplet energy host materials is essential for efficient solution-processible OLEDs. When carbazole units are used as building blocks in the main chain of the conjugated polymers, the resulting polymer hosts typically have triplet energies around 2.5-2.6 eV. Host materials with higher triplet energies are difficult to obtain using this approach due to the decrease of triplet energy with the increase of conjugation length. We hereby report a different way to incorporate carbazole units into polymers through side-chains. Using the new approach, the triplet energies of the polymers can remain the same as that of their small molecular building blocks. Polymers with triplet energies as high as 3.0 eV have been obtained. OLEDs have been fabricated, showing external quantum efficiencies of 4-5 %.


10:30 AM *O1.8
Synthetic And Processing Strategies To High Performance Organic Thin-Film Transistors. Antonio Facchetti, Northwestern University, Evanston, Illinois.

The general design and synthesis of new rylenes and oligo/thiophenes functionalized with a variety of phenacyl, alkylcarbonyl, and perfluoroalkylcarbonyl is presented. These organic semiconductors exhibit low-lying LUMOs allowing efficient electron injection/transport under ambient atmosphere. Organic thin-film transistors (OTFTs) fabricated via conventional methods exhibit electron mobilities as high as 2 cm^2 V^-1 s^-1 for vapor-deposited films and 0.3 cm^2 V^-1 s^-1 for solution-cast films, with current modulation as high as 10^8. Furthermore, unconventional processing approaches to addressing the realization of well-defined supramolecular architectures with precise, nanometer-level control of bulk electronic properties are described. The ultimate goal is the realization of inexpensive electronic circuits employing unconventional materials classes and simple fabrication techniques.


11:00 AM O1.9
In-plane Molecular Alignment in Thin Films of Pentacene Grown by Solution Casting and Performance of Thin Film Transistors. Takashi Minakata¹, Yutaka Natsume¹, Yuji Yoshida² and Yasukiyo Ueda³; ¹Central R&D Laboratories, Asahi-KASEI Corporation, Fuji-shi, Shizuoka-ken, Japan; ²National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki, Japan; ³Kobe University, Kobe, Hyogo, Japan.

Thin film preparation of organic semiconductor via solution process attracts significant attention since it is essential to fabricate large-area and low-cost electronic devices, such as display thin film transistors, identification tags, and sensors. We have reported the direct formation method for pentacene thin-films with solution casting 1). In this method, no particular precursor molecule(s) have been used during the process and thin films of pentacene can be fabricated directly using solution of pentacene. The solution-processed pentacene thin-films showed higher crystallinity than that of sublimed films and exhibited good transistor performance with a carrier mobility above 1cm2/Vs by selecting fabrication conditions. In this report, we have studied about molecular alignment in the solution-processed thin films of pentacene by several kinds of structural analysis such as atomic force microscopy, polalizing microscopy, in-plane grazing incidence X-ray diffraction and transmition diffraction electron beam diffraction. Crystalline structure of solution-grown thin films of pentacene is almost identical to bulk phase of pentacene. Unexpected results indicated that in-plane molecular alignment in crystalline domain in the films was not isotropic and strong in-plane crystalline anisotropy in the films was observed by grazing X-ray diffraction. By polarized microscopy, oriented large domains in the films were identified. Then, we have determined molecular orientation in a single oriented domain of solution grown thin film of pentacene by transmition electron beam diffraction. It turned out that priority growth along b-axis of pentacene crystal from the solution was confirmed. Trials to grow large crystalline domains in the films were made by controlling fabrication conditions. Thin films with regularly oriented domains were successfully fabricated using one directionally growth from solution casting. Thin film transistors (TFTs) with bottom contact and bottom gate structure were formed applying the pentacene solution on the substrates with source-drain electrode patterned on surface oxidized silicon. The observed maximum carrier mobility of the FETs was 2.7 cm2/Vs and this is identical to the single crystal value 2). It indicated the possibility to eliminate grain boundaries in the FET channel and to form single crystalline domain along the channel. The solution-grown platelet crystals of pentacene tend to form cracks parallel to crystal growth direction in the films. Preferred performance of FETs with platelet crystals grown perpendicularly to the channel was observed, since the damage by the cracks formed parallel to the channel was minimized in the film. These results were assumed to be useful for improving the performance and the reliability of printing FETs for the future applications. 1)T. Minakata and Y. Natsume, Synthetic Metals, 153, 1 (2005) 2)J. Y. Lee, S. Roth and Y. W. Park, Appl. Phys. Lett., 88, 252106 (2006)


11:15 AM O1.10
High Field-Effect Mobilities for Diblock Copolymers of rr-P3HT Genevieve Sauve and Richard D. McCullough; Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania.

One important challenge in the field of organic field-effect transistors (OFET) is to better understand the relationship between structure and transport properties of conjugated polymers. Much work has focused on regioregular poly(3-hexylthiophene) (rr-P3HT), because of its excellent electrical properties and ease of synthesis. To further tune the structure and properties of rr-P3HT, we have synthesized several well-defined diblock copolymers of rr-P3HT, where the second block is a coil polymer. Here we present surprising results of high mobilities for our diblock copolymers of rr-P3HT and poly(methylacrylate) (PMA), despite the presence of various amounts of insulating PMA. This was observed when the silicon dioxide dielectric layer was treated with octyltrichlorosilane. In contrast, mobilities went down with PMA content when the silicon dioxide was not treated. It is therefore possible to control the self-assembly of these block copolymers at the dielectric layer-semiconductor interface by using different SiO2 surface treatments. These results also demonstrate the promise of using block copolymers as organic semiconductors in plastic electronics.


11:30 AM O1.11
Abstract Withdrawn

SESSION O2: Materials Physics and Characterization I
Chair: Alberto Salleo
Tuesday Afternoon, April 10, 2007
Room 2002 (Moscone West)
1:30 PM *O2.1
Spectroelectrochemistry Studies of the Charging and Discharging of Single Conjugated-Polymer Nanoparticles. Paul F Barbara, Rodrigo E. Palacios, Allen J. Bard and Fu-Ren F. Fan; Department of Chemistry and Biochemistry, Center for Nano and Molecular Science and Technology, University of Texas, Austin, Texas.

The unique ability of single molecule spectroelectrochemisry, SMS-EC, to unravel complex electrochemical process in heterogeneous media is used to study the oxidation of nanoparticles of the practically important conjugated polymer poly (9,9-dioctylfluorene-co-benzothiadiazole) (F8BT). Two main processes have been observed, an irreversible chemical reaction on the surface of the oxidized F8BT nanoparticles, and a reversible hole-injection charging process. The latter occurs primarily by initial injection of shallow (untrapped) holes, but soon after the injection, the holes become deeply trapped. Good agreement between experimental data and simulations strongly supports the presence of deep traps in the studied nanoparticles and highlights the ability of SMS-EC to study deep traps in organic materials at the nanoscale.


2:00 PM O2.2
On the Validity of Continuum Drift-Diffusion Device Models for Organic Light Emitting Diodes. B. Ramachandhran¹, M. A. Uijttewaal², M. Bouhassoune¹, P. A. Bobbert¹, R. A. De Groot², G. A. De Wijs² and Reinder Coehoorn^3,4; ¹Polymer Physics Group, Department of Applied Physics, Eindhoven University of Technology, Eindhoven, Netherlands; ²IMM, Radboud University, Nijmegen, Netherlands; ³Philips Research Laboratories, Eindhoven, Netherlands; ⁴Molecular Materials and Nanosystems Group, Department of Applied Physics, Eindhoven University of Technology, Eindhoven, Netherlands.

It has recently been demonstrated by Pasveer et al. that the energetic disorder of amorphous polymer organic semiconductors used in organic light emitting devices (OLEDs) can give rise to a strong carrier concentration dependence of the mobility, and that this can have a significant effect on the current density (J) versus voltage (V) curves of polymer-based hole-only devices [1]. In ref. 1, the mobility was obtained from a numerically exact Master-Equation (ME) approach, assuming a Gaussian density of states (DOS). For obtaining the J(V) curves, a continuum solution of the coupled transport and Poisson equations was used, assuming ohmic contacts and neglecting diffusion. However, one might question under which conditions such a continuum approach is valid. In actual OLEDs, large carrier concentration gradients can occur near external and internal interfaces, and the current density can have a strongly filamentary nature [2,3]. The conductivity is then a non-local property, so that the concepts of ‘mobility’ and ‘diffusion coefficient’ could no longer be useful. In this contribution, we present the results of a numerical investigation of the validity of the continuum approach, by making a comparison with the results from ME calculations for complete metal/organic/metal devices. We have varied the (uncorrelated) Gaussian disorder (width σ), the layer thicknesses (20 to 100 nm), and the interface conditions. For the continuum drift-diffusion calculations, the compact expressions for the mobility given in [1] were used, with the diffusion coefficients as obtained using the generalized Einstein equation. We confirm the highly filamentary nature of the current density and present a quantification of the current density distribution. We also present a 3D visualization of the non-uniform current density. Even for highly disordered systems (σ = 6k_BT) and for layer thicknesses of only 20 nm, for which the current is shown to be strongly filamentary, the continuum model is found to be surprisingly well obeyed. We have investigated the effect of injection barriers (and image potentials), by making use of two types of models: (i) by assuming a fixed carrier concentration at the outermost planes of the organic layer (as in [2]), (ii) by self-consistently calculating all carrier concentrations in all layers (beyond [2]). A critical comparison is given of the calculated J(V) curves and the predictions as obtained from a continuum drift-diffusion model, including the Scott-Malliaras [4] and Arkhipov [5] models for injection in an ordered system and in a system with a Gaussian DOS, respectively. [1]. W.F. Pasveer et al., Phys. Rev. Lett. 94 (2005), 206601. [2]. E. Tutiš, I. Batistić and D. Berner, Phys. Rev. B 70, 161202 (2004). [3]. K.D. Meisel et al., Phys. Stat. Sol. 3, 267 (2006). [4]. J.C. Scott and G.G. Malliaras, Chem. Phys. Lett. 299, 115 (1999). [5]. V.I. Arkhipov et al., J. Appl. Phys. 84, 848 (1998).


2:15 PM O2.3
Photoinduced Charge Transfer at Hybrid Semiconductor Interfaces. Juan Cabanillas-Gonzalez, Hans Joachim Egelhaaf, Guglielmo Lanzani, Alberto Brambilla, Marco Finazzi, Lamberto Duo and Franco Ciccacci; Physics, Politecnico di Milano, Milano, Italy.

We monitor in real time photoinduced charge injection at the interface between a fluorinated copper phthalocyanine layer (CuPcF16) deposited by thermal evaporation on top of a p - doped GaAs (100) wafer. Preliminary information on the electron affinity of CuPcF16 (5.2 eV respect to vacuum level) combined with photoemission measurements indicates an energy offset of 1.1 eV for the GaAs conduction band respect to the CuPcF16 LUMO level. This suggests that charge transfer from inorganic to organic is feasible. We study bilayers of GaAs and CuPcF16 thin films (100 nm) by pump - probe spectroscopy with 200 fs time resolution. Pump photons at 2.3 eV excite the GaAs substrate resonantly through the organic layer whereas probe photons in the visible range, reflected by the GaAs surface, monitor induced changes at the interface. We observe a strong photoinduced absorption band centered around 2.2 eV which appears during the pulse duration, shows a build-up dynamics and persists beyond 0.5 ns. This band cannot be attributed to single material contribution, excluded by test experiments with single layers. By applying charge modulation spectroscopy we identified charge state absorption in CuPcF16 in the same spectral region of the photoinduced absorption band. We thus assign our transient dynamics to formation of CuPcF16 ions at the interface, following charge injection. On account of the rapid charge formation we identify this system as a potential candidate for the fabrication of hybrid photodiodes.


2:30 PM *O2.4
Comparing Theory and Hypotheses to Electric Force Microscope Data: Are We Thinking about Injection, Trapping, and Transport Correctly in Organic Electronic Materials? John A. Marohn¹, Michael Jaquith¹, Showkat Yazdanian¹, Michael Chabinyc² and Tse Nga Ng^1,2; ¹Dept. of Chemistry and Chemical Biology, Cornell University, Ithaca, New York; ²Palo Alto Research Center, Palo Alto, California.

In order to determine energetic disorder’s role in facilitating charge injection from gold into a molecularly doped polymer, we have examined the dependence of current on local electric field, measured using electric force microscopy (EFM), at temperatures ranging from 250 to 330 K. From these data we infer, for the first time in a single experiment, the temperature dependence of the main factors governing charge injection: the electric-field induced lowering of the image-potential barrier, the interfacial charge density, and the mobility. In this system, the Schottky effect is anomolously large, and the interfacial charge density is larger than expected and strikingly non-Arhennius. Our analysis indicates that these effects are all a consequence of the Gaussian density of states in the organic. We have also used EFM to study charge trapping in as-deposited polycrystalline pentacene. In films with comparatively large grains, EFM images reveal that charges trap primarily at grains near the pentacene-metal interface. By contrast, in films with small grains, EFM images show that long-lived holes traps are dispersed throughout the film. These traps, however, do not appear to be associated with grain boundaries, as is often assumed. We find that charge traps require 100’s of milliseconds to form, implying that traps do not form instantaneously by the emptying of levels but instead arise from a reaction having finite activation energy. These experiments show that a new, improved microscopic view of charge injection and trapping can be gained by making scanned probe measurements of local electrostatic potential and capacitance. It would therefore be exciting if a local measurement of carrier mobility or diffusion constant could also be made. We have recently shown that low-spring-constant cantilevers can be used to observe electric field fluctuations arising from thermal dielectric fluctuations in polymers. In this talk we will show that ultrasensitive cantilevers can likewise measure the local charge diffusion constant via the effect of the associated electric field fluctuations on cantilever frequency and ringdown time. Analytical scaling laws and numerical simulations of the electric field power spectrum resulting from the thermal motion of holes in a N,N'-diphenyl-N-N'-bis(3-methylphenyl)-(1,1'-biphenyl)-4,4'-diamine (TPD) / polystyrene field effect transistor suggest that the local hole diffusion constant can be inferred in this system from measurements of cantilever frequency and ringdown time as a function of tip height and charge density. Progress towards directly testing the Einstein relation by comparing the locally measured charge diffusion constant to the bulk field effect transistor mobility will be reported.


3:30 PM O2.5
Effects of Different Length-Scale Organic-Organic Heterojunctions on Photophysical Processes. Annamaria Petrozza¹, Igor Avilov², Richard Friend¹ and Ji-Seon Kim¹; ¹Physics, University of Cambridge, Cambridge, United Kingdom; ²Laboratory for Chemistry of Novel Materials, University of Mons-Hainaut, Mons, Belgium.

Recent works have highlighted the importance of organic-organic semiconductor heterojunctions, in particular interchain heterojunctions formed in blend systems. However, there has been very little work to address the photophysical processes which depend on the nature of the organic interfaces (i.e. interchain vs intrachain) and the length-scale of these interfaces (i.e. from micron-scale in blends to molecular-scale in random copolymers). Here, we report the detailed studies of photophysical processes including triplet absorption and decay dynamics on a range of well-controlled organic-organic interfaces, which are formed both as a function of the polymer chain-length (molecular weight, MW) and of the different nature of molecular units present within the polymer chain. We have studied spin-coated films of poly (9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) of different molecular weights (Mn = 9K - 255K) both in the pristine and annealed states and F8BT random copolymer (RC) synthesised by random copolymerisation with Poly(9,9’-dioctylfluorene-co-N-(4-butylphenyl)diphenylamine) (TFB), which result in the presence of both strong electron acceptor and donor units within the polymer chain. We observe that the triplet absorption increases in the pristine films as the MW of F8BT decreases; upon annealing above the glass transition temperature of F8BT triplet dynamics differ depending on the MW: high MW samples show a strong enhancement of the triplet absorption, whereas the lowest MW sample shows a decrease in the intensity of the PIA signal; RC present a much stronger triplet absorption and the main T1-->Tn transition band is blue-shifted compared both to pure F8BT and F8BT:TFB blends. We conclude that the triplet generation efficiency based on the inter-system crossing (ISC) mechanism is significantly reduced in high molecular weight pristine films, and the triplet lifetimes are enhanced upon annealing. We consider that these changes are strongly correlated with a solid state packing of F8BT molecules which varies as a function of molecular weight and upon annealing, i.e. different packing structures will induce different relative population of the emissive states which are differently coupled to the non-emissive triplet states. Previous studies have demonstrated that the ISC process in blends systems is efficiently mediated by interfaces induced excited states, such as very short lived inter-chain charge transfer (CT) states and long lived exciplex states. We consider that in the RC the ISC process is even more efficient than in blend systems due to its long lived intra-chain CT state and a molecular-scale phase separation. Our results clearly demonstrate the importance of the nature of the organic-organic interfaces and their length-scale on photophysical processes. A potential control of these well-defined structures provides a powerful tool for the smart design of organic semiconductors for diversified applications.


3:45 PM *O2.6
Microstructure Foundations of High Carrier Mobility in Polymers. Dean Michael DeLongchamp¹, R. Joseph Kline¹, Eric K. Lin¹, Daniel A. Fischer¹, Youngsuk Jung¹, Lee J. Richter¹, Iain McCulloch², Martin Heeney² and John E. Northrup³; ¹Polymers Division, National Institute of Standards and Technology, Gaithersburg, Maryland; ²Merck Chemicals, Southampton, United Kingdom; ³Palo Alto Research Center, Southampton, California.

The microstructure of organic semiconductor films can impact charge carrier mobility because it defines the persistence and quality of π overlap in the source-drain plane. Important aspects of microstructure include the intermolecular packing arrangement within crystals, the substrate-relative crystal orientation, and the overall crystal size and connectivity. We combine complementary microstructure measurements including polarized absorption spectroscopies (infrared, visible, and X-ray), scanning probe techniques, and X-ray diffraction (XRD) to investigate the microstructure details of polymer semiconductors for organic thin film transistors (OTFTs). The investigations establish correlations between primary chemical structure, processing, film microstructure, and carrier mobility. These fundamental relationships yield practical guidelines for synthesis and processing. <p>Here we demonstrate this approach by solving the packing arrangement of a polymer semiconductor, poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophenes) (pBTTTs), with hole mobility of 0.2 to 0.6 cm²/Vs. XRD of pBTTT films indicates lamellar packing, and atomic force microscopy (AFM) reveals terraces several microns wide. Spectral ellipsometry (SE) indicates that polymer long axes are confined to the film plane. The XRD, AFM, and SE evidence supports a narrow orientation distribution, which permits tilt angle assignment from linearly polarized spectroscopies. Near edge X-ray absorption fine structure (NEXAFS) spectroscopy indicates a conjugated plane tilt that strongly impacts electron and hole bandwidths. NEXAFS combined with Fourier transform infrared spectroscopy (FTIR) reveals nearly all-trans side chains that strongly tilt; reconciled with the XRD lamellar spacing it proves that vertically adjacent layers interdigitate. A general consideration of side chain configuration reveals a striking signature packing motif that sets high performance polymers such as pBTTT apart from the lower performance poly(3-alkylthiophenes). A simple model provides a synthetic design rule describing the side chain graft density necessary to achieve this signature packing. Similar analyses explain the influences of dielectric chemistry, dielectric roughness, and thermal history, and the mechanisms by which each impacts semiconductor microstructure and OTFT performance.


4:15 PM O2.7
Potential Mapping on Organic Devices: 3D Simulations and Experiments. Dimitri S.H. Charrier¹, Martijn Kemerink¹, Barry E. Smalbrugge², Tjibbe de Vries² and Rene A.J. Janssen¹; ¹Applied physics, Eindhoven University of Technology, Eindhoven, Netherlands; ²COBRA Research Institute, Eindhoven University of Technology, Eindhoven, Netherlands.

Scanning Kelvin Probe Microscopy and Electric Force Microscopy offer a unique opportunity to measure local (surface) potentials with nanometer resolution on actual devices. Therefore, these techniques are becoming more and more popular for characterizing physical aspects of organic thin film devices [1] like charge transport in polymer transistors [2] and charge generation in organic solar cells [3]. Unfortunately these techniques are extremely sensitive to the entire experimental geometry which severely limits the obtainable resolution. Here we present a numerical model that enables a quantitative description of experimental curves on a number of relevant organic devices. In addition, a strong and unexpected charging of common dielectric materials is found. We have focused our experiments and simulations on a study of the surface potential of samples with a standard organic transistor layout without active layer. A potential difference was applied between two flat electrodes, i.e. source and drain in FET operation, separated by an insulating micron-sized channel. It appeared that the measured potential profile strongly depends on whether the tip is parallel or orthogonal to the channel. Moreover, only a fraction (~0.6-0.9) of the applied voltage was observed in measured voltage traces. In order to understand the electrostatic interaction between the tip and the surface, and to explain these results we simulated the entire system of tip consisting of apex, cone and cantilever and sample in three dimensions using a commercial finite element program. The 3D simulations work for arbitrary potentials and realistic tip and cantilever geometries. It is shown to nicely reproduce the experimental features and yields valuable information on experimentally obtainable resolution versus tip-sample distance and tip-channel orientation. The increasing resolution with decreasing tip-sample distance is found to be due to the relative increase of the contribution of the apex area to the total probe-sample capacitance. The simulations fit the experimental surface potential using reasonable numerical parameters. Using the numerical tool to interpret experimental potential profiles of channels on different substrate materials, we found, to our surprise, that supposedly charge-free, insulating substrates do not at all behave as ideal dielectrics, but show significant charging effects when bias-stressed under ambient conditions. [1] S. Palermo, M. Palma, and P. Samorì, Adv. Mater. 2006, 18, 145. [2] L. Bürgi, T. Richards, M. Chiesa, R.H. Friend, and H. Sirringhaus, Synth. Met. 2004, 146, 297. [3] D.C. Coffey, and D.S. Ginger, Nature Mater. 2006.


4:30 PM O2.8
Vibrational Spectroscopy Reveals Field and Electrochemical Doping in Organic Thin Film Transistors. Loren G. Kaake¹, Ying Zou¹, Matthew J. Panzer², C. Daniel Frisbie² and Xiaoyang Zhu¹; ¹Chemistry, University of Minnesota, Minneapolis, Minnesota; ²Chemical Engineering & Materials Science, University of Minnesota, Minneapolis, Minnesota.

The nature of charge carriers and traps in organic thin film transistors (OTFTs) has been a subject of much speculation and debate. We have developed a simple strategy to record vibrational spectra of buried interfaces in OTFTs. In this approach, an OTFT is fabricated on an IR waveguide (Si or Ge) and the evanescent field probes the molecular species in the transistor under gate bias. In particular, we use a polymer electrolyte dielectric material to obtain a very high electric field at the dielectric-organic interface. In the case of a PTCDI based OTFT, we show molecular evidence for two distinctive regions of doping: at low gate voltages (< 3V), the organic semiconductor is under field doping as evidenced by a reversible loss of vibrational intensity associated with the neutral PTCDI molecule. At high gate voltage (3-7 V), the film is electrochemically doped, as evidenced by the conversion of neutral PTCDI molecules into anions. The electrochemical doping region is less reversible and is accompanied by chemical changes to the organic film. This demonstrates the strategy as capable of elucidating the chemical consequences of gate doping.


4:45 PM O2.9
Electrical Properties of Conjugated Polymers under High Pressure Sheena K. Elliott, Jeroen Cottaar and Neil C. Greenham; Department of Physics, University of Cambridge, Cambridge, United Kingdom.

The application of hydrostatic pressure allows the role of intermolecular interactions in conjugated polymers to be studied. We use a pressure cell to operate a polymer light-emitting diode (LED) or transistor at up to 8 kbar. In contrast to optical measurements using a diamond anvil cell, this method allows the pressure dependence of the charge carrier mobility to be studied. In LEDs based on amorphous poly(p-phenylenevinylene) derivatives, we see a small increase in mobility up to 1 kbar, followed by a decrease at higher pressures due to increasing disorder. In the polyfluorene derivative F8BT, however, we see a much stronger increase in mobility with pressure, leading to an increase in device current by up to a factor of 10 at 6 kbar. We attribute this increase to a change in chain conformation at high pressures leading to improved interchain contact between the benzothiadiazole units. We will discuss the implications of these results for the inter-chain transfer integrals in conjugated polymers, and will also present measurements of the electroluminescence spectrum as a function of pressure.


SESSION O3: Poster Session: Materials, Devices and Characterization I
Chairs: Ana Claudia Arias, J. Devin MacKenzie, Alberto Salleo and Nir Tessler
Tuesday Evening, April 10, 2007
8:00 PM
Salon Level (Marriott)
O3.1
Abstract Withdrawn


O3.2
Performance Enhancement of Top-emitting Organic Light-emitting Diodes Using Self-Assembled Monolayer-Modified Ag Anode. Lai-Wan Chong, Yuh-Lang Lee and Ten-Chin Wen; Chemical Engineering, National Cheng Kung University, Tainan, Taiwan.

Ag anode with self-assembled monolayers (SAM) of 4-flourothiophenol (4-FTP) is employed to construct an efficient top-emissive polymer light-emitting diodes. The reflectivity of Ag not only does not decrease by the 4-FTP SAM, but also is slightly enhanced. A high brightness of 68981 cd/m2 in the forward direction of the substrate and an electroluminescence efficiency of 10.3 cd/A are achieved for the 4-FTP device. The improved performance is attributed to the presence of fluorine atom on 4-FTP. In order to cross check the effect of 4-FTP, the device with the TP SAM was fabricated to make a comparison.


O3.3
Abstract Withdrawn


O3.4
Deposition and Patterning of Conductive Carbon Black Thin Films. Jaime C. Grunlan^1,2,3, Matthew Walton¹, Yeon Seok Kim^1,3, William Neal Everett^1,3, C. Jason Jan^1,3 and Woo-Sik Jang^1,3; ¹Mechanical Engineering, Texas A&M University, College Station, Texas; ²Materials Science and Engineering, Texas A&M University, College Station, Texas; ³Polymer Technology Center, Texas A&M University, College Station, Texas.

Using cationic polyethylenimine (PEI) and anionic poly(acrylic acid) (PAA), the effect of compositional variables on construction of thin carbon black polymer composites using layer-by-layer (LbL) deposition was studied. Processing variables include polymer ratio variation, polymer ratio mismatch, absence of conductive filler from one solution, and ionic strength manipulation. Films made with 0.25 wt% carbon black in 0.05 wt% PAA, alternated with plain 0.1wt% PEI, attained the lowest sheet resistance (~325 Ω/sq) with bulk resistivity of approximately 5 S/cm. These films are typically 0.5 - 5 μm thick and can be patterned using traditional photolithography. The relatively high conductivity results from a high carbon black concentration (> 40 wt%) in the films combined with relatively low porosity for such a large filler concentration. This combination of good conductivity and patternability is ideal for flexible electronics.


O3.5
Semiconducting Organic Thin Film Devices with Large Room-temperature Magnetoresistance. Yugang Sheng^1,2, Tho Duc Nguyen¹, Govindarajan Veeraraghavan², Omer Mermer³, Thomas Lee Francis² and Markus Wohlgenannt¹; ¹Physics & Astronomy, University of Iowa, Iowa City, Iowa; ²Electrical & Computer Engineering, University of Iowa, Iowa City, Iowa; ³Institute of Polymers and Organic Solids, University of California, Santa Barbara, Santa Barbara, California.

We performed a comprehensive study on a recently discovered, large room-temperature magnetoresistance (MR) effect in sandwich devices comprised of nonmagnetic electrodes and organic thin films. The devices were fabricated from pi-conjugated polymers and small molecular weight compounds in combination with different electrode materials, and characterized extensively at different voltages, temperatures, and at weak magnetic fields from DC up to 100 kHz in frequency. The MR effect shows only weak temperature dependence and is independent of the sign and direction of the applied magnetic field. In some materials, the effect reaches up to 10% in a magnetic field of 10 mT at room temperature. To the best of our knowledge, the discovered effect is not adequately described by any of the MR mechanisms known to date, and therefore poses a significant scientific puzzle. To explore the possibility that OMAR is caused by spin-dynamics induced by the hyperfine interaction, we deduce a simple fitting formula from the hyperfine Hamiltonian that relates the saturation field of the OMAR traces to the hyperfine coupling constant. We compare the fitting results to literature values for this parameter. By varying the injection efficiency for minority carriers in the devices, we show experimentally that OMAR is caused by an effect of spin-dynamics on the carrier mobility, rather than carrier density (recombination). We discuss a possible mechanism. Since the devices we describe can be manufactured cheaply, our devices hold promise for applications such as magnetic sensors, magnetic random access memories, organic LED (OLED) touch screens, etc. To illustrate the potential application of the effect in touch screen displays, we demonstrate an 8x8 pixel OLED pen-input touch screen using the MR effect, where the position of a magnetic pen that is held over the display is detected.


O3.6
Engineering Carrier Transport Across Organic Heterojunctions by Interface Doping. Sai Wing Tsang¹, Zheng Hong Lu¹ and Ye Tao²; ¹Material Science and Engineering, University of Toronto, Toronto, Ontario, Canada; ²Institute for Microstructural Sciences, National Research Council of Canada, Ottawa, Ontario, Canada.

It is found that injection current at an organic heterojunction can be effectively tuned over several orders of magnitude by engineering a thin (2 nm) p-doped interlayer at the junction. Two common hole transporting materials, 4,4’,4’’-Tris(N-3-methylphenyl-N-phenyl-amino) triphenylamine (MTDATA) and N,N’-diphenyl-N,N’-bis(1-naphthyl)(1,1’-biphenyl)-4,4’diamine (NPB), and p-type dopant tetra-fluorotetracyanoquinodimethane (F4-TCNQ), were used to build various test devices. For a hole-only heterojunction device, ITO / MTDATA / NPB / Ag, both experimental data and theoretical modeling showed that the current flow in the device is limited by the heterojunction potential barrier or band-offset at the MTDATA/NPB interface. It is found that the device current can be modulated dramatically to increase or decrease by introducing a 2 nm p-doped NPB:F4-TCNQ or MTDATA:F4-TCNQ interlayer, respectively. Quasi Fermi energy level realignment at the doped/undoped organic-organic interface is proposed as the working principle for the observed phenomena. By employing the interface doping technique, the turn-on voltage of an organic light-emitting diode (OLED) can be efficiently tuned.


O3.7
Comparative Studies on the Stability of Polymer Gate Dielectrics for Pentacene Thin-film Transistors. Sang Chul Lim, Seong Hyun Kim, Jae Bon Koo, Yong Suk Yang, Jung Hun Lee, Chan Hoe Ku, Gi Heon Kim, Kyung Soo Suh and Taehyoung Zyung; ICCL, ETRI, Daejeon, South Korea.

In this study, we report the effects of instability with gate dielectrics of pentacene organic thin-film transistors (OTFTs) inverter circuits. We used to the UV sensitive curable resin and poly-4-vinylphenol(PVP) as the gate dielectrics. Pentacene OTFTs channel layers were deposited by thermal evaporation and total thickness is 100 nm. The inverters have a driver gate width of 16,000 mm, a driver gate length 50 mm, a load gate width of 2,000 mm, and a load gate length of 50 mm. The inverter supply bias is VDD= -40 V. For a given dielectric thickness and applied voltage, pentacene OTFTs with inverter circuits measurements field effect mobility, on-off current ratio, Vth, gain. All devices and circuits are characterized in ambient air. The field effect mobility 0.03~0.7 cm2/Vs, and the threshold voltage is -3.3 V~-8.8 V. The on- and off-state currents ratio is about 103~106. The inverter has very large gain of 25, 32 and matching input and output levels. From the OTFT device and inverter circuit measurement, we observed hysteresis behavior was caused by interface states of between the gate insulator and the pentacene semiconductor layer.


O3.8
Abstract Withdrawn


O3.9
Abstract Withdrawn


O3.10
Different Interactions of Tris 8-hydoxyquinoline Aluminum and Cesium Chloride on Al Substrate as a Function of the CsCl Thickness in OLED. K.H. Cho, Yeonjin Yi, Kyul Han, Kyoung jin Park, Pyung eun Jeon, Hyun bok Lee, In seung Jeong, Ju heyuck Baeck and Kwangho Jeong; Department of Physics, Yonsei university, Seoul, South Korea.

The intervening of CsCl electron injection layer (EIL) shows comparable luminance-voltage (LV) characteristics to the LiF EIL. We have investigated the interaction of Alq3 and electron injection layers (CsCl ) on Al substrate as a function of the CsCl thickness, using X-ray and ultraviolet photoemission spectroscopy (XPS and UPS). The results revealed that the core level shift of Cs 3d as gradually deposited Alq3 on CsCl(3Å)/Al heads toward high binding energy and Cs 3d peak in the case of the CsCl(30Å)/Al moves toward low binding energy. However, when Alq3 is deposited on CsCl (20.0 Å)/Al, shifts of the Cs 3d and Cl 2p core peak move toward high binding energy with submonolayer of the Alq3 deposited, and move toward low binding energy as thickness of the Alq3 deposition in gradually increased up to over submonolayer. It is evidences that CsCl 20Å on Al gives dual effects to Alq3. One is the Al surface control layer (CsCl+Al) and the other is CsCl layer itself.


O3.11
Bistable Characteristics of the Organic Device with Heterojunction Szu-Yuan Chen¹, Tzu-Yueh Chang² and Po-Tsung Lee^1,2; ¹Department of Photonics & Display Institute, National Chiao-Tung University, Hsinchu, R.O.C., Taiwan; ²Department of Photonics & Institute of Electro-Optical Engineering, National Chiao-Tung University, Hsinchu, R.O.C., Taiwan.

Tris-(8-hydroxyquinoline)-aluminum (Alq3) is one of the famous organic electron transfer semiconductor materials. In this work, we fabricate an organic electrical bistable device with Al/Alq3 deposited on N-doped Si substrate and with a heterostructure between N-doped Si and Alq3. Current-voltage characteristic similar to that of metal/organic semiconductor/metal structure, which is usually used for memory devices, is obtained. Initially, the device exhibits low conductance (OFF state). Till the voltage sweeps past the threshold voltage (3.5V), the device can be switched from low conductance to high conductance (ON state) and then it remains in this state even after the applied voltage is turned off. This device exhibits two different conductive states at the same applied voltage, and it is found to exhibit distinct bistability with an on/off current ratio over 1E+3. We believe that interface traps and heterojunction may play important roles on conductance bistability. Owing to its simple structure, we could embed novel organic electronic memory device in conventional fabrication processes. And it shows promise for high-density, low-cost memory applications in future organic nanoelectronics.


O3.12
Analysis of the Metal Impurities in the Organic Semiconductor Materials for Molecular Electronics by Inductively Coupled Plasma - Mass Spectrometry (ICP-MS). Svitlana Prada¹, Diethard Bohme¹ and Vladimir Baranov²; ¹Physics Department, Chemistry Department, York University, Torotno, Ontario, Canada; ²IBBME, Chemistry Department, University of Toronto, Toronto, Ontario, Canada.

We study the trace metal impurities in the organic semiconductor material such as Tetracene, Antracene, Pentacene and Rubrene. These materials are among the most interesting organic semiconductors for use in molecular electronic devises such as Organic Thin-Films Transistors (OTFTs) and Light Emitting Diodes (LEDs). Recent studies have shown that the major part of the problem with instability in time of the current-voltage characteristics of organic electronic device comes from the presence of metal impurities. Thus, the purity of the starting material is one of the most important parameter for fabrication of a molecular electronic device. The impurities are initially present in as-purchased organic materials and they are still present till certain extend in the organic material after several chemical purification cycles. We report the application of ICP-MS for impurity analysis in the organic semiconductors both as-purchased and after several chemical purification cycles. ICP-MS is a well established analytical technique in the semiconductor industry for essential trace metal characterization in chemical reagents and silicon wafers. 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 provides a characterization of the trace metal impurities in the organic semiconductor material. The results on elemental impurities in organic semiconductors are important for further improvement of organic semiconductor devices and experimental reproducibility.


O3.13
Electronic Structure of Copper Hexadecafluorophthalocyanine (F16CuPc) Measured using Soft X-ray Spectroscopies. Yufeng Zhang¹, Alex Demasi¹, Ian Reid¹, Leyla Colerkerol¹, Kevin Smith¹ and Anne Y. Matsuura²; ¹Boston University, Boston, Massachusetts; ²Air Force Office of Scientific Research, Arlington, Virginia.

We report studies of the electronic structure of thin films of the organic semiconductor copper hexadecafluorophthalocyanine (F16CuPc) using synchrotron radiation-excited resonant soft x-ray emission spectroscopy (XES), soft x-ray absorption spectroscopy (XAS) and x-ray photoemission spectroscopy (XPS). XES measures directly the element specific partial density of states (PDOS) in solids. When excited at resonance with selected core levels, XES measures the PDOS associated with particular chemical sites or environments. We have recently reported a resonant XES study of thin films of copper phthalocyanine, CuPc, and showed that by using this non-ionizing spectroscopy, the electronic structure near the Fermi level in organic systems can be accurately measured. The F16CuPc films were grown in an ultra-high vacuum organic molecular beam deposition system, and transferred under vacuum to the spectrometer system on the beamline X1B at the National Synchrotron Light Source. As with our earlier study of CuPc, the F16CuPc films were discovered to be highly susceptible to synchrotron radiation beam damage. We successfully circumvented this effect by continuous translation of the films during measurement. The results of XES measurement of the PDOS associated with the F, N, and C atoms in F16CuPc will be presented. We find that the measured C 2p PDOS for F16CuPc differs from that of CuPc, and will discuss the possible origins of this result. Supported in part by U.S. AFOSR under FA9550-06-1-0157, and by the donors of the Petroleum Research Fund, administered by the ACS. Experiments were performed at the NSLS which is supported by the U.S. DOE, Divisions of Materials and Chemical Sciences.


O3.14
Microscopic View of the Transition from Layer-by-layer to Rapid Kinetic Roughening in Organic Growth of DIP on SiO₂. XueNa Zhang¹, Esther Barrena^1,2, Dimas G. de Oteyza1¹ and Helmut Dosch^1,2; ¹Prof. Dosch, Max Planck Institute, Metal Research, Stuttgart, Germany; ²Institut für Theoretische und Angewandte Physik, Suttgart, Germany.

Although it has been recognized that the optimization of performance in organic devices demands control over the structure and morphology of the films, the knowledge of the physical principles of ordering and growth of organic films is still very poor and a general framework of their growth has not emerged yet. Recently, thin films of diindenoperylene (DIP) have been investigated showing an extraordinarily high structural order on silicon dioxide making this molecule a prospective candidate for application in electronic devices and an interesting system for growth studies [1-4]. It has been shown that this system belongs to a class of systems which display the phenomenon of rapid roughening [4]. In this work, by combining atomic force microscopy with surface-sensitive x-ray diffraction, we have been able to get a detailed in-situ insight into the microscopic processes which take place during the growth of the first stages of the DIP deposition on SiO2. This study discloses significant strain relaxation phenomena in the in plane structure during the growth of the first four layers concomitant with a transition in the growth mode from perfect layer-by-layer to rapid roughening. [1] A.C. Dürr, et al. Appl. Phys. Lett. 81, 2276, (2002). [2] F. Schreiber, Physica Status Solidi (A), Applied Research, 201, 1037-1054 (2004). [3] S. Kowarik, et al. Phys. Rev. Lett. 96, 125504 (2006). [4] A.C. Dürr, et al. Phys. Rev. Letters, 90, 016104, (2003).


O3.15
High-performance, Microscale Field-effect Transistors for the Probing of Charge Transport in Molecular Crystals. Colin Reese, Wook-Jin Chung, Toshihiro Okamoto, Ming Lee Tang and Zhenan Bao; Chemical Engineering, Stanford University, Stanford, California.



O3.16
Structural Investigation of Pyrene Discotics for Organic Field Effect Transistors. Leah Lucas¹, Dean M. DeLongchamp², Lee J. Richter², Daniel A. Fischer², Eric K. Lin², Brigitte Wex¹, Bilal R. Kaafarani³ and Ghassan E. Jabbour¹; ¹Flexible Display Center, Arizona State University, Tempe, Arizona; ²National Institute of Standards and Technology, Gaithersburg, Maryland; ³Department of Chemistry, American University of Beruit, Beruit, Lebanon.

Synthetic tailoring of organic semiconductors is a powerful means to manipulate their microstructure, orbital overlap, and charge carrier mobility. Directed synthetic variation for device improvement, however, requires a comprehensive understanding of how primary chemical structure directs crystal packing and microstructure. We investigate these relationships in a series of solution-processable discotics based on a 6,7,15,16-tetrakis(alkylthio)quinoxalino[2',3':9,10]-phenanthro[4,5-abc]phenazine, TQPP-[SR]₄ core where R= C₁₂H₂₅, C₁₀H₂₁, C₈H₁₇, and C₆H₁₃. The room temperature microstructure of the TQPP-[SR]₄ series was characterized using a combination of techniques including near edge X-ray absorption fine structure spectroscopy, atomic force microscopy, and X-ray diffraction. The extent of order in the films is dependent on the length of the aliphatic chains attached to the core edge. Longer chains appear to lead to greater three dimensional order and higher carrier mobilities. The mechanism by which heating to a mesophase increases domain size will also be described. These relationships between primary chemical structure, microstructure, and carrier mobility provide guidance for further directed synthetic improvement of this class of organic semiconductors.


O3.17
Electrochemical Fluorescence Modulation of s-triazine Bridged p-phenylene Vinylene Oligomers. Eunkyoung Kim, Taechange Kwon, Jeongmok You and Bhimrao D. Sarwade; Chemical Engineering, Yonsei University, Seoul, South Korea.

Two s-triazine bridged p-phenylene vinylenes (ST-PPV) oligomers were synthesized by the Wittig polycondensation reaction. Both the oligomers are soluble in common organic solvents such as chloroform, tetrahydrofuran, methylene chloride and toluene. They absorbed around 340 and 390 nm and behaved as blue-greenish light emitting materials in both solutions and films. Film fluorescence emission maxima were in the range of 460 to 491 nm and quantum yields 0.57 to 0.63 in solution. The fluorescence of the oligomers was modulated by electrochemically with a switching voltage of 1.5 V. The cyclability was longer than 1,000 cycles under a switching time of 4 s.


O3.18
New p-type 1st-generation Dendrimers Bearing Thiophenyl Peripheral Moieties for Organic Field Effect Transistor. Dong Hoon Choi¹, Kyung Hwan Kim¹, Minju Cho¹, Jinsoo Joo², Hansaem Kang² and Miyeon Cho²; ¹Chemistry, Korea University, Seoul, South Korea; ²Physics, Korea University, Seoul, South Korea.

Organic semiconductor materials based on extended linear π-conjugated systems have been very intriguing and significant development has been achieved in these materials over the last several years. For developing a flexible field effect transistor, many soluble poly- and oligothiophenes have been suggested. In particular, dialkylsexithiophene and its derivatives have been intensively employed as an active layer material in OFET. We also have designed π-conjugated dendrimers bearing bithiophenyl or thienothiophenyl peripheral groups, which exhibit 2-dimensional planar geometry for ease in crystallization. The solubility problem in linear conjugated oligothiophenes was completely overcome in these dendrimers. In this study, we prepared four-armed crystalline dendrimers containing 5-hexyl-5-propenyl-[2,2’]bithiophenyl or the 2-hexyl-5-propenyl-thieno[3,2-b]thiophenyl group as a dendritic wedge. First, 5’-hexyl-[2,2’]bithiophenyl-5-carbaldehyde was prepared by Vilsmeier reaction of 5-hexyl-[2,2’]bithiophenyl. 5-Hexyl-5’-vinyl-[2,2’]bithiophenyl was synthesized by the Wittig reaction between 5’-hexyl-[2,2']bithiophenyl-5-carbaldehyde and methyltriphenylphosphonium iodide. Same methods were utilized for synthesizing the intermediates for 4(HPTT)-benzene. Two dendrimers were synthesized by a Pd(0)-catalyzed Heck coupling reaction of the 1,2,4,5-tetrabromo-benzene and the 5-hexyl-5-propenyl-[2,2’]bithiophenyl or 2-hexyl-5-propenyl-thieno[3,2-b]thiophene peripheral group in a yield of 30-40%. They display a p-type semiconducting behavior and a considerably better solubility than the linear conjugated oligothiophene. Two crystalline dendrimers such as 4(HPBT)-benzene and 4(HPTT)-benzene showed a carrier mobility as high as 6.2 * 10-3 and 2.4 * 10-4 cm2V-1s-1 respectively.


O3.19
High Performance Functionalized Asymmetric Linear Acenes for p-type Organic TFTs. Ming L. Tang¹, Toshihiro Okamoto² and Zhenan Bao²; ¹Chemistry, Stanford University, Stanford, California; ²Chemical Engineering, Stanford University, Stanford, California.

Pentacene is the workhorse of the organic semiconductor industry, with reliably high mobilities, and decent on-off ratios. In this work, we present a series of novel linear acenes with fused thiophene units. These acenes have conjugation lengths between anthracene and pentacene. They are characterized by elemental analysis, mass spectrometry, differential scanning calorimetry, cyclic voltametry and thermogravimetric analysis. Thin films of these linear molecules were characterized by ultra-violet spectroscopy, x-ray diffraction, atomic force microscopy (AFM) and field-effect transistor measurements. Mobilites as high as 0.45cm2V-1s-1 have been found. We will present interesting halogenated derivatives of these asymmetric linear acenes.


O3.20
Ambipolar Transport of Organic Field Effect Transistors Based on P3HT and PCBM Composite Films. Keiichi Kaneto, Miho Shibao, Takeomi Morita and Takashima Wataru; Life Science and Systems Engineering, Kyushu Institute of Technology,, Kitakyushu, Japan.

Field effects in composite films of poly(3-hexylthiophene), P3HT and [6,6]-phenyl-C61-butyric methyl ester, PCBM have been studied to confirm the ambipolar transport and the light illumination effect. The composite films were prepared by spin coating of mixed chloroform solutions. A p-type transport originating from P3HT showed that the mobility of 6x10^-3 cm^2/Vs at 0 content of PCBM decreased with increasing the PCBM content. At the ratio of P3HT:PCBM=1:3, ambipolar transport appeared, and the mobility of n- and p-type carriers were 1.4x10^-4 and 4.0x10^-4 cm^2/V, respectively. With increase of PCBM content, the mobility of n-type carrier increased up to 1.1x10-2 cm2/Vs at pure PCBM. The ambipolar behavior was observed at the ratio of P3HT:PCBM = 1:3~1:12. It was found that light illumination to the FET at the P3HT:PCBM=1:3 enhanced the drain current, which mostly originated from the shift of threshold voltage.


O3.21
Extremely Low-voltage Organic Light-emitting Diodes with p-doped Alpha-sexithiophene Hole Transport and n-doped Phenyldipyrenylphosphine Oxide Electron Transport Layers. Toshinori Matsushima¹ and Chihaya Adachi^2,1; ¹JST-CREST, Tokyo, Japan; ²Center for Future Chemistry, Kyushu University, Fukuoka, Japan.

Organic light-emitting diodes with p-doped alpha-sexithiophene hole transport and n-doped phenyldipyrenylphosphine oxide electron transport layers are fabricated. In the doped diodes, we demonstrate an extremely low driving voltage of 2.9 V at a current density of 100 mA/cm2 and very high luminance at a low driving voltage: 1 000 cd/m2 at 2.4 V, 10 000 cd/m2 at 2.8 V, and 920 000 cd/m2 at 4.5 V. Such lowered driving voltages and enhanced luminance characteristics are attributed to formation of charge-transfer complexes in the doped layers, resulting in an increase in electrical conductivities and formation of ohmic contacts at metal/organic interfaces. These excellent carrier transport characteristics of the undoped alpha-6T layer originate from higher hole mobility than that of the alpha-NPD layer. The hole mobilities of alpha-6T18 and alpha-NPD19 films were previously reported to be 3.4 x 10-2 and 8.8 x 10-4 cm2/Vs, respectively. A small hole injection barrier of 0.1 eV at the ITO/alpha-6T interface also leads to efficient carrier injection from the ITO contacts compared with a large hole injection barrier of 0.5 eV at the ITO/alpha-NPD interface. The work function of ITO and the ionization potential energies of alpha-6T and alpha-NPD films were measured to be -5.0, -5.1, and -5.5 eV, respectively. Hole-only single-layer devices with the structure of glass substrate/ITO (100 nm)/undoped alpha-NPD, 2-mol%-F4-TCNQ-doped alpha-NPD, undoped alpha-6T, or 2-mol%-F4-TCNQ-doped alpha-6T (50 nm)/Ag (5 nm)/Al (100 nm) were fabricated to understand the improved carrier transport characteristics of the p-i-n OLEDs. In this structure, a high-work-function Ag layer was used to prevent electron injection from the metal electrodes. The J-V characteristics of the single-layer devices are shown in Fig. 4. Obviously, the driving voltage for the F4-TCNQ-doped alpha-6T device was the lowest compared with the other devices, and the driving voltages for the single-layer devices were decreased in a manner similar to the driving voltages for the OLEDs shown in Fig. 1. Moreover, the J-V characteristics of the F4-TCNQ-doped alpha-6T device exhibited Ohm's law (J x V). The electrical conductivity calculated from the J x V region of the doped alpha-6T device was 8.5 x 10-6 S/cm. These improved carrier injection and transport characteristics were attributable to an increase in free-carrier concentration, which was caused by the formation of CT complexes in the doped layers. In a separate experiment, we confirmed that CT bands peaked at ≈ 830 nm for a F4-TCNQ:alpha-6T film and ≈ 600 nm for a F4-TCNQ:alpha-NPD film in their visible/near-infrared absorption spectra while undoped films had no CT bands.


O3.22
Pentacene-based Thin Film Transistors with Titanium Oxide-polystyrene/polystyrene Insulator Blends: Influence of Permittivity and Morphology on Transistor Performance. Ashok Maliakal, Cecil Jung, Alex Sidorenko and Theo Siegrist; Bell Labs, Murray Hill, New Jersey.

High K titanium oxide-polystyrene nanocomposite (TiO2-PS) has been blended with polystyrene (PS) to generate gate dielectric films with permittivities ranging from 2.5 to 8 in order to investigate permittivity effects on pentacene TFT performance. An order of magnitude increase in saturation mobility is observed for TiO2-PS (K = 8) as compared to PS devices (K =2.5). Morphological differences for pentacene grown on TiO2-PS/PS dielectrics are thought to be responsible for the observed mobility enhancements. The high performance of pentacene on TiO2-PS devices suggests that high permittivity films are not incompatible with high mobility devices.


O3.23
Hybrid Polymer and CdSe Nanocrystal OLED. Ting Zhang, Gao Liu, Dacheng Zhao, Amanda S Simens and Andrew Minor; Lawrence Berkeley National Laboratory, Berkeley, California.

Hybrid polymer nanocrystal composites have drawn immense attention in the OLED and solar cell research communities by combination of easy processibility of the polymers and superior optical and electronic properties of the inorganic nanoparticles. We report the incorporation of 1 nm size CdSe nanocrystal into poly(9-vinylcarbozole) (PVK), and its OLED device performance. The electroluminescent of host polymer PVK was changed after doping nanocrystals. Furthermore, this change was related to the aging time of the polymer-nanocrystal solution. This could be due to the interaction between polymer and the nanocrystals surface. The PVK polymer amine sites can bind with the unpassivated surface sites on nanocrystals. FT-IR spectroscopes were carried out to study the nature of interaction. Bright white light-emitting OLEDs were obtained with the nanocrystal devise. Further mechanism study on interaction between polymer and inorganic nanocrystal are in progress. Understanding of the interaction between polymer host and nanocrystal is significant in order to improve the efficiency of both hybrid OLEDs and solar cells.


O3.24
Influence of Deposition Chamber Pressure and Substrate Temperature on the Properties of Fluorescent Blue and Phosphorescent Red OLED Deposited by OVPD. Philipp van Gemmern¹, Christoph Zimmermann¹, Phenwisa Niyamakom², Matthias Wuttig², Sabine Brand³, Holger Schwab³, Heinrich Becker⁴, Rocco Fortte⁴, Michael Heuken^1,5, Holger Kalisch¹ and Rolf H. Jansen¹; ¹Institute of Electromagnetic Theory, RWTH Aachen, Aachen, Germany; ²Institute of Physics (IA), RWTH Aachen, Aachen, Germany; ³Philips Technologie GmbH, Aachen, Germany; ⁴Merck OLED Materials GmbH, Frankfurt/Main, Germany; ⁵AIXTRON AG, Aachen, Germany.

Organic vapor phase deposition (OVPD) is a promising alternative to vacuum thermal evaporation (VTE) for the deposition of organic films. OVPD offers a variety of precisely controllable process parameters such as substrate temperature (T) and deposition chamber pressure (P). Due to the fact that OVPD is a comparably novel technique, only little is known about their influence on the properties of organic films and organic light-emitting devices (OLED). T and P are expected to influence the morphology and microstructure of the films possibly also modifying carrier injection and mobility as well as radiative properties. Within this work, the effect of T for injection/transport materials and of T and P for emissive layers was investigated. Fluorescent blue devices as well as phosphorescent red devices employing the emissive guest/host systems MBG:MBH and MRG:MRH, respectively, were fabricated by OVPD on ITO-coated glass substrates. MHIL, N,N'-diphenyl-N,N'-bis(1-naphthyl)-1,1'-biphenyl-4,4'-diamine (NPB) and tris-(8-hydroxyquinoline) aluminum (Alq₃) were used as hole injection layers (HIL), hole transport layers (HTL) and electron transport layers (ETL), respectively. MHIL, MBG, MBH, MRG, and MRH were supplied by Merck OLED Materials GmbH. The cathode contact was formed by LiF/Al metallization. At first, the influence of a modified T on the hole injection/transport materials of a red OLED was examined. OLEDs with both MHIL/NPB as well as NPB only as HIL/HTL were deposited once at 75°C, once at 100°C. It was found that the device with NPB deposited at 100°C showed identical performance like the devices with MHIL/NPB at both temperatures. The device with NPB deposited at 75°C showed an efficiency which was 50% less. We conclude that the deposition of NPB at 100°C allows the omittance of the additional HIL (possibly due to an intimate contact between ITO and NPB) for these devices, leading to a simplified device stack. For the blue-emitting OLEDs, the process parameters for the deposition of the HIL, HTL and ETL were kept constant. The thicknesses of the organic layers as well as the doping concentration in the emission layer were identical for all devices. For the deposition of MBG:MBH, P was varied between 0.6 and 1.3 mbar, T between 60°C and 100°C. It was found that both have a strong influence on the efficiency of the devices. The resulting current efficiencies covered a range from 4.3 to 6.7 cd/A at a current density of <nobr>25 mA/cm²2

O3.25
Crystalline F8T2 Domains Hosted in Nematic Aligned Matrix. Oliver Werzer¹, Kurt Matoy¹, Roland Resel¹, Kurt Stubenrauch², Gregor Trimmel² and Peter Strohriegl³; ¹Inst. for solid state physics, University of Technology Graz, Graz, Austria; ²ICTOS, Graz University of Technology, Graz, Austria; ³Macromolecular Chemistry I, University of Bayreuth, GrazBayreuth, Germany.

Conjugated polymers especially such with liquid crystalline behavior are of great interest for optical and electronic applications. The liquid crystalline octylfluorene-bithiophene copolymer (F8T2) is a promising material for commercial application since it shows relative high field effect mobility and long time stability. The use of rubbed surfaces like rubbed polyimid gives the opportunity to introduce an alignment of the F8T2-backbones with respect to the rubbing direction. This leads directly to anisotropy of the optical and electrical properties of the film. The F8T2 polymer itself shows a complex phase behavior from the ambient bulk phase to isotropic liquid phase. With temperature dependent x-ray diffraction (XRD) combined with differential scanning calorimetry (DSC) measurements the phase transitions are determined at 370K (glass transition), 500K (crystal-mesophase transition) and 560K (clearing temperature). Based on the pre-investigations aligned F8T2 films on rubbed polyimid surfaces are produced by bringing the sample to the nematic phase. With a subsequent quenching of the film down to ambient temperatures the information from the nematic phase is frozen. From UV/VIS spectra dichroic ratios in the range from 10 to 13 are calculated for different films. X-ray pole figures (PFs) are measured on films thicker than 250nm. The investigated scattering angles are taken from specular XRD which shows two broad peaks belonging to a mean intermolecular distance of 17Å and 4.4Å. The PFs show enhanced intensities in the direction perpendicular to the rubbing direction for both intermolecular distances (directional order) but no alignment in respect to the surface normal is observed. The director representing the polymer backbone is therefore along the rubbing direction. Such a behavior is expected from a nematic aligned film but was never shown before for F8T2. The degree of alignment is 25 deg calculated from the full width at half maximum (FWHM) of an axial scan. The nematic aligned film is heat treated at 440K to bring the film in the crystalline state. Sharp diffraction features are observed with XRD representing intermolecular distances of 15.7Å, 5.8Å, 5.6Å and 4.9Å. PF measurements show again the nematic streaks but with additional enhanced poles. From the enhanced poles an orthorhombic unit cell of a=17Å, b=5.6Å and c=40Å is calculated. Since two phases of F8T2 are observed at the same time it is concluded that the crystalline F8T2 is embedded within the nematic matrix.


O3.26
Investigation of Epitaxial Order of Pentacene on Cu(110)-(2x1)O Using X-ray Diffraction. Markus Koini¹, Thomas Haber¹, Oliver Werzer¹, Roland Resel¹, Martin Oehzelt², Stephen Berkebile², Georg Koller² and Michael G. Ramsey²; ¹Institute of Solid State Physics, Graz University of Technology, Graz, Steiermark, Austria; ²Institute of Physics, Karl Franzens University, Graz, Steiermark, Austria.

Pentacene, an aromatic compound consisting of five fused benzene rings, has emerged as a viable candidate for the semiconducting transport layer in organic devices. It is a very promising material due to its relatively high electronic mobility. It crystallizes in a layered structure with a herringbone arrangement within the layers. By reason of the pronounced anisotropy of its electronic properties, an investigation of the epitaxial structure and morphology of the thin film is necessary. The difficulty with Pentacene is its tendency to polymorphism, when deposited onto substrates. Within this study, the structure of an approximately 30 nm thick Pentacene film on Cu(110)-(2x1)O is investigated by x-ray diffraction (XRD) using specular scans, rocking curves and polefigures for orientation and structure determination. The study reveals, that most of the Pentacene molecules align edge-on the substrate surface along particular surface directions, namely along the Cu(001) - direction which is equal with the direction of the oxygen rows. The copper surface serves as an atomically ordered two fold symmetric surface consisting of oxygen rows that might induce epitaxial growth of Pentacene. It was possible to identify two different phases of Pentacene with two different orientations each. The dominating phase is the vapour phase with contact plane (022). The molecule rods are aligned along the oxygen rows. The same applies accordingly for a crystal phase orientation, where the contact plane is (1-10). In case of the second crystal phase orientation (110), the long molecular axis lies symmetrically around the oxygen row direction. It was possible to show that a specific crystal orientation, namely the (001) direction is equal with the oxygen row direction. There was also a small amount of a standing vapour phase observed, where a classical lattice match condition seems to be possible. Except for the standing vapour phase, lattice match analysis reveals that this alignment does not lead to commensurability. This implies the conclusion, that independent of the surface lattice dimensions and the contact planes of the crystals, an expitaxial growth is present and it is determined by the alignment of the initially adsorbed molecules along corrugated surface directions. Concerning thin film growth, the lattice match condition in organic epitaxy is relaxed and the growth can be dominated by different effects. Especially rod - like molecules such as Pentacene align their long molecular axes along distinct surface directions of the substrate, independent of size and ratio of the surface unit cell.


O3.27
Epitaxial Order of Rod-like Molecules Determined by Surface Corrugation. Thomas Haber¹, Martin Oehzelt², Markus Koini¹, Georg Koller², Stephen Berkebile², Jan Ivanco², Marcello Campione³, Adele Sassella³, Michael G Ramsey² and Roland Resel¹; ¹Institute of Solid State Physics, Graz University of Technology, Graz, Austria; ²Institute of Physics, Karl-Franzens University Graz, Graz, Austria; ³Department of Materials Science, University of Milan Bicocca, Milano, Italy.

Electronic and optical properties of organic semiconductors are, due to the nature of the molecules, usually highly anisotropic. Thus the molecular orientation and the crystal order are decisive for organic device applications. Rod like molecules such as oligo-phenylenes, oligo-thiophenes and oligo-acenes regularly form molecular crystals of single crystalline nature and the understanding and control of their thin film growth is important. In this work we investigate the organic crystallite orientations that grow on various atomically ordered single crystal substrates; with particular emphasis given to the role of the substrates anisotropy (or the lack of it) on the resulting epitaxial relationships. Here the growth of pentacene, sexithiophene and sexiphenyl on atomically clean Cu(110), (2x1) oxygen reconstructed Cu(110), TiO2(110) and the organic single crystal surface of KAP(010) (potassium acid phthalate) is investigated. Both, in-situ and ex-situ grazing incidence x-ray diffraction investigations (using synchrotron radiation at the beamlines ID10B and BM32, ESRF, Grenoble) from mono-layer (0.3nm) to thin film thickness (30nm) are presented as well as XRD pole figures obtained with laboratory equipment for thin films. These results are supported by morphological measurements (STM/AFM). The rectangular surface unit cells of the substrates are all extremely different with unit cell vectors in the range of 0.36x0.26 to 0.96x0.65 nm. Despite this, similar organic crystallite orientations are observed. It is argued that the atomic surface corrugations of the substrates direct the orientation of the molecules in the first monolayer and consequently the crystalline orientations of the subsequent thin films. These results are contrasted with the results obtained on an atomically disordered surface where the in-plane order of the organic films is completely lost. Therewith it is shown that the epitaxial order of organic thin films, at least for rod like molecules, is induced by the alignment of initially adsorbed molecules along surface corrugations, while lattice match, that is the dominating factor for inorganic epitaxy, is not an important driving force for this type of organic epitaxy.


O3.28
Investigation of the Influence of OVPD Process Parameters on the Properties of α-NPD and Alq₃ Layers. Christoph Zimmermann¹, Philipp van Gemmern¹, Hans-Peter Loebl², Phenwisa Niyamakom³, Matthias Wuttig³, Holger Schwab⁴, Michael Heuken^1,5, Holger Kalisch¹ and Rolf H. Jansen¹; ¹Institue of Electromagnetic Theory, RWTH Aachen, Aachen, Germany; ²Philips Research Laboratories, Aachen, Germany; ³Institute of Physics (IA), RWTH Aachen, Aachen, Germany; ⁴Philips Technologie GmbH, Aachen, Germany; ⁵AIXTRON AG, Aachen, Germany.

Organic light emitting diodes (OLEDs) have the potential to cover a wide range of illumination applications as efficient and flexible light sources. Much research effort is presently spent on determining the best production method for cheap, stable and efficient OLEDs. One promising production method is organic vapor phase deposition (OVPD), which offers several precisely controllable process parameters such as substrate temperature, chamber pressure and carrier gas flow and the possibility to deposit well defined material mixtures. For matching the requirements of industrial production of OLEDs, it is crucial to understand the influence of the process parameters on the properties of the deposited films and devices. We have fabricated hole-only devices consisting of N,N'-diphenyl-N,N'-bis(1-naphthylphenyl)-1,1'-biphenyl-4,4'-diamine (α-NPD) with ITO as anode and aluminium as cathode and electron-only devices consisting of tris-(8-hydroxyquinoline) aluminum (Alq₃) with aluminum as anode and lithium fluoride and aluminum as cathode at different deposition rates, reactor pressures and substrate temperatures. I-V and mobility measurements were performed and compared to devices which have been fabricated by vacuum thermal evaporation (VTE). The deposition rate for the Alq₃ samples was varied from 2 to 5 Å/s, for α-NPD it was varied from 2 to 20 Å/s. The reactor pressure for both materials was varied from 0.4 to 2.3 mbar and the substrate temperature was varied from 40°C to 80°C. For some deposition parameters the diodes did not conduct at all. For the others it was found that the mobilities of the OVPD and the VTE samples were of the same order of magnitude and that the mobilities of the OVPD samples were nearly not influenced over the mentioned range of deposition parameters. The injection currents on the other hand significantly depend on the deposition parameters. For Alq₃ samples, the combination of low substrate temperature and high deposition rate led to a reduced steepness of the I-V curve and strong scattering between the diodes whereas at high temperatures the influence of the deposition rate disappeared.The α-NPD samples showed the same negative effects of a high deposition rate, but these negative effects could be even overcompensated by reducing the reactor pressure. The optical appearance of the samples clearly showed that the inefficient injection for disadvantageous deposition parameters is caused by the roughness of the deposited layers which also leads to coarse cathodes deposited on these layers. Even the α-NPD devices with steep and reproducible I-V curves were injection limited (in contrast to VTE samples) whereas most Alq₃ samples were close to the space charge limited case like the VTE samples. The experiments performed so far indicate that the transport parameters of the materials will probably remain constant when optimizing the deposition parameters.


O3.29
Abstract Withdrawn


O3.30
Surface-induced Alignnment of Liquid Crystalline Semiconductors Using a “Command Surface” for Organic Transistors. Takenori Fujiwara, Jason Locklin and Zhenan Bao; Chemical Engineering, Stanford University, Stanford, California.

Recently the control of orientational order and crystallization of organic semiconductors have been proven to be important factors for determining the performance of organic thin film transistors (OTFTs). Several methods have been reported in order to control the orientational order of organic semiconductor liquid crystalline (LC) molecules for the anisotropic characteristics of carrier mobility. These include mechanical stretching, liquid-crystalline self organization, rubbing and surface treatments of dielectric layers. We demonstrate that the alignment of nematic liquid crystalline semiconductor films is achieved by a thin film of polymethacrylate with p-cyanoazobenzene side chains as a command layer, which was exposed to linearly polarized light (LPL) in advance. The thin film of the polymer with p-cyanoazobenzene side chains displays high thermostability and enhancement of photo-anisotropy at elevated temperatures. We expect that the command surface could align LC semiconductor materials even if they possess a thermotropic, nematic LC phase in high temperature. We demonstrate that a main-chain of polyfluorene copolymer film is aligned perpendicular to LPL direction. We will report that field-effect mobility of the OTFTs used command layer as a gate dielectric insulator.


O3.31
Charge Transport Measurements on Anisotropic Polythiophene Thin Films Fabricated via Directional Crystallization. Leslie H Jimison¹, Roger Tsai² and Alberto Salleo¹; ¹Materials Science, Stanford University, Stanford, California; ²Materials Science and Engineering, University of California, Los Angeles, Los Angeles, California.

Recently, there has been considerable interest in the development of semiconducting polymers for use in printable electrical devices, such as transistors for display backplanes. These materials offer a cost effective alternative to conventional semiconductors, with key advantages of organic materials being their low processing temperature and possibility of solution processing in ambient leading to roll-to-roll printing. Performance over the past few years has improved, but charge transport in these materials is not yet fully understood. These conjugated polymers generally form a semicrystalline microstructure, consisting of lamellar crystalline regions separated by amorphous grain boundaries. A mobility edge model has been proposed that suggests charge delocalization in the crystallite regions where mobile states exist, with the effective mobility limited by the localized states within the disordered grain boundaries and at defects. We have used a means of controlling the orientation and size of crystallites in the plane of the substrate to explore the relationship between trap density within grain boundaries and charge transport.Regioregular poly-3-hexylthiopene (P3HT) and poly(2,5-bis-alkylthiophen-2-yl)thieno[3,2-b]thiophene (PBTTT) are the materials under investigation. In both cases we have fabricated films on glass and silicon substrates by starting with a heated solution of polymer and 1,3,5-tricholorobenzene (TCB), a solvent that is crystalline at room temperature. As we bring the liquid down in temperature, nucleation of the TCB is induced. The TCB forms elongated crystals. The polymer is continuously excluded from the TCB into the remaining solution until it is deposited on top of the TCB crystals, which act as a substrate for epitaxy. Apon TCB removal, an oriented polymer film is left behind, consisting of large (mm2) domains where the film extinguishes uniformily under crossed polarizers. This suggests long range orientation of the polymer chain axis. Absorption measurements confirm anisotropy in microstructure. Characterization with an AFM reveals stacked lamellar structures, with approximate dimensions of 50 nm by 100 nm. Charge transport in the directionally crystallized film was probed by measuring in plane mobilities. Both bottom contact and top contact TFTs were made with n-doped silicon substrate that serve as the common gate electrode. A 200 nm thermal oxide treated with octadecyltrichlorosilane (OTS) was used as the gate dielectric. Devices were made with different relative orientations between the channel and the polymer film. Transport measurements as a function of charge density and temperature for different orientations of our film were used to explore the effect of anisotropy and microstructure Dimitrakopoulos et al, Adv. Mat. v.14, 2 p. 99-117 (2002) Street et al, PR B 71 165202 (2005) Brinkmann et all, Adv. Mat. 18 p. 860-863(2006)


O3.32
Nanoscale Observation of Electrical Properties of Polypyrrole Film During its Aging Using Conductive AFM. Hosup Jung¹, Dennis E Tallman^1,2 and Gordon P Bierwagen¹; ¹Coating and Polymeric Materials, North Dakota State University, Fargo, North Dakota; ²Chemistry, North Dakota State University, Fargo, North Dakota.

Conducting polymers (CPs) have been extensively investigated for their application in many areas, such as gas sensors, solar-batteries, nanoscale electronic devices, coating systems and so on. For many years, coating systems based on CPs have attracted much attention for corrosion protection of active metals such as iron, steel, aluminum or aluminum alloy. The electrical properties of CPs films play an important role for corrosion protection. However, it is difficult to measure the electrical properties of electrodeposited CPs film by typical methods, such as four-probe technique, scanning tunneling microscopy and the Kelvin probe method. In this study, we report the observation of the electrical properties of polypyrrole (PPy) film containing various dopants during its aging under atmospheric conditions using conductive atomic force microscopy (C-AFM). For many applications of CPs, the stability is a very important factor. Here we investigate the time evolution of the current image of the PPy films which were prepared with 4,5-dihydroxy-1,3-benzenedisulfonic acid disodium salt (Tiron), dodecylbenzene sulfonate (DBS) and polystyrenesulfonate (PSS ) as dopants on aluminum alloy as a function of aging time employing C-AFM technique. The as-prepared PPy films showed very high conducting characteristics regardless of the dopant. PPy films doped with DBS and PSS exposed to air for 48 h exhibited significantly diminished conductivity. After 192 h, significant currents were detected only at a few spots, with most of the surface exhibiting rather poor conductivity. However, the PPy film doped with Tiron exhibited significant stability of conductivity, even after exposure to air for 3 months. These results indicate that the PPy film doped with Tiron on aluminum alloy maintained conductivity for a long period without a significant decrease of electrical properties.


O3.33
Fixed p-i-n Junction Polymer Light-emitting Electrochemical Cells Based on Self-assembled Doping Monolayers. Daniel T. Simon, David B. Stanislowski and Sue A. Carter; Physics, University of California, Santa Cruz, Santa Cruz, California.

Polymer light-emitting electrochemical cells (LECs) have been given much attention as an enhancement over polymer light-emitting diodes (pLEDs). By incorporating doping ions within the active polymer layer, a p-i-n junction is formed under bias, which increases efficiency and overall light emission. We report on a new approach to LEC fabrication employing charged self-assembled monolayers (SAMs) that aims to overcome a major hurdle in performance: the relatively long time needed to move the dopant ions to establish the p-i-n structure, and the tendency to lose the p-i-n structure when the bias is removed. The devices consist of a Super Yellow-based LEC blend spun onto a trimethoxysilylpropy-tri-n-butylammonium chloride (TMS3BACl) SAM-modified indium-tin oxide (ITO) bottom electrode, with an evaporated Ag top electrode. Unlike the standard configuration of ITO anode and Ag cathode — chosen based on workfunction considerations — the devices are operated with the ITO-SAM electrode as the cathode due to the n-doping nature of the TMS3BA⁺ SAM. The LEC blend is the PPV-derivative Super Yellow plus the dopant species tetra-n-butylammonium tetrafluoroborate (TBABF₄) and polymer electrolyte poly(ethylene oxide) (PEO). TMS3BACl was chosen based on its structural similarity to the commonly used TBA⁺ dopant ion. In comparison to the same structure without the SAM, the time to full brightness decreased from minutes (without SAM) to seconds (with SAM), and the quantum efficiency more than doubled. Spectral response shows an increase in peak emission, indicating a shift in the recombination zone towards the ITO electrode and thus suppressed self-absorption. Examined as a photovoltaic, both the open circuit voltage and short circuit current increase by more than a factor of two. We discuss these results, as well as ongoing research on LECs based on anchored dopant species.


O3.34
Synthesis and Processing of Soluble Oligothiophene Semiconductors. Ajay Virkar, Jason Locklin and Zhenan Bao; Chemical Engineering, Stanford University, Stanford, California.

Organic semiconductors which can be deposited from solution represent the unique possibility of mass-produced, cheap electronics, and deposition on flexible plastic substrates. Highly conjugated organic molecules, can be cast from organic solvents by introducing solubilizing side chains. Charge carrier transport is facilitated by the &pi;-&pi; stacking between neighboring molecules. We have focused on oligothiophene molecules which are synthesized using Suzuki or Stille coupling mechanisms. Inks are created by dissolving the organic semiconductor in common organic solvents which can then be deposited onto device wafers by drop-casting or spin coating. As the solvent evaporates, the organic molecules crystallize into a solid film. Device performance is strongly influenced by the morphology of the resulting film. The fabricated transistors were probed for electrical performance like field effect mobility, On/Off ratio, and threshold voltage. For good performance, large two dimensional crystal growth is preferred, with strong a overlap of the &pi;-electron system. Desirable properties can be achieved both by molecular design and processing conditions. Several of the devices exhibit high performance with mobilities approaching 0.1cm²/Vs, and On/Off ratios of 10⁴.


O3.35
Effect of Self-Assembled Monolayer modified Electrodes on Bottom-Contact Organic Field-Effect Transistors Jong Won Lee, Ki Pyo Hong, Sang Yoon Yang, Kwonwoo Shin, Hayoung Jeon, Se Hyun Kim and Chan Eon Park; Chemical Engineering, POSTECH, Pohang, South Korea.

Pentacene-based organic field effect transistors (OFETs) have been widely studied as potential applications for low-cost, large area, flexible electronics such as information display, chemical sensor, electronic papers etc. To produce commercial OFET devices, fine lithography is needed and top contact OFETs are difficult for it due to damage of active layer. Therefore, bottom contact type OFETs are more desirable for a manufacturable process. However, it has been demonstrated that the bottom contact configuration gives inferior performance to the top contact one because of relative high contact resistance which were originated from the formation of injection barrier at the metal-organic semiconductor (OSC) interface. It has been shown previously that, by self-assembled molecular layer with intrinsic electric dipole moment, the work function of metal electrodes can be lowered or raised, affecting the size of the injection barrier at the metal-OSC interface. However, limited testing have been made to use this approach to engineer injection barriers in OFET field. In this study, gold (Au) surfaces modified with 1-Decanethiol (DT - electron donating group), Heptadecafluoro-1-decanethiol (HDFDT - electron withdrawing group), Tridecafluoro-1-octanethiol (TDFOT - electron withdrawing group) were characterized by He(I) Ultra-violet photoelectron spectroscopy (UPS). We have tuned the Au work functions by chemically modifying the Au surface through the formation of chemisorbed self-assembled monolayers (SAMs) having electron donating group and electron withdrawing group. The ordering in the SAMs creates an effective, molecular dipole at the metal/SAM interface, which increased the work function of Au (bare Au~4.9 eV) to 5.5~5.7 eV for electron withdrawing group. On the other hand electron donating group shifted work function of Au to 4.1 eV. The hole injection barriers at the pentacene/Au, pentacene/DT/Au, pentacene/HDFDT/Au and pentacene/TDFOT/Au were also determined using in-situ thin film deposition in combination with X-ray and ultraviolet photoelectron spectroscopy at PAL-4B1 beam line. We have obtained that hole injection barrier between electron withdrawing group SAM modified Au surface and pentacene was 0.59 eV which was 0.4 eV smaller than that between bare Au and pentacene. By using these SAMs to engineer the effective Au work function, we found that the charge injection process can be markedly affected. In addition, we have fabricated bottom contact pentacene-based organic field effect transistors (OFETs) by using these SAMs to investigate the effect of hole injection barriers at interface between the metal electrode and the vacuum deposited pentacene films on the performances of OFETs.


O3.36
Performance of PLED with Anode of Transparent Conductive ZnO_{x < 1} Thin Film Made at Low Temperature Yuhua Lee^1,2, Sun-Huei Huang², Jen-Fa Min¹, Jung-Chuan Lee¹, Shih-Ting Lin² and Wei-Yang Chou²; ¹Physics department, National Cheng Kung University, Tainan, Taiwan; ²Institute of Electro-Optical Science and Engeneering, National Cheng Kung University, Tainan, Taiwan.

The invention of polymeric light-emitting diode (PLED) has aroused a revolutionary change in display devices. Display devices made of PLED are much thinner, lighter and even flexible. The performance of the device depends on the use of transparent conductive electrodes. Films of indium oxide doped with tin (ITO) have small resistivity and have wide and long-term commercial uses. However, its component material indium is scarce. Therefore there is an urgent need to find a substitute to satisfy the fast growing market demands. Many wide band-gap metal-oxides are suitable alternatives. Among these, zinc oxide (ZnO) is cheaper, abundant and more friendly to the environment. Besides, ZnO is a II-VI n-type semiconductor with a band-gap energy of ~ 3.3 eV at room temperature. Thin films of ZnO_x can be made to have high electrical conductivity as well as high optical transparency by controlling a proper value x (< 1) of the relative atomic ratio of oxygen to zinc through deposition parameters. Therefore, we decide to make ZnO_x<1 thin film as anode for fabrication of PLED. Ion-beam deposition was adopted in this study because of its low operation pressure (~ 7 x 10^-5 torr). It is expected to have better microstructure and, thus, small resistivity at low substrate temperature. The substrate was not subjected to intentional heating and had a temperature of ~ 50 ^oC. Beam voltages V_b = 400, 500, 600 and 700 V were used for each of the beam current I_b = 10, 20 and 25 mA. Film thickness was kept at ~ 200 nm. All films are transparent with average transmittance in visible region T_av > 80 % and conductive with resistivity ~ 2 to 6 x 10^-3 ohm-cm and carrier density ~ 1 to 5 x 10<sip>20 cm^-3. Both atomic-concentration of oxygen and band-gap energy are roughly constant, at ~ 30% and 3.3 eV respectively. Hall mobility decreases clearly with increasing either V_b or I_b. Films made at I_b = 10 mA and V_b = 400 to 700 V were used as anode for fabrication of PLED. PEODT:PSS film (40 nm) and PT layer (100 nm) were used as hole transport and electron transport/emitting layers respectively. The cathode contact was an alloy of Ca (60 nm)/Al (120 nm). The active area of the device is ~ 2.5 × 2.5 mm. The current-voltage (I-V) and the luminance-voltage (L-V) curves show that forward biasing current (I_f) decreases and threshold voltage (V_th) increases respectively with increasing V_b. The commercially made ITO film (9.83 × 10^-5 ohm-cm in resistivity and 250 nm in thickness) shows V_th ~ 2.3 V and I_f ~ 40 mA at forward bias V_f = 5 V. Comparing with ITO film, ZnO film of V_b = 400 V shows larger V_th (~ 3 V) and smaller I_f at V_f =5 V (~ 1.5 mA). However, increasing the film thickness to 500 nm we observe smaller V_th (~ 2.3 V) and larger I_f at V_f =5 V (~ 7.2 mA). Our results show that ion-beam deposition is a proper technique to fabricate transparent conductive ZnO film for future development of flexible display device.


O3.37
Low-temperature, Solution-processed, High-mobility Polymer Semiconductors for Thin-film Transistors Hualong Pan¹, Yuning Li², Yiliang Wu², Ping Liu², Beng S Ong², Shiping Zhu¹ and Gu Xu¹; ¹Materials Science and Engineering, McMaster University, Hamilton, Ontario, Canada; ²bMaterials Design & Integration Laboratory, Xerox Research Centre of Canada, Mississauga, Ontario, Canada.

a new class of polymer semiconductors which self-assemble into higher structural orders without thermal annealing provided excellent field-effect transistor performance with mobility up to 0.25 cm^2V^-1s^-1 when used as a solution-processed thin-film semiconductor in thin-film transistors.


O3.38
On Charge Transport at the Interface of Organic Semiconductors. Xiaojiang Yu, Jianbin Xu, Jia Gao, Wing Yiu Cheung and Ning Ke; Electronic Engineering Department, The Chinese University of Hong Kong, Hong Kong, China.

CuPc/CoPc OTFT in sandwich configuration has demonstrated a strikingly high apparent-mobility of 0.11 cm²/Vs ^[1]. Our investigation on CuPc/CoPc OTFT reveals that there is an energy barrier between two layers of metallophthalocyanines even though CuPc and CoPc have very similar molecule packing. It is found that charge carriers can transport faster at the interface of CuPc/CoPc than in pure CuPc. Very similarly, other metallophthalocyanine-based heterostructure OTFTs, e.g. CuPc/FePc and TiOPc/CoPc OTFTs, can also exhibit higher mobilities than those in single-layer OTFTs. The highest mobility of metallophthalocyanine-based heterostructure OTFTs in our experiments could reach 0.2 cm²/V-s, which is comparable to the mobility of amorphous silicon. The mechanism for these heterostructure OTFTs still remains elusive and is subject to investigation. We tentatively propose that a high carrier density and a low density of electrical traps are probably formed at the interface of organic semiconductors, leading to a faster transport of charge carriers. This work is partially supported by the Research Grants Council of Hong Kong SAR, particularly, via Grant No. CUHK4172/06E. [1] J. Zhang, J. Wang, H. B. Wang, et al. Appl. Phys. Lett. 84, 142 (2004).


O3.39
A Metal Nanoparticles Incorporated Low Leakage Polymer Dielectric Film for Low Voltage Organic Thin Film Transistor Zingway Pei¹, Heng Tien Lin^2,3 and Yi Jen Chan^2,3; ¹Department of Electrical Engineering, National Chung Hsing University, Taichung, Taiwan; ²Electronics and Opto-electronics Research Laboratories (EOL), Industrial Technology Research Institute (ITRI), Hsinchu, Taiwan; ³Department of Electrical Engineering, National Central University, Chungli, Taiwan.

In recent year, the flexible electronics including flexible display and flexible electronic circuit attracted extensive research focus for its variety of applications. Among flexible electronics, organic thin film transistor (OTFT) is one of the major components either as the driving transistor in liquid crystal display (LC) and organic light-emitting diode (OLED) or as the basic element in organic circuit. However, instead of inorganic material, organic dielectric generally exhibit low dielectric constant and higher leakage current that limit the development on OTFT for low voltage operation. Several methods were demonstrated having low leakage current at low voltage including self-assembled monolayer, highly cross-linked polymer and multi-polymer layers. Those techniques are either hardly controlled or with complicated process. Here we reported a simply method for low leakage at thin organic dielectric film by incorporated metal nanoparticles into the organic dielectric materials. The appropriate metal nanoparticles in the organic material can be treated as carrier traps that limit carrier transport through the dielectric layer unless the traps are filled. With appropriated size and distribution of the metal nanoparticles, the traps will not be filled before dielectric breakdown. In experiment, the leakage current shows nearly three order of magnitude reduction after incorporation of metal nanoparticles.


O3.40
Relationship Between ITO Surface Properties and Device Performances of C60 Modifed Organic Light Emitting Diodes. Sung Hyun Kim², Jyongsik Jang² and Jun Yeob Lee¹; ¹Department of Polymer Science and Engineering, Dankook University, Seoul, South Korea; ²School of chemical and biochemical engineering, Seoul National University, Seoul, South Korea.

Relationship between ITO surface characteristics and device performances of C60 modified organic light emitting diodes was investigated. Surface properties of ITO were controlled by changing the degree of oxidation of ITO surfaces and it was correlated with the device performances of organic light emitting diodes. Interfacial energy barrier lowering was observed in C60 modified devices with highly oxidized ITO, while interfacial energy barrier was not affected by C60 modification in the case of bare ITO. Driving voltage of organic light emitting didoes could be lowered by more than 1 V by modifying highly oxidized ITO with C60. In addition, light emitting efficiency of organic light emitting diodes could be improved by more than 70 % by using C60 modified ITO as electrodes for organic light emitting diodes. In summary, high efficiency and low driving voltage could be obtained in organic light emitting diodes by modifying bare ITO and highly oxidized ITO with C60 and it could be used as an universal material to get high power efficiency irrespective of surface properties of ITO.


O3.41
The Effect of Interfacial Roughness on the Thin Film Morphology and Charge Transport of High Performance Polythiophenes. Youngsuk Jung¹, R. Joseph Kline¹, Eric K Lin¹, Daniel A Fischer¹, Dean M DeLongchamp¹, Martin Heeney² and Iain McCulloch²; ¹NIST, Rockville, Maryland; ²Merck Chemicals, Southampton, United Kingdom.

Roughness at the semiconductor-gate dielectric interface significantly influences the performance of organic field effect transistors (OFETs). However, controlled studies of the effect of interfacial roughness on semiconducting polymers have not been reported. In this work, we control and vary the interfacial roughness between a silicon oxide gate dielectric and a high performance polymer semiconductor, poly(2,5-bis(3-alkylthiophen-2-yl) thieno[3,2-b]thiophene) (pBTTT). We investigate the effects of controlled interfacial roughness on the pBTTT thin film morphology and OFET performance. The silicon oxide roughness was controlled by varying exposure to a reactive ion etch gas. The roughness was then characterized with atomic force microscopy (AFM) for both root mean square (RMS) roughness and power spectral density (PSD) before and after functionalization with octyl trichlorosilane (OTS). pBTTT forms large, highly oriented terraced domains with high hole charge carrier mobility when it is cast upon flat, low surface energy substrates and heated to a mesophase. For surfaces with RMS roughness of greater than 0.5 nm, we find significant changes in the morphology of the pBTTT active layer and large reductions in charge carrier mobility. The morphology of the pBTTT films on rough surfaces show significantly less order than that observed on smooth dielectric surfaces through characterization with AFM, near-edge x-ray absorption fine structure (NEXAFS) spectroscopy, and x-ray diffraction (XRD) measurements. These results show that even small RMS interfacial roughness can severely compromise the performance of OFETs.


O3.42
Nanoimprinting of Ferroelectric Polymer Films Jiangyu Li¹ and Lei Zhang^1,2; ¹Univ of Washington, Seattle, Washington; ²University of Nebraska, Lincoln, Nebraska.

The design and fabrication of nanostructured electronic materials has attracted considerable amount of interests in recent year. In this talk, we report our recent work on patterning ferroelectric P(VDF-TrFE) thin films by nanoimprint lithography (NIL). Various patterns have been created with the feature size approaching 10s nanometers, and very good pattern transfer between the molds and films has been observed. Systematic characterizations including AFM, SEM, XRD, FTIR and dielectric spectroscopy confirm that the imprinted P(VDF-TrFE) is ferroelectric with good dielectric, piezoelectric, and ferroelectric properties, which could find important applications as sensor arrays and actuator arrays.


O3.43
Air-stable Operation of Organic Field-effect Transistors on Plastic Films using Organic/Metal Hybrid Passivation Layers Tsuyoshi Sekitani and Takao Someya; Univ. of Tokyo, Bunkyo-ku, Tokyo, Japan.

Employing novel organic/metal-hybrid passivation layers, we have drastically improved the electric stability of pentacene field-effect transistors (FETs) on plastic films in air. The change in mobility was less than 5% and the shift in Vth was less than 4 V after the application of continuous voltage biases (V_DS = V_GS = - 40 V) for 10 days in air, demonstrating that the stability of the organic FETs is much superior to that of amorphous silicon transistors [1,2]. Organic FETs are manufactured by a vacuum evaporation process. A gate electrode consisting of 100-nm-thick Au was prepared in a vacuum evaporator on a 75-μm-thick polyimide film. Then, polyimide was spin-coated to form 800-nm-thick gate dielectric layers. Purified pentacene was deposited through a shadow mask in the evaporator to form 50-nm-thick semiconductor layers. A 60-nm-thick Au drain and source electrodes were evaporated through a shadow mask. The typical channel length and width were 50 μm and 500 μm, respectively. Finally, these FETs were coated with organic/metal hybrid passivation layers consisting of 800-nm-thick parylene/150-nm-thick Cu/ 10-μm-thick parylene. The FETs with passivation layers exhibited a high mobility of 0.6 cm^2/Vs, Vth of -11 V, and on/off current ratio of 10^6 with the off current defined at V_GS = 0 V. The FETs without passivation layers also exhibited a high mobility of 0.5 cm^2/Vs, Vth of -8.0 V, and on/off current ratio of around 10^2 with the off current defined at V_GS = 0 V. We measured the transistor characteristics of the FETs with and without passivation layers after storage in air (temperature: 26 oC, humidity: 60%) for two months. After two months, the FETs with passivation layers exhibited slight changes in Vth from -11 V to -10 V, while the change in mobility was less than 2%. The on/off current ratio exceeded 10^4 even when the off currents were defined at V_GS = 0 V. On the other hand, the FETs without passivation layers exhibited significant changes in Vth from -8 V to 3 V and a decrease in mobility from 0.5 cm^2/Vs to 0.3 cm^2/Vs. The on/off current ratio degraded to around 10^2. We also investigated the electrical stability under continuous voltage biases in air for 10 days. The pentacene FETs with the passivation layers exhibited the slight changes in mobility less than 5%, the Vth-shift less than 4 V, and the on/off ratio with the off current defined at V_GS = 0 V exceeded 10^5. This work was supported by Special Coordination Funds for Promoting and technology, JST-CREST, and TOKUTEI(15073204). [1] F. Taghibakhsh, J. Vac. Sci. Technol. A 24, 866 (2006). [2] C. -S. Yang, J. Vac. Sci. Technol. B 18, 683, (2000).


O3.44
NMR Studies of Hexafluorophosphate (PF6-) Doped Polypyrrole. Jenny Chien-Hsin Tso and Carl Michal; Physics & Astronomy, University of British Columbia, Vancouver, British Columbia, Canada.

Polypyrrole is a promising candidate for use as a mechanical actuator and as an electrode for supercapacitor applications. The ion content and dynamics of hexafluorophosphate (PF6-) doped polypyrrole films are examined by a variety of NMR techniques in different oxidation states. The ion content decreases linearly with decreasing electrochemical potential, directly confirming the ion insertion mechanism of polymer actuation. With known ion content and deposition current, a doping level (dopant ion/pyrrole) of 0.26 was determined for the as-grown film. A T1 relaxation study reveals that the rotational correlation time of PF6- ions in the oxidized film (10.76 ps) is similar to that in the solvent (8.08 ps), suggesting the ions are located inside solvent pockets rather than at stable sites in the polymer matrix. 1D Nuclear Overhauser Effect (NOE) difference experiments confirm the solvated ion environment in oxidized films, but reveal a drastic decrease in the NOE enhancement factor in reduced films, implying that polypyrrole undergoes a significant structural change when reduced. This change leads to a much less solvated environment as experienced by the dopant ions in the reduced state. Translational motion of the PF6- ions in the oxidized films at two orientations is probed via self-diffusion coefficient measurements made using pulsed-field gradient (PFG) NMR. The D values obtained at different diffusion times range from 3x10-8 cm2/s to 5x10-9 cm2/s. The time dependence of the D values suggests an upper limit on the pore size of 0.4 um in the oxidized films.


O3.45
Lateral J-aggregate Photodetectors. John C Ho, Michael Scott Bradley, Yasuhiro Shirasaki, Jonathan R. Tischler and Vladimir Bulovic; MIT, Cambridge, Massachusetts.

We fabricated an efficient, nanostructured photodetector using a thin film of J-aggregated cyanine dye molecules, which impart a spectrally narrow, tunable (across visible and near-IR) detection bandwidth. The strong dipole-dipole coupling of individual cyanine dye molecules forms the J-aggregates and yields narrow spectral linewidths (~ 10-20 nm full width half max in thin film) and record-high oscillator strengths [Bradley, et al. Adv. Mater. 2005, 15, 1881]. Layer-by-layer growth promotes a high density of J-aggregates to form in thin film structures leading to high optical absorption in only a few monolayers. Our device structure is a bi-layer heterojunction consisting of an optically active J-aggregate and a charge-transport layer arranged in a lateral photoconductor-style device with bottom contacts. The contacts are a series of gold interdigitated fingers (W x L = 1500 µm x 4 µm) spaced 10 µm apart. The gold electrodes are photolithographically defined on glass before the J-aggregate is deposited layer-by-layer, followed by the thermal evaporation of the organic layers. The lateral heterojunction physically separates the charge transport mechanism from the optical properties of the device, taking advantage of the J-aggregates’ unique, optical properties in this new class of photosensitive devices. In addition to developing a novel photodetector, we are using this structure to study charge transport and exciton dynamics in J-aggregate thin films.


O3.46
Towards Planar and Smooth Microarray Electrodes. Nuria Queralto Gratacos^2,1, Mark Roberts¹, Wolfgang Knoll² and Zhenan Bao¹; ¹Chemical Engineering, Stanford University, Stanford, California; ²Max Plank Institute for Polymer Research, Mainz, Germany.

The formation of defect-free self-assembled monolayers for direct applications in transistor fabrication or as templates for the (epitaxial) growth of ultrathin organic semiconductors in hybrid devices suffers from substrate-induced structural imperfections. The typical roughness of evaporated Au substrates can easily reach a few nanometers. So-called template stripped Au (TSG) has been used as substrates, which can generate surfaces with a mean roughness of only a few tenths of a nanometer and extending over several cm2. These TSG substrates have been used for the epitaxial growth of organic semiconductors. Alternatively, an Au/polymer hetero-substrate has been developed by template-stripping protocol. This approach is an important step toward an all-plastic device. Polymer layer has been spun coated on top of a silicon substrate partially covered with Au-electrodes. A smooth transition from gold to the insulator polymer has been achieved. Bottom contact organic field effect transistors with regular and heteroTSGsubstrates were compared.


O3.47
Naphthalimide Bearing 4-Arylamino as Red-Light-Emitting Materials for Electroluminescence Applications Sung Ouk Jung¹, Dong Min Kang¹, Jong-Won Park¹, Sang Yong Nam¹, Sung-Gap Lee¹, Jong-Taek Je³, Yun-Hi Kim² and Soon-Ki Kwon¹; ¹Shool of Nano-Advaced Materials Engineering, Gyeongsang National University, Jinju, GyeongNam, South Korea; ²Department of Chemistry, Gyeongsang National University, Jinju, GyeongNam, South Korea; ³SFC CO., LTD, Cheongwon, Chungbuk, South Korea.

Organic light-emitting devices (OLEDs) have attracted much attention because of their potential applications in low-cost, large-area, and flexible devices. For realization of full color, red-, green-, and blue-light-emitting materials with sufficiently high efficiency, good color purity and luminance are required. In comparison with green and blue emitters, the red emitters remain one of the greatest challenges. One of the reasons is low emitting efficiency that they have small energy gap because non-radiative relaxation of the excited states. The other is a result of color-impurity, owing to the strong intermolecular dipole-dipole interactions or intermoleculars-stacking, which leads to high tendency of aggregation and, so-called concentration quenching. We designed new red emitting material based on donor-acceptor-substituted naphthalimide containing arylamino moieties, which are non-planar and morphological characteristics in forming amorphous molecular glasses. The photophysical, elctrochemical properties of these materials, and OLEDs fabricated using these compounds are investigated.


O3.48
Organic Field-Effect Transistor Based on Polyalkylanthracene Jong-Won Park¹, Sung Ouk Jung¹, Jang-Yeol Beak¹, Jin-Ouk Ju¹, Mi-Hye Yi³, Taek Ahn³, Yun-Hi Kim² and Soon-Ki Kwon¹; ¹School of Nano-Advanced Materials and Engineering Research Institute, Gyeongsang national university, chinJu, gyeongnam, South Korea; ²Department of Chemistry, Gyeongsang National University, chinju, gyeongnam, South Korea; ³Polymeric Nanomaterials Laboratory, Korea Research Institute of Chemical Technology, Daejeon, South Korea.

Organic Thin-Film transistors (OTFTs) based on oligomeric and polymeric organic semiconductors are applied to active-matrix display, organic light emitting diodes, and electronic paper displays. A valuable characteristic to decide the performance of OTFTs is the field effect mobility of the charge carriers in the organic semiconductor layer and by the efficiency of injecting and extracting carriers at the source and drain contacts. OTFTs ma be fabricated by common solution deposition/patterning techniques (e.e., coating, stamping, offset printing, inkjet printing, etc.) and on plastic substrates to enable flexible electronics. Molecular ordering is believed to play a large role in the performance of devices based on organic active layer materials, for Polymer OTFT materials, the amount of π-orbital overlap is expected to have a strong influence on field-effect mobility. In this paper, we report the synthesis and characterization of poly(9,10-dialkyl substituted anthracene) for their behavior as FET semiconductors. The new polymer was prepared by a Yamamoto coupling reaction. The materials were measured by DSC, TGA, FT-IR spectrometer, and GPC etc.


O3.49
Polymeric OTFT Materials Containing Dithienothiophene Derivatives. Mai Thi Tuyet Dang¹, Qing-Hua Zhao¹, Yi-Nan Li¹, Moon-Hak Park¹, Yun-Hi Kim², Chan Eon Park³, Dae-Sung Chung³ and Soon-Ki Kwon¹; ¹Department of Chemistry, Gyeongsang National University, Chinju, South Korea; ²Chemical Chemistry, Gyeongsang National University, Chinju, South Korea; ³Chemical Engineering Department, Chemical Engineering Department, Pohang, South Korea.

Polymeric semiconductors have been considered as active materials for organic thin film transistors (OTFTs) due to their advantages including good process-ability, stability as well as compatible with plastic substrates to flexible electric devices. Among the large class of semiconductors, organic semiconducting oligomers containing dithienothiophene are the most promising for high stability and oxidation potentials. Also, dithienothiophene has π-π stacking orderness for efficient charge carrier transport to obtain high field-effect transistor mobility. Apart from the combination with 3-alkylthiophene or 9,9’-dialkylfluorene units is an improving solution processability. Therefore, the polymers based on dithienothiophene and 3-alkylthiophene or 9,9’-dialkylfluorene have been synthesized, and determined from FT-IR, NMR, and elemental analysis. Furthermore, physical, optical and electrochemical properties will be discussed. The polymers will have performed for OTFT device with expecting high mobility.


O3.50
Synthesis and Characterization of Diphenylaminodiphenyl Stryl-Based Alternating Copolymers Qinghua Zhao¹, Yi Nam Li¹, Dong Min Kang¹, Moon Hak Park¹, Yun Hi Kim², Jin Uk Ju¹, Sung Chul Shin² and Soon Ki Kwon¹; ¹School of nano and advanced materials engineering, Gyeongsang National University, Jinju, South Korea; ²Chemistry Department, Gyeongsang National University, Jinju, South Korea.

Since the first discovery of poly(phenylenevinylene)-based light emitting dioed(LED) in 1990’1, much effort have been made toward the development of new electroluminescent (EL) polymers. The advantage of EL polymers is that their solubility properties and highest occupied molecular orbital(HOMO) and lowest unoccupied molecular orbital(LUMO) energy levels can be adjusted by varying the molecular structure of the polymers. Poly(p-phenylenevinylene)(PPV)2 and poly(p-phenylene)(PPP) were the typical EL polymers used in PLED. PPV and PPP are attractive, owing to high thermal stability and suitable color tunability. However, much efforts have to be done to emit blue light-emission and increase ability of hole-injection or hole-transporting in order to improve device efficiency. Usually, triarylamine group was used to increase hole-injection and hole-transporting ability. On the same time, Polyfluorene(PF) was well known to emit blue light emitting. Thus, we synthesized BDAV3-based alternating polymers with phenyl or fluorene. BDAV had bulky triphenyl group and was expected to increase ability of hole-injection or hole-transporting. The PLED devices would be fabricated and discussed further. [1] J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. Mackay, R. H. Friend, P. L. Burns, A. B. Holmes, Nature(London), 1990, 347, 539 [2] D. Clery, Science, 1994, 263, 1700 [3] Y. H. Kim, Q. H. Zhao, S. K. Kwon, Journal of Polymer Science: Part A, Polymer Chemisty, 2006, 442, 172


O3.51
Efficient Green-Electroluminescent Devices Using [1,3,4]-Oxadiazole Derivatives as Hole-Blocking and Electron Transporting Materials Sung Jin Park¹, Dong Min Kang¹, Jae-Sang Kim², Hyoung-Yun Oh³, Sung Ouk Jung¹, Yun-Hi Kim² and Soon-Ki Kwon¹; ¹School of Nano-Advanced Materials, Gyeongsang National University, Jinju, South Korea; ²School of Nano-Advanced Materials, Gyeongsang National University, Jinju, South Korea; ³LG Electronics Institute of Technology, Seoul, South Korea.

The application of organic light-emitting devices (OLEDs) has reached low-cost, full-color flat panel displays due to their advantage of high brightness, easy fabrication, and wide range of emission colors. However, some important and fundamental challenges in achieving a high-resolution full color display. Particularly hole-blocking material is one of the significant factors of the device performance. For the hole-blocking material, large energy gap, high ionization potential, readily sublimable, and forming uniform amorphous films. Some organic materials have been reported that heteroatom-containing small molecular/polymeric compound, boron-containing or aluminum-containing materials. However, a hole-blocking and electron transporting layer still require improvement for efficient OLEDs. Therefore, as a new hole-blocking and electron transporting material, we have synthesized [1,3,5]-oxadiazole derivatives with high current efficiency and power efficiency of 45 cd/A and 17.7 lm/W in 10 mA/cm2, which is superior to the result of the device using BAlq (current efficiency: 31.5 cd/A and power efficiency: 13.5 lm/W in 10 mA/cm2) as well known hole blocker. The ITO/DNTPD/NPD/6% Ir(ppy)3 doped CBP/[1,3,5]-oxadiazole derivatives as both hole blocking and electron transporting layer/Al device showed efficiency of 45 cd/A and maximum brightness of 3000 cd/m2 in 10 mA/cm2.


O3.52
Polymeric OTFT Materials Containing Anthracene Derivatives Yinan Li, Mai Thi Tuyet Dang, Qinghua Zhao, TaeHoon Kim, SungJin Park, Yun-Hi Kim, Soon-Ki Kwon, Seung-Hon Han and Jin Jang; School of Nano-Adavanced Materials and Engineering, Gyeongsang National University, Jinju, South Korea.

Organic semiconducting molecules were very attractive for the fabrication of organic thin film transistors (OTFT) in various molecular electronics applications, such as flexible display, smart card, RF tag, and electronic paper. As we know, poly(3-hexylthiophene) and anthracene exhibit a high charge-carrier mobility, and the alkyl chains on the anthracene moiety can help promote self-organization. On the other hand, fluorene has a number of advantages, such as the thermal and chemical stability. Thus, we have selected anthracene and 3-alkylthiophene or dialkylfluorene to synthesis polymers, expecting to be used in organic thin film transistor (OTFT) and achieve high mobility. The polymers were confirmed by FT-IR, 1H-NMR. The physical and optical properties of the polymers were characterized by differential scanning calorimetry (DSC), cyclic voltammetry (CV), and optical absorption and photoluminescence (PL) spectroscopy. The OTFT will be fabricated and discussed further.


O3.53
Design, Synthesis and Characterization of Novel Fused Aromatic Semiconductors Containing Alkyl Bithiophene. Tae-Hoon Kim¹, Jong-Won Park¹, Sung-Ouk Jung¹, Seung-Moon Pyo⁴, Mi-Hye Yi³, Yun-Hi Kim² and Soon-Ki Kwon¹; ¹School of Nano & Advaned Materials and Engineering Research Institute, Gyeongsang National University, Jinju, South Korea; ²Department of Chemistry, Gyeongsang National University, Jinju, South Korea; ³Polymeric Nanomaterials Laboratory, Korea Research Institute of Chemical Technology, Daejeon, South Korea; ⁴Department of Chemistry, Konkuk University, Seoul, South Korea.

Organic thin-film transistors (OTFTs) using organic semiconductors as an active layer are of interest for their use in low-cost, lightweight and flexible electronic products. Although the field-effect mobility of OTFTs is still lower than those of inorganic thin-film transistor, the advantages of easy manufacturing and processing make them suitable for selected applications. In this study, the effect of fused aromatic units (anthracene, naphthalene and bivinylphenyl) having hexyl bithiophene on the performance of OTFTs have been investigated. The oligomers were synthesized by Suzuki coupling reaction and Wittig reaction, etc. The novel oligomers were chosen to organize more ordered and symmetric molecular structure, which facilitates molecular packing necessary for high mobility as well as to give oxidation stability. The oligomers were characterized by Infrared(IR), proton nuclear magnetic resonance(H-NMR), elemental analysis(EA), and mass spectra(MS). Their thermal properties were investigated by differential scanning calorimetry(DSC). Also, OTFT device using the thin film of new obtained material vacuum-deposited on the Si/SiO2 substrate showed typical p-channel TFT characteristic.


O3.54
In situ Real-time Synchrotron Study of Small Molecule Organic Thin Film Growth from Hyperthermal Molecular Beams. Aram Amassian^1,3, Sugandha Bhargava^2,3, Arthur Woll³, Sukwon Hong², John D Ferguson^4,3, Aravind S Killampalli², Todd W Schroeder², George G Malliaras¹ and James R Engstrom²; ¹Materials Science and Engineering, Cornell University, Ithaca, New York; ²Chemical Engineering, Cornell University, Ithaca, New York; ³Cornell High Energy Synchrotron Source, Cornell University, Ithaca, New York; ⁴Applied and Engineering Physics, Cornell University, Ithaca, New York.

We have examined in situ and in real time the growth of pentacene thin films deposited on SiO₂ and hexametyldisilazane (HMDS)-modified SiO₂ using a supersonic molecular beam (SSMB) source producing hyperthermal (E_i = 1.5 - 6.7 eV) kinetic energies. The growth was probed by X-ray anti-Bragg scattering for a wide range of deposition rates (0.002 to 2 ML/s) and substrate temperatures (T_s) using point and area detectors. In situ out-of-plane X-ray scattering measurements were followed up with ex situ in-plane structural characterization using grazing incidence x-ray scattering (GIXD) and grazing incidence wide area x-ray scattering (GIWAXS). In both the monolayer and multilayer regimes of pentacene growth we find that the growth rate at a fixed incident flux decreases as E_i is increased from 1.5 to 6.7 eV, consistent with trapping-mediated adsorption. In the submonolayer regime the data is well explained by nucleation theory, where the critical island size is found by atomic force microscopy to be 4.5 ± 1.3 and the so-called “thin-film” phase (out-of-plane spacing of 15.4 Å) is preferred. The behavior of monolayer and multilayer films was found to be more complex and exhibited unexpected trends with increasing deposition rate near room temperature. Whereas the crystalline order of thermally deposited pentacene - and of most known organic and inorganic systems - degrades with increasing deposition rate as a result of a higher nucleation density and a tendency to breakdown epitaxial growth, we find that SSMB deposition of pentacene at very high rate (>0.2 ML/s) both enhances and in some cases sustains layer-by-layer growth (up to 15 MLs), resulting in drastic improvement of both out-of-plane and in-plane crystallinity. Also, the bulk phase of pentacene (out-of-plane spacing of 14.5 Å) appears to form in the thin films deposited at higher rate, even though it is considered to be more thermodynamically stable than the “thin-film” phase. These results clearly demonstrate that energy and flux tunable molecular beams can be used to produce substantial changes in the growth habit of organic thin films, which are unexplored as of yet.


O3.55
Functional Self-Assembled Monolayers for Large Photoinduced Charge Transfer in Organic Field Effect Transistors. Paul Evans, Padma Gopalan, Byoungnam Park, Peerasak Paoprasert, Insik In and Jodi Zwickey; Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin.

The development of molecular electronics and the integration of organic semiconductors into devices fundamentally involve creating and characterizing functional electronic interfaces. The design of these interfaces includes the opportunity to incorporate functions that are not possible in inorganic structures, where the range of available materials is much smaller. We describe a structure designed to optimize photoinduced charge transfer in organic thin film devices. Thin films exhibiting photoinduced transfer have traditionally been envisioned for photovoltaic devices using bulk mixtures of semiconductors and molecules with electron acceptors. We have produced a highly controlled nanometer-scale implementation of these molecular mixtures in modified pentacene thin film transistors with a functional self-assembled monolayer at the interface between the pentacene and the SiO₂ gate insulator. This arrangement is optimized to produce a large electronic signature of photoinduced charge transfer. Charge densities on the order of 10¹³ cm^-2 can be induced by the photoinduced charge transfer effect at the interface between a pentacene thin film and a gate dielectric functionalized with a C₆₀-terminated γ-amino propyl trimethoxy silane. Atomic force microscopy, x-ray photoelectron spectroscopy, and infrared spectroscopy support a structural model based on an interface between the pentacene film and a single layer of the functionalized monolayer. Subsequent developements based on functionalized monolayers at the gate-insultator/semiconductor insulator have the potential to further optimize this effect and could lead to similar effects in at a wide range of interfaces.


O3.56
Synthesis and Characterization of Biphenyl End-capped Anthracene as a Core(BP2An) and Anthracene End-capped Biphenyl as a Core(An2BP) for OTFT Materials. Moon-Hak Park¹, Hyong-Sun Kim¹, Young-Hee Park³, Jong-Won Park¹, Ki-Yul Yang³, Mi-Hye Yi⁴, Yun-Hi Kim² and Soon-Ki Kwon¹; ¹School of nano and advance Materials Engineering, Gyeong Sang National University, Jin-Ju, Gyeong Nam , South Korea; ²Department of Chemistry, Gyeong Sang National University, Jin-Ju, Gyeong Nam , South Korea; ³Department of Chemical Education, Gyeong Sang National University, Jin-Ju, Gyeong Nam , South Korea; ⁴Polymeric Nanomaterials Lab., Korea Research Institute of Chemical Technology, Daejeon, South Korea.

Small molecule semiconductors have been considered as active materials for Organic Thin Film Transistors (OTFTs), because of their advantages including good process-ability, easy synthesis, good crystallization. The materials with high charge carrier mobility have currently been received a many interested, but the importantly focus on thiophene and pentacene derivatives. But, in this type of materials are limited to use by semiconductors because of stability problem. While anthracene and biphenyl have π-π stacking orderness for efficient charge carrier transport to obtain high field-effect transistor mobility and high oxidation stability. Among them, 2,6-substitution of anthracene is better than 9,10-substitution of anthracene because of the highest planarity and the most extended πconjugation. Therefore, we have two type materials designed that 2,6-Bis-biphenyl-4-yl-anthracene(An2BP) and 4,4'-di(anthracene-2-yl)biphenyl (BP2An) and synthesized by Suzuki reaction. And, we have compared An2BP with BP2An that calculated mobility in theory, measured mobility composing OTFT device, and observed oxidation stability for 1 month by IR. New small molecule materials were characterized by 1H-NMR, elemental analysis, and mass spectra. Their thermal properties were investigated by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA). And their Spectroscopic and Electrochemical properties investigated by cyclic voltammetry (CV), UV-vis and Photoluminescence (PL).


O3.57
Abstract Withdrawn


O3.58
Ion-pair Monomer Dependence and Correlation to Performance in Chemically-fixed Junction, Polymer Light-emitting Electrochemical Cells. Deanna B. Rodovsky, Janelle M. Leger and Glenn P. Bartholomew; Chemistry, University of Washington, Seattle, Washington.

Light-emitting electrochemical cells provide a low-cost processing alternative to multilayer organic and polymer light-emitting devices. For LECs to reach their full potential, a fixed, rather than dynamic, junction is required. Recently, we demonstrated a self-assembled, chemically-fixed, p-n junction in an organic semiconductor. We achieve this junction by using ion-pair monomers that can be chemically fixed in situ and form covalent bonds to retain the desired charge distribution where ions are spatially confined after removal of a bias. Preliminary results of comparative, yet distinct, ion-pairs indicate a correlation of structure, size, and concentration of these materials to device performance. Here we report our findings based on investigation of these ion-pairs that elucidates their role in optimal device performance. An important discovery is the strong correlation between higher concentration of ion-pair monomer and greater light output, limited ultimately by solubility of the ion-pair monomer. These results point to the need for additional processing strategies for these new materials. We will enumerate the factors necessary to further optimize the advantages offered by a chemically-fixed p-n junction.


SESSION O4: Device Physics and Materials
Chair: J. Devin MacKenzie
Wednesday Morning, April 11, 2007
Room 2002 (Moscone West)
8:30 AM *O4.1
Simulations of State of the Art P-OLED Devices. Matthew Roberts, Sumation, Cambridge, United Kingdom.

High device efficiencies are essential for low power consumption and thermal management of P-OLED flat panel displays. Higher efficiencies can also contribute to extending device lifetime, in particular within a given structure, where the half-life typically varies quadratically with brightness. Simulations of planar P-OLED structures highlights some key factors governing device efficiency, and gives design rules for optimised structures. In this paper, the key factors governing device efficiency are identified and discussed alongside experimental techniques suitable for measuring relevant parameters, and example results from current state of the art P-OLED devices.


9:00 AM O4.2
Characteristics of an Organic Light-Emitting Diode Utilizing a Phosphorescent, Shallow Hole Trap Ian H. Campbell and Brian K Crone; Materials Physics & Applications, Los Alamos National Lab, Los Alamos, New Mexico.

We demonstrate the effects of incorporating a phosphorescent, shallow hole trap in an organic light-emitting diode. We present device properties as a function of trap concentration including: electron only, hole only, and bipolar current-voltage (I-V) characteristics, electroluminescence (EL) and photoluminescence spectra, and diode quantum efficiency. We specifically considered poly (9,9-dioctylfluorene) doped with an Ir phosphor. Built-in potential and I-V measurements were used to determine that the phosphor is a shallow hole trap. The EL spectrum is dominated by phosphor emission for concentrations above 0.1 wt%. The effects of incorporating the phosphor are shown to be consistent with quasi-equilibrium statistics.


9:15 AM O4.3
Bipolar Transport and Device Performance - Solar Cells and Transistors. Lars Mattias Andersson and Olle Inganas; Biomolecular and Organic Electronics, Department of Phycics, Chemistry and Biology, Linköpings universitet, Linköping, Sweden.

Conjugated polymers are rapidly approaching mainstream commercialisation in a wide variety of electronic applications such as transistors and solar cells. Both of these structures are possible to use for electrical characterisation, and although there are several fundamental differences, such as film anisotropy and carrier concentrations that influence transport, there is still a substantial common ground where observations on the different structures can give a more thorough understanding of the transport properties than can a single type of experiment. This is especially true for blend materials such as the bulk heterojunctions used in most high performance solar cells, where transport is bipolar and where there is ample room for morphological variations. Electrical data, mainly collected from charge extraction measurements on solar cells and transistor i/v measurements, for a wide range of materials, most of which are based on alternating copolymers of fluorene and donor-acceptor-donor segments blended with substituted C60 and C70 fullerenes, will be shown. The polymers have varying electron affinities and ionisation potentials with band gaps as low as 1 eV. Solar cells from these materials reach power conversion efficiencies of 3.5 % at high band gap and 2.2 % with lower band gap materials. Among the results are resolved electron and hole transients, observed for the first time in optimised organic bulk heterojunction solar cells, where the stoichiometry dependence of electron and hole mobility can be followed and correlated to transistors with good agreement. Both solar cells and transistors reveal a high percolation threshold for electron currents, and that both the hole and electron mobilities are severely dependent on the polymer / fullerene system rather than just the constituents. This has obvious implications for solar cell design and also indicates that composite systems can be useful in transistor applications where the polarity of a transistor can be tailored from p-type to ambipolar to n-type. The good correlation between the solar cell and transistor measurements indicate that the same theoretical models should be applied to both systems. To this end, some remarks on how the Gaussian disorder model, which has found widespread use for diodes but only been sparingly applied to transistors, can explain many experimental observations such as hysteresis and light enhanced stress recovery in transistors, will be given.


9:30 AM *O4.4
Hybrid Organic/inorganic Materials For White Light-Emitting Diodes. Gitti L Frey, Eyal Aharon and Michael Kalina; Materials Engineering, Technion, Haifa, Israel.

Conjugated polymers have received a great deal of interest for a variety of applications, especially in displays; however, the production of white light has been problematic because the width of an emission peak from a single polymer species is generally not broad enough to cover the entire spectrum of visible light. As a consequence, pure white emission from a polymer light-emitting diode requires that separate red, green, and blue chromophores be present in the active layer and emit simultaneously. Here, a new and general strategy for white photoluminescence and electroluminescence from a single material is demonstrated. In this material, red, green, and blue emitting conjugated polymers are confined within the galleries of a layered semiconducting host matrix. The host, SnS2, not only supports the transport of charge carriers, but also hinders polymer π-π interactions which are responsible for the energy transfer between polymer chains. Consequently, emission from the three chromophores is observed simultaneously resulting in white photoluminescence. The efficacy of the nanocomposites is demonstrated in simple single-layer white-emitting polymer diodes. The mechanism suggested here for white light generation, supported by extensive luminescence measurements, is in contrast to that previously reported in white-emitting polymer diodes where efficient energy transfer between polymer chains was essential for obtaining white light, but also contributes processing complications and poor color stability. Facile integration of the conjugated polymer/layered semiconductor nanocomposites into optoelectronic devices shows their suitability to open additional approaches towards the development of functional nanocomposite materials for solid-state lighting applications.


10:30 AM *O4.5
Air-stable Polymer Electronic Devices. Alan Heeger, Kwanghee Lee and Jin Young Kim; UCSB, Santa Barbara, California.

Despite promising expectations of technological impact, electronic devices based on semiconducting and metallic polymers are not yet utilized in large scale commercial applications.The thin film form factor of polymer devices (thicknesses of approximately 100 nm) inevitably leads to vulnerability to the diffusion of oxygen and water vapor into the active layers. Moreover, most semiconducting polymer materials degrade when exposed to humidity and/or oxygen, and photo-oxidation can be a serious problem. By introducing a solution-based titanium oxide (TiOx) layer between the active layer and the aluminum cathode in polymer light-emitting diodes (PLEDs) and polymer solar cells (PSCs), we have demonstrated devices with excellent air stability and with enhanced performance. The TiOx layer acts as a shielding and scavenging layer which prevents the intrusion of oxygen and humidity into the electronically active polymers, thereby improving the lifetime of unpackaged devices exposed to air by nearly two orders of magnitude.


11:00 AM O4.6
Long Lifetime Polymer Light Emitting Electrochemical Cells Yan Shao^1,2, Guillermo C. Bazan^1,2 and Alan J. Heeger^1,2; ¹CPOS, University of California, Santa Barbara, Santa Barbara, California; ²MC-CAM, University of California, Santa Barbara, Santa Barbara, California.

Polymer light-emitting electrochemical cells (LECs) with long operating lifetimes are reported. A soluble phenyl-substituted poly(para-phenylene vinylene) (PPV) copolymer (“superyellow”) was used as the host light-emitting polymer and methyltrioctylammonium trifluoromethanesulfonate, an ionic liquid, was used to introduce a dilute concentration of mobile ions into the emitting polymer layer. The ions inside the luminescent semiconducting polymer can be spatially redistributed by applying a voltage at a temperature above the melting point of the ionic liquid. These ionic liquid based LECs, with stable aluminum metal as cathode, exhibit excellent current-rectification diode properties, a single-phase active layer, short response time, and long continuous operating lifetime.


11:15 AM O4.7
A Quantitative Morphological Model for the Conductivity of PEDOT:PSS. Alexandre Mantovani Nardes¹, Martijn Kemerink¹, Rene A.J. Janssen¹, Jolanda A.M. Bastiaansen², Nicole M.M. Kiggen², Bea M.W. Langeveld², Albert J.J.M. van Breemen² and Margreet M. de Kok³; ¹Applied Physics, Eindhoven University of Technology, Eindhoven, Netherlands; ²TNO Science and Industry, Eindhoven, Netherlands; ³Philips Research Laboratories, Eindhoven, Netherlands.

Poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonic acid) (PEDOT:PSS) is one of the most widely used materials in the field of organic electronics. Yet, the charge transport in spin cast films of PEDOT:PSS is rather poorly understood. By combining scanning-probe techniques and charge transport measurements, we obtained a morphological model that quantitatively accounts for the (highly anisotropic) conductivity of device-type PEDOT:PSS layers. In this work, the dc conductivity of commercial (Baytron-P) and specially synthesized batches of PEDOT:PSS has been studied as a function of temperature in the range from 5 to 300 K. Electrical measurements of the dc conductivity σ 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 neighboring pixels in organic displays. The temperature dependence was found to follow σ = σ₀exp[-(T₀/T)^α], with the characteristic temperature T₀, the exponent α and the prefactor σ₀ all dependent on the direction of the current. The physical reason for this unexpected behavior is the microscopic morphology of spin cast films. From low-current scanning-tunneling microscopy images we found that the films consist of flattened PEDOT-rich particles embedded in a PSS-rich matrix of low conductivity [1,2]. By performing phase-sensitive AFM on cryogenically cleaved samples we were able to obtain a cross-sectional view of the morphology. From this, it was found that the PEDOT-rich particles are actually organized in horizontal, i.e. parallel to the substrate, layers that are separated by quasi-continuous nm-thick PSS lamellas. In the normal direction, the PSS lamellas enforce nearest-neighbor hopping between the quasi-metallic PEDOT particles, leading to a strongly reduced conductivity and a temperature exponent α = 1 [3], both in agreement with experiment. The absence of substantial barriers in the lateral direction enables variable range hopping between the PEDOT-rich particles, explaining the experimentally observed exponent α = 1/4 [3]. This picture was corroborated by electric field-dependent measurements, from which the characteristic hopping distance d in both directions could be extracted. In the normal and lateral directions we found d ≈ 1 nm and d ≈ 70 nm, respectively, in excellent agreement with nearest-neighbor hopping over a thin barrier and variable-range hopping between 20-30 nm sized particles. [1] S. Timpanaro, M. Kemerink, F. J. Touwslager, M. M. de Kok, and S. Schrader, Chem. Phys. Lett. 2004, 394, 339. [2] G. Greczynski, T. Kugler, M. Keil, W. Osikwicz, M. Fahlman, W. R. Salaneck, J. Electron Spectrosc. Relat. Phenom 2001, 121, 1.


11:30 AM O4.8
Device Model for Light-Emitting Field-Effect Transistors with Organic Semiconductor Channel. P.Paul Ruden¹ and Darryl L. Smith²; ¹University of Minnesota, Minneapolis, Minnesota; ²Los Alamos National Laboratory, Los Alamos, New Mexico.

Recent experiments have demonstrated ambipolar channel conduction and visible light generation through radiative recombination in conjugated polymer field effect transistors.[1,2] The devices have source and drain contacts biased to inject negative charge carriers (electron polarons or electrons) from one contact and positive charge carriers (hole polarons or holes) from the other. In the ambipolar mode of operation, the gate potential lies between the potentials of the electron and hole injecting contacts, so that electrons dominate the channel conductance near the electron injecting contact and holes dominate channel conductance near the hole injecting contact. The injected charge carriers propagate along the field-effect-induced channel and recombine in regions where both types of carriers are present. The location and intensity of maximum recombination and light emission are controlled by the voltages applied to the three transistor terminals. In this work a new device model for ambipolar organic field effect transistors based on the gradual channel approximation is presented. Trapping of injected carriers in localized states within the band gap is shown to be an important mechanism in these devices. The model includes the effect of trapping through carrier density dependent mobilities.[3] Because the carrier densities depend on the local gate-to-channel voltage, the mobilities vary throughout the channel. Recombination is limited by a mobility dependent Langevin mechanism. A non-linear differential equation for the channel potential is derived and solved numerically. Subsequently, charge carrier density profiles and recombination profiles are determined. In addition, the limit of high recombination probability, which leads to analytical results, is discussed. The results of the device model are in good agreement with the recently published[1,2] experimental data for polymer light emitting field effect transistors. References: [1] J.S. Swenson, C. Soci, and A.J. Heeger, Appl. Phys. Lett. 87, 252511 (2005). [2] J. Zaumseil, R.H. Friend, and H. Sirringhaus, Nature Materials 5, 69 (2006). [3] A. Salleo, T.W. Chen, A.R. Volkel, Y. Wu, P. Liu, B.S. Ong, and R.A. Street, Phys. Rev. B 70, 115311 (2004).


11:45 AM O4.9
Self-assembled, Chemically Fixed pn Junctions in Doped Organic Semiconductors. Glenn Bartholomew, Janelle Leger and Deanna Rodovsky; Chemistry, University of Washington, Seattle, Washington.

We describe a method for self-assembling a fixed junction in a conjugated polymer that relies on the formation of covalent bonds to retain the desired charge distribution. We achieve this by replacing the salt in traditional light-emitting electrochemical cells with ion-pair monomers that can be polymerized in situ. This method allows for unipolar light emission and a linear relationship between current and radiance, and has been demonstrated with three major classes of emissive polymers: polyphenylenevinylenes, polyfluorenes, and copolymers thereof. We have also extended this idea to photovoltaic devices with success. Efforts to characterize the nature of these junctions and the dopant gradients present will be discussed.


SESSION O5: Materials Physics and Characterization II
Chair: Alberto Salleo
Wednesday Afternoon, April 11, 2007
Room 2002 (Moscone West)
1:30 PM *O5.1
Excited States at Polymer Semiconductor Heterojunctions. Astrid Gonzalez-Rabade, Johanna Schmidke and Richard Friend; Cavedish Laboratory, Cambridge, United Kingdom.

Bound charge-transfer excitons (exciplexes) can form at appropriate polymer-polymer Heterojunctions, and for some material combinations can still show radiative emission (with strongly increased radiative lifetimes and large red-shifts). Long-range charge separation, as required in a photovoltaic diode, needs an external field. We show that the reduction in exciplex formation (detected as quenching of exciplex luminescence) due to an external field matches quantitatively the increase in current collected in the external circuit. The strength of interchain interactions can be increased under pressure, and measurements of absorption and time-resolved luminescence under hydrostatic pressure will be reported. These reveal the changing character of the exciplex at the heterojunction.


2:00 PM O5.2
Organic Field-Effect Transistor with Low Operating Voltages Using Anodized Ta₂O₅ Gate Dielectric. Yeon Taek Jeong^1,2, Byungwook Yoo^1,3 and Ananth Dodabalapur^1,3; ¹Microelectronics Research Center, The University of Texas at Austin, Austin, Texas; ²Materials Science & Engineeing, The University of Texas at Austin, Austin, Texas; ³Electrical & Computer Engineering, The University of Texas at Austin, Austin, Texas.

Organic field-effect transistors (OFET’s) have been attracting considerable attention because of their potential applications in active matrix display drivers, radio-frequency identification tags, sensors, etc. Although there have been significant advances in fabricating OFET’s with reasonably high electron and hole mobility, their generally high operating voltages still remain a major drawback to practical applications. In organic electronics, plastic substrates are essential to make the best use of the unique properties of organic devices such as low temperature process and flexibility. To address the problem of high operating voltages and implement a low temperature process compatible with plastic substrates, anodization was performed on Ta thin film at room temperature, thereby obtaining high-k Ta₂O₅ (ε_r: 25.0), where 0.01 M of citric solution was used as the electrolyte. Top contact devices were fabricated using pentacene and gold as channel and source/drain contact materials, respectively. The channel length was 50 µm, and width 500 μm. Anodized Ta₂O₅ of 1,700 Å was obtained from the Ta thin film sputtered and annealed on a p-Si wafer substrate. The roughness of anodized Ta₂O₅ measured by AFM was around 22.9 Å, which was much larger than that of commonly-used thermally-grown SiO₂ 2.6 Å. The AFM images also showed that the grain size of the pentacene layer was relatively small and highly dependent on the surface roughness of the anodized Ta₂O₅ layer. Nonetheless, the device showed significant potential in that its saturation mobility μ_sat, threshold voltage V_T, I_on/I_off ratio at V_ds = -5 V and V_g = 0 & -2.5 V, are 1.09 cm²/V.s, -1.28 V, 1.64 × 10², respectively. The gate leakage is -110 nA at V_ds = 0 V and V_g = -2.5 V, and it decreases by 40 % when the insulator is subjected to an HMDS treatment. The HMDS treatment also results in slightly enhanced saturation mobility, decreased threshold voltage by a factor of two, and increased I_on/I_off ratio by more than an order of magnitude, which suggests that the characteristics of an anodized Ta₂O₅ device can be improved by a proper surface treatment. In summary, we demonstrated that high-performance anodized OFET’s with Ta₂O₅ gate dielectric have the potential to attain low operating voltages and a low temperature process compatible with plastic substrates. The HMDS treatment resulted in slightly enhanced saturation mobility, decreased threshold voltage by half, and increased I_on/I_off ratio by more than an order of magnitude. Future research will be focused on improving the characteristics of the anodized Ta₂O₅ device by achieving a smoother surface, and reducing the gate leakage current with suitable surface treatments other than HMDS. The realization of devices on plastic substrates is another area of interest, where Ta films will be formed on low-temperature-compatible substrates by e-beam evaporation without high temperature annealing.


2:15 PM O5.3
Bifunctional Field-Effect Transistors Based on Ambipolar Organic Semiconductors Thomas D Anthopoulos¹, Paul H. Woebkenberg¹, Edsger C. P. Smits^2,3, Paul W. M. Blom² and Dago D. M. de Leeuw³; ¹Department of Physics, Imperial College London, London, United Kingdom; ²Materials Science Centre, University of Groningen, Groningen, Netherlands; ³Philips Research Laboratories, Eindhoven, Netherlands.

Recent years have seen tremendous advances in the area of organic opto-/electronic devices and several previously envisioned applications are now reaching the stage of commercial exploitation. Organic field-effect transistors (OFETs) are among these devices and can be arguably viewed as a possible alternative to their inorganic counterparts in a range of low-cost, high-volume applications. Traditionally, OFETs have been used as pixel switches in active matrix displays and as the building blocks in integrated circuits where mechanical flexibility and low-cost fabrication are two prerequisites [1]. Recently, novel bifunctional OFETs have also made their debut where in addition to their classical current modulating function, light-emission as well as light-sensing has been reported [2-4]. Such electro-optical transistors are interesting for two reasons. First, the ability of combining opto-electronic functionality in a single device increases the number of potential applications, and secondly, it provides an ideal experimental platform for the study of various fundamental physical processes within organic semiconductors. Here we report on two types of bifunctional OFETs based on ambipolar organic semiconductors. The first device type is near infrared light-emitting OFETs (LEOFETs). The second device type is photo-sensitive OFETs. It is anticipated that integration of such bifunctional devices may lead to all-organic electro-optical switches and circuits and can be viewed as a significant breakthrough. In this work, LEOFETs are realised using a single ambipolar organic semiconductor. Due to the narrow band gap of the semiconductor simultaneous injection of electron/holes is possible allowing the formation of a recombination zone from which light emission occurs. We are able to accurately model the position of the recombination zone using a new model based on variable range hopping theory [5]. Based on experimental data and theoretical predictions we are able to estimate the distance at which metal-induced electroluminescence (EL) quenching occurs as the recombination zone approaches the source/drain electrodes. When recombination occurs at some distance away from the metal electrodes, EL efficiency remains constant. By selecting suitable ambipolar semiconductors we are able to realise light-sensitive OFETs with fast frequency response. Here device current is modulated by the incident light due to photogenerated carriers. By measuring the frequency response we are able to calculate the maximum operating speed of our light-sensing ambipolar OFETs. Such device provides significant advantages over conventional diode-type photodetectors due to their intrinsically low RC constants. [1]G. H. Gelinck, et al. Nat. Mater. 3, 106 (2004). [2]A. Hepp, et al. Phys. Rev. Lett., 91, 157406 (2003). [3]J. Zaumseil, et al. Nat. Mater. 5, 69 (2006). [4]N. Marjanovic, et al. Org. Ele. 7, 188 (2006). [5]E. C. P. Smits, et al. Phys. Rev. B., 73, 205316 (2006).


2:30 PM *O5.4
Material Design And Structure - Property Relation Of A Novel Low-Bandgap Polymer - Correlation Between The Electro-Optical Properties And Photovoltaic Device Performance. Christoph J. Brabec, M. Scharber, M. Koppe, M. Morana, Z. Zhu, R. Gaudiana and D. Waller; Konarka Technologies Inc., Linz, Austria.

One promising way to further develop the performance of organic solar cells is the design of low-bandgap copolymers, allowing to capture a larger fraction of the solar spectrum. However, the charge transport in most of these materials is frequently too low, probably a consequence of the electronic distortion along the backbone which is necessary to reduce the bandgap (at least for donor - acceptor (D-A) type polymers). Additional, the high degree of disorder, partially related to the alternating structure of D-A type copolymer is a reason for low mobility observed in these material classes.. We present a study of the transport properties of a novel low-bandgap conjugated polymer with high photovoltaic quantum efficiencies in the near infrared spectral region. The polymer poly BBTBT, consisting of alternating electron-rich diethyl hexyl bridged bithiophene units (BBT) and electron-deficient 2,1,3-benzothiadiazole (BT) units has a bandgap of 1.35 eV and a mobility in the order of 10-2 cm2/Vs or higher. Based on this specific donor, a whole class of low bandgap D-A polymers can be built. We discuss the electro-optical properties of this material class, investigate the structure - property relation and outline the impact on the photovoltaic performance.


3:30 PM *O5.5
Micron to Nanometer Scale Pentacene Thin Film Transistors Cherie R Kagan^1,2, Ali Afzali¹, George S Tulevski^3,1, Qian Miao³, Colin Nuckolls³ and Teresita Graham¹; ¹IBM T. J. Watson Research Center, Yorktown Heights, New York; ²University of Pennsylvania, Philadelphia, Pennsylvania; ³Columbia University, New York, New York.

We report the device characteristics of micron and nanometer scale pentacene thin film transistors (TFTs) fabricated using a solution processable precursor route. The solution processable precursor route to pentacene TFTs retains the desirable mobility of ~0.5-1 cm²/V-s and current modulation >10⁷ found in evaporated pentacene devices. In this talk, we describe measurements of repeated stressing and environmental exposure on the operational stability of pentacene TFTs, necessary for their application. We show that device degradation is consistent with thermal-oxidation of the channel that can be limited by reducing the operational power (through device scaling) or by limiting device exposure to ambient air. We also address a key difference in the thin film morphology of pentacene films deposited by evaporation versus from a solution-processable precursor that results in greater stability of the solution-processed devices. Scaling devices is important not only to limit degradation, but to achieve high speed and high output current devices. We report well-behaved and electrostatically scaled sub-100 nm channel length pentacene transistors and describe the physics of organic transistor electrostatics and contacts. The effects of contact resistance, already observable in micron scale devices, become dominate in the characteristics of nanometer scale channels. We show that engineering the metal-molecule interface by self-assembling a pentacene-thione on the electrode surface prior to thin film deposition dramatically improves device performance. This particular electrode surface modification uniquely provides a structurally and electronically complementary structure to the pentacene thin film that supports the improved device characteristics.


4:00 PM O5.6
Effect of Shallow Traps on Polaronic Transport in OFETs. Mattew Calhoun, Christine Hsieh and Vitaly Podzorov; Physics Department, Rutgers University, Piscataway, New Jersey.

Recent development of single-crystal organic field-effect transistors (OFETs) with significantly reduced disorder enabled the realization of intrinsic (not dominated by trapping) polaronic transport at organic surfaces and interfaces (see, e.g., [1,2]). Nevertheless, the majority of OFETs is still dominated by shallow traps. Here, we propose a method of shallow trap characterization, based on photo-induced charge transfer across the semiconductor-dielectric interface in OFETs [3]. The photo-induced electron and hole transfer has been investigated in trap dominated p-type OFETs. It was observed that the transfer of electrons into the dielectric results in a decrease of the field-effect mobility of polarons, suggesting that additional shallow traps are generated in the conduction channel. Using this effect, the dependence of the field-effect mobility on the density of shallow traps, μ(N_shallow), has been measured, which allowed to estimate the average polaron trapping time, τ_tr = 50 ± 10 ps, and the density of shallow traps, N_shallow = (3 ± 0.5)10^11 1/cm^2, in the channel of single-crystal tetracene OFETs. [1]. R. W. I. de Boer, M. E. Gershenson, A. F. Morpurgo and V. Podzorov, Phys. Stat. Solidi, 201, 1302 (2004); [2]. M. E. Gershenson, V. Podzorov, and A. F. Morpurgo, “Colloquium: electronic transport in single-crystal organic transistors”, Rev. Mod. Phys. 78, 973 (2006); [3]. V. Podzorov and M. E. Gershenson, Phys. Rev. Lett. 95, 016602 (2005).


4:15 PM O5.7
Frequency-response of the Accumulation Channel in Organic MISFETs David Martin Taylor, David Llewellyn John and Justin R Lawrence; School of Electronics, University of Wales, Bangor, Gwynedd, United Kingdom.

It is customary to evaluate organic MISFET transistors by measuring their DC output and transfer characteristics. From these measurements important parameters such as the carrier mobility and on-off ratios may be determined. While allowing the DC operation of devices to be modelled and their intrinsic cut-off frequency to be estimated, this approach does not provide a means of determining the response of the device over a range of frequencies, which is an essential requirement for circuit modelling. We have already shown that admittance measurements can be used to great effect for investigating trapping of both majority [1] and minority charge carriers [2] in interface states. Following the approach by Nicollian and Goetzberger [3], we have now derived the frequency-dependence of the susceptance and conductance of the gate-voltage-induced accumulation channel in an organic MISFET under different operating conditions e.g. modulation of the source voltage with fixed gate voltage and grounded drain i.e. in the linear regime. As expected, the results show a strong dependence on (a) the sheet resistance, Rs, of the channel, which in turn is determined by the carrier mobility and gate voltage through control of the induced charge, (b) the gate capacitance per unit area, Ci and (c) device dimensions. At low-frequency, both the resistance and capacitance of the channel are constant and given by RsL/W and CiLW/3 respectively, where L and W are the channel length and width. The latter compares with 2CiLW/3 for the gate-channel capacitance for an enhancement-mode silicon MOSFET operating in saturation when the gate voltage is modulated. Above the cut-off frequency, both the capacitance and loss (conductance/angular frequency) become equal and decrease as f^(-0.5). In addition to predicting the frequency response of the device, application of the model to data from real devices enables a number of device/material parameters to be determined e.g. the dependence of Rs and hence carrier mobility on gate voltage in the linear regime, where mobility is trap-controlled. This is particularly important in hopping systems where the mobility determined under dc conditions may differ from that under ac conditions, because of the different time scales involved. Furthermore, the technique may be applied to n-channels formed during photocapacitance measurements [2] to determine electron mobilities without the need for a low work function electrode to encourage electron injection as is the case in an ambipolar transistor. [1] I. Torres and D. M. Taylor, 2005, J Appl Phys, 98, Art. No. 073710. [2] D. M. Taylor, J. A. Drysdale, I. Torres and O. Fernández, 2006, Appl Phys Lett, (in press). [3] E. H. Nicollian and A. Goetzberger, 1965, IEEE Trans ED-12, 108


4:30 PM O5.8
Electric Field Induced Gap States in Pentacene. Dietmar Knipp¹, Amare Benor¹, Arne Hoppe¹, Veit Wagner¹ and Armin R. Voelkel²; ¹School of Engineering and Science, International University Bremen, Bremen, Germany; ²Electronic Materials and Devices Laboratory, Palo Alto Research Center, Palo Alto, California.

Despite the realization of polycrystalline pentacene transistors with high mobility the electronic transport is not fully understood. In particular the creation of gap states and the influence of gap states on the charge transport are still under investigation. In order to study the creation of electronic defects electrical in-situ measurements of pentacene TFTs were carried out. The devices were exposed to oxygen and moisture to study the influence on the device characteristic. The polycrystalline pentacene films transistors were prepared by Organic Molecular Beam Deposition with hole mobilities ranging from 0.2 - 0.5cm²/Vs. Exposing the transistor to oxygen does not lead to a change of the transistor characteristic. Only if an electric field is applied while exposing the devices to oxygen a change of the device characteristic is observed. Applying voltages to the device 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 0.6V/decade. The threshold voltage and the mobility of the transistors are only slightly affected by the oxygen contamination. Furthermore, the electrical characteristics were numerically simulated using a density of state charge transport model. The simulations reveal that the onset of the drain current for positive gate voltages is caused by acceptor-like defect states deep in the bandgap. A Gaussian distribution of defect states was assumed for the simulation of the I/V characteristic. A good agreement between the numerical simulation and the experiment was observed. The energetic level of the defect distributions are in good agreement with first-principles pseudopotential density functional calculations of oxygen related defects in pentacene.


4:45 PM O5.9
Metallic Transport in Electrolyte-Gated Polymer Field-Effect Transistors Anoop S. Dhoot¹, Jonathan D. Yuen¹, Martin Heeney², Iain McCulloch², Daniel Moses¹ and Alan J. Heeger¹; ¹Center for Polymers and Organic Solids, University of California, Santa Barbara, California; ²Merck Chemicals, Southampton, United Kingdom.

We have studied the carrier transport in conjugated polymer field-effect transistors (FETs) using electrolytes as the gate and gate dielectric. An easily removable, aqueous salt electrolyte is used to show that conventional bulk electrochemical doping of the semiconducting polymer is unimportant and that, instead, very high field-induced carrier densities, ~10²² charges/cm³, present in the device channel, lead to metallic conductivity, typically ≈10²-10⁴ S/cm at room temperature. At 4.2 K, devices gated using a polymer electrolyte exhibit metallic transport and support a remarkable current-carrying capacity without any noticeable degradation; the current density in the transistor channel exceeds 2×10⁷ A/cm².


SESSION O6: Poster Session: Materials, Devices and Characterization II
Wednesday Evening, April 11, 2007
8:00 PM
Salon Level (Marriott)
O6.1
Abstract Withdrawn


O6.2
A New Organic-Solvent-Free Process for High Stability Flexible/Organic Thin Film Transistors Jeng-Hua Wei, HorngJiunn Lin and Ying-Ren Chen; Electronics Engineering, Ching Yun University, Jung-Li, Taiwan.

In this paper, we report a new soluble, organic-solvent-free process for organic thin film transistors (OTFTs). In general, the semiconducting and insulating layers of OTFT are dissolved in organic solvents and these organic solvent will damage the underlying layers, including the gate insulator and plastic substrate. It will cause undesirable leakage current. Besides, the water and oxygen molecule in the air usually oxidize the organic base material and the performance of OTFTs will degrade evidently. In our OTFTs, both the gate insulator & semiconducting channel are dissolved in the water and each layer is formed by spun-on process. The drawbacks of organic solvent can be removed and the highly air-stable OTFTs are completed. For the gate insulator, the silicon oxide film is deposited by a special liquid- phase deposition (LPD) process at low temperature. In this LPD process, the water and hydrofluorosilicic acid (H2SiF6) are mixed for the oxide deposition. For the semiconducting layer, the single-wall carbon nanotubes (SWNTs) are used as the channel materials for OTFTs. In this device, the SWNTs were dissolved in the mixture of water and Sodium dodecyl sulfate (SDS). In the OTFT, first the bottom gate electrode is fabricated, and then the LPD oxide is deposited on the Si wafer or plastic substrate. Subsequently, the SWNTs network is coated on the LPD-SiO2 at room temperature. By controlling the density and uniformity of SWNTs networks, the process will be optimized to meet the high mobility and high on/off current ratio requirements. Finally, the Au electrodes are formed by the shadow mask method and the top contact structure OTFT is finished. The I-V characteristics of this organic-solvent-free OTFT are measured. In this device, all the process and measurement are made in air. We found this device demonstrates a highly stabile performance. As keeping the devices in the air for the several days, no evident drain current degradation is found.


O6.3
Fullerene/phthalocyanine as Ambipolar Transport Materials for Organic Light Emitting Devices. Yanyan Yuan¹, Zhenghong Lu¹, Wenli Jia² and Suning Wang²; ¹University of Toronto, Toronto, Ontario, Canada; ²Queen's University, Toronto, Ontario, Canada.

The combination of copper phthalocyanine(CuPc) and C60 was studied extensively in organic solar cells. In the present work, this material system, either employed as C60/CuPc bilayer or as CuPc:C60 composite, was investigated systematically for its applications in organic light emitting devices (OLEDs). It was found that they could be used for electron injection and transport, for hole injection and transport, and as an ambipolar interlayer interposed between two emission units. Although typically employed as a hole injection layer in OLEDs, CuPc has an electron mobility that is higher than common electron transport materials. However, a large electron injection barrier usually exists from CuPc to adjacent emission layers such as tris (8-hydroxyquinolinato) aluminum (Alq). Hung et.al addressed this barrier by placing either a Li layer[1] or LiF/Al bilayer[2] between CuPc and Alq. However, both methods have potential problem of Li diffusion, which may cause device degradation. In this work, C60 was used to facilitate electron injection from CuPc to Alq3, either by inserting a C60 layer or by mixing C60 with CuPc. For example, compared to a control device with Alq as electron transport layer (ETL): ITO/CuPc(25nm)/NPB(45nm)/Alq(40nm)/LiF/Al, the device with C60/CuPc bilayer as ETL had lower driving voltage: ITO/CuPc(25nm)/NPB(45nm)/Alq(20nm)/C60(15nm)/CuPc(10 nm)/LiF/Al. Moreover, a significant increase of CuPc thickness from 10nm to 52nm did not affect the current density -voltage (I-V) characteristics, due to the high electron mobility of CuPc. In addition, replacing the C60/CuPc bilayer with a 25nm CuPc:30wt%C60 composite did not change the I-V characteristics either, indicating the high electron transport ability of the composite. However, the devices using CuPc:C60 composite as ETL have substantially lower current efficiency than the devices with C60/CuPc bilayer ETL. This low current efficiency was attributed to high leakage hole current, since our previous study found that CuPc:C60 composite transports holes efficiently. Furthermore, the fact that CuPc:C60 composite transport both electrons and holes inspired us to explore the possibility of using CuPc:C60 as ambipolar interlayer between two emission zones. The following device as fabricated: ITO/NPB/Alq/CuPc:30wt.%C60/NPB/BNPB/TPBi/LiF/Al, where Alq is a green emitter, and BNPB is a blue emitter[3]. It was found that the electroluminescence (EL) spectrum of the device combines the EL of Alq and BNPB. This observation proves the CuPc:C60 composite is capable of transporting holes and electrons simultaneously in OLEDs, which is useful for making white OLEDs with multiple emission units. 1. L. S. Hung and C. W. Tang, Applied Physics Letters 74, 3209 (1999). 2. L. S. Hung and M. G. Mason, Applied Physics Letters 78, 3732 (2001). 3. W. L. Jia, X. D. Feng, D. R. Bai, et al., Chemistry of Materials 17, 164 (2005).


O6.4
Dielectric Materials for Low Operating Voltage Organic Thin-film Transistors. Mark E. Roberts¹, Nuria Queralto^2,1, Stefan C.B. Mannsfeld¹, Zhenan Bao¹ and Wolfgang Knoll²; ¹Chemical Engineering, Stanford University, Stanford, California; ²Max Planck Institute for Polymer Research, Mainz, Germany.

Over the past few decades, interest in organic materials for electronic applications has been steadily increasing due to their distinct advantages for inexpensive, large area processing. While vast attention has been paid to organic materials as the active semiconductor layer, modest consideration has been given to the gate-dielectric material. Some of the earlier reports have shown great improvements in device performance using polymer blends as the dielectric materials . Furthermore, low operating voltage devices have been realized with thin dielectric layers consisting of cross-linked networks of polymers in an alkylsilane matrix . In our research, we show alternative materials and methods for cross-linking polymer layers integrated into organic thin-film transistors (OTFTs) and continuing work toward low operating voltage devices. 2.2/Vs with an on-off ratio of greater than 10⁴ on OTS treated dielectric substrates.2/Vs, with on/off ratios greater than 10³ for top-contact devices. OTFTs are also demonstrated on flexible substrates using commercially available aluminum foil, which serves as the gate electrode. Flexible devices with pentacene active layers are measured in the flat state, followed by bending to radii of curvature of 7 mm and 2 mm. Slight variations in transfer curve are observed through the bending cycle.


O6.5
Method for Determining the Depth Profile of Emission Dipoles in Organic Light Emitting Devices from Experiment M. Megens¹, M. Bartyzel¹, S. L.M. Van Mensfoort², H. Greiner³ and Reinder Coehoorn¹; ¹Philips Research Laboratories, Eindhoven, Netherlands; ²Applied Physics Department, Eindhoven University of Technology, Eindhoven, Netherlands; ³Philips Research Laboratories, Aachen, Germany.

The external quantum efficiency of organic light emitting devices (OLED) depends strongly on the shape of the distribution of the emission dipoles across the semiconducting layer. A predominant contribution to this microcavity effect is formed by exciton quenching near the electrodes. For OLEDs that are based on a single semiconducting layer, this leads to the requirement that the electron and hole mobilities should be not too dissimilar, in order to avoid a situation in which recombination takes place predominantly near one of the electrodes. So far, semi-quantitative information about the shape of the dipole emission profile for single-layer OLEDs was only obtained from electronic device modelling, using measured parameters describing the electron and hole mobilities, injection, and exciton diffusion [1]. Such analyses are based on extensive sets of electrical measurements, for single and double carrier devices, and the emission profiles can be sensitive to the details of the transport model that is assumed. We have developed an alternative method for obtaining the depth profile of the emission, using the measured angular and polarization dependence of the emission spectrum, and using the predicted dipole position dependence of these spectra as obtained from a thin-film microcavity outcoupling model [2]. No use is made of electrical modelling. A weighed least squares fit method is employed to obtain the dipole emission intensity in typically 10 intervals across the organic layer thickness, and to obtain a parameter describing the dipole orientation distribution. We apply the method to OLEDs of the type (glass/PEDOT:PSS/LEP/Ba:Al), containing a blue emitting polyfluorene-based light emitting polymer (LEP) described in ref. 3. Experimental results for devices with thicknesses in the range 80-160 nm were analyzed, as a function of the applied voltage. As compared to the analysis given in ref. 3, which predicted that for a wide range of conditions the emission takes place very close to the anode, a wider distribution is obtained. An extensive analysis was made of the sensitivity on the input parameters (the layer thicknesses and refractive indices), and on the model used for optical self-absorption (mainly at small wavelenghts) in the LEP layer. This is a fundamental issue on which there is no full consensus in the literature [4]. We discuss the potential of the method, and some challenges. [1]. D.W. Markow and P.W.M. Blom, Appl. Phys. Lett. 87, 233511 (2005), and references therein. [2]. K.A. Neyts, J. Opt. Soc. Am. A 15, 962 (1998). [3]. R. Coehoorn et al., Proc. SPIE 6192 (2006), 61920O. [4]. M.S. Tomaš and Z. Lenac, Phys. Rev. A 56, 4197 (1997); G. Juzeliunas, J. Phys. B: At. Mol. Opt. Phys. 39, S627 (2006).


O6.6
Work Characteristics of Conducting Polymer Actuators using Cation Driven Polypyrrole Keiichi Kaneto, Hisashi Fujisue, Tomokazu Sendai, Kentaro Yamato and Wataru Takashima; Life Science and Systems Engineering, Kyushu Institute of Technology,, Kitakyushu, Japan.

Conducting polymers deform upon electrochemical oxidation and reduction, including the change of electrical conductivity. The deformation can be utilized as a soft actuator driven by voltage application. In this paper, results on the load dependence of polypyrrole (PPy) actuator are mentioned with a discussion of the energy conversion efficiency. Polypyrrole tubes were electrodeposited on gold coated plastic rods of acryl resin with the diameter of 2.0 mm in an aqueous solution of pyrrole and dodecylbenzensulfonic acid (DBS). The actuator was operated in aqueous 1M NaCl, and the change of tube length was measured as the function of load stress. The PPy/DBS tube was expanded upon reduction and contracted by oxidation, since the DBS is too large anion and can’t be dedoped. Hence, the tube did work to lift a weight by oxidation. It was found that the initial strain of ca.3 % at a load zero decreased to the half at 5MPa, and the current response varied with increasing stress, as the current lasted longer at higher stress.The electrical input energy was calculated to be about 0.26 J at 3-4 MPa. The mechanical out put energy was estimated from the work of lifting weights to be 0.8-0.85 μJ at 4-5 MPa load stress. The maximum energy conversion efficiency of PPy/DBS was obtained to be 0.035%. The overall results will be discussed, taking the Young’s modulus of PPy/DBS tubes in consideration. References [1] S. Hara, T. Zama, W. Takashima and K. Kaneto, Smart Mat. Struct., 14 (2005) p1502. [2] K. Yamato and K. Kaneto, Analytica Chimica Acta 568 (2006) pp133-137.


O6.7
Soluble Naphthalenetetracarboxylic Dianhydride (NTCDA)-Based Semiconductor Materials for Organic Electronics Ya-Lien Lee and Tri-Rung Yew; MSE, National Tsing-Hua University, Hsin-Chu, Taiwan.

The 1,4,5,8-naphthalene-tetracarboxylic dianhydride (NTCDA)-based organic material as an n-type semiconductor layer for organic electronics was investigated. The NTCDA-based material was spin-coated on SiO2/p+Si or polyimide/p+Si substrate using toluene, chloroform, dimethyl formamide (DMF), or methanol as a solvent. The p+ Si was used as a backgate electrode. The Au or Al layer was deposited on the NTCDA-based semiconductor layer as source/drain electrodes. The organic electronic devices were fabricated and electrically characterized in air. To enhance electrical properties, various approaches including the annealing and surface treatment of NTCDA, and interface buffer layer insertion between NTCDA-based semiconductor layer and source/drain electrode were studied. The physical properties of NTCDA-based semiconductor layers were inspected by atomic force microscopy (AFM), X-ray diffraction, and Fourier transform infrared spectrum (FTIR) for the correlation with electrical characteristics.


O6.8
Micropatterning of Organic Semiconductors by Surface Modification with SAMs for Patterning of Organic Semiconductors Yutaka Ito, Jason Locklin and Zhenan Bao; Chemical Engineering, Stanford University, Stanford, California.

Nonlithographic patterning technique of organic semiconductors is important because organic semiconductors tend to degrade under conventional photolithography conditions. Recently, we have demonstrated the use of polydimethylsiloxane (PDMS) stamps to fabricate patterns of different conjugated thiols on gold and silanes on silicon oxide substrates that could be used to template the selective growth of crystals of organic semiconductors from solution and showed higher performance of arrays of organic field-effect transistors (OFETs) compared to unpatterned OFETs. Here, we improved this process to get patterned monolayers on oxides, which have very high surface smoothness and investigated its influence on crystal growth. This newly developed preparation method of monolayers and patterns is presented. The monolayers were characterized by AFM, contact angle measurements, and ellipsometry. Also patterning procedure of OFETs is demonstrated.


O6.9
Effects of Mechanical Stress on Printed Polymer-Based TFTs on Flexible Display Backplanes Julia Rosolovsky Greer, William S Wong, Rene Lujan, Brent Krusor and Robert A Street; Electronic Materials and Devices Laboratory, Palo Alto Research Center, Palo Alto, California.

Additively printed polymer thin-film transistors (TFTs) arrays on plastic substrates can enable low cost displays with new functionality and performance. Here, we discuss some of the challenges arising from the integration of ink-jet printing and polymeric semiconductors to fabricate display backplanes on flexible substrates. Specifically, we present the results of in-situ electrical performance of printed bottom-gate TFTs subjected to a known mechanical strain during both static and dynamic loading. These TFTs were fabricated on flexible polyester substrates, with a polythiophene derivative PQT-12 as the semiconducting polymer and conventional inorganic metal and dielectric layers. Transfer/output characteristics and field-effect carrier mobility as a function of elastic strain for several device dimensions and geometries are evaluated and compared with those of amorphous Si-based TFTs. Transistor performance during high- and low-cycle fatigue is compared to that during incremental static loading, and the effects of device dimensions and orientation on their performance while mechanically deformed are also discussed. We found that for all deformation modes, the TFTs functioned in compression down to the radius of curvature of 4 mm, corresponding to 2% strain, with little degradation in performance, while in tension the transistors failed due to dielectric cracking at ~ 1% strain.


O6.10
Organic Field-effect Transistors Based on Pi-congugated Materials. Yunqi Liu, Institute of Chemistry, Chinese Academy of Sciences, Beijing, China.

Organic field-effect transistor (OFET) is a three-terminal device whose characteristics can be modulated by the electrical field. It is composed of organic conjugated molecules as active channels, inorganic or polymer insulators as dielectric layers and metals or conductivity polymer as electrodes. Since the first OFET was reported in 1986, OFETs have drawn more and more attention because of their low cost, flexibility as well as the capability for large area preparation. The design and synthesis of novel materials that possess a low threshold voltage, a high on/off ratio, and high mobility and stability under ambient and operating conditions are the major challenges in organic semiconductor research. Pi-conjugated organic molecules are the subject of considerable current research interest in organic semiconductors. OFETs commonly have good performance (taking mobility as an example, 1.5 cm2/Vs on chemically modified SiO2/Si substrates for pentacene, 1.1 cm2/Vs for alkyl-substituted oligothiophene and beyond 0.1 cm2/Vs for regioregular polythiophene. Currently, the series of pi-conjugated organic materials used in OFETs are limited. Thus, to fabricate and synthesize stable organic semiconductors with high performance can enhance the development of OFETs. In this presentation, we have successfully fabricated OFETs, exploring a series of pi-conjugated molecules as the organic semiconductors, including linear, planar, star-shaped compounds and carbon nanotubes. [1-10] References 1. Yanming Sun, Yunqi Liu and Daoben Zhu, J. Mater. Chem., 2005, 15, 53. 2. Lei Fu, Zhimin Liu, Yunqi Liu, Buxing Han, Pingan Hu, Lingchao Cao and Daoben Zhu, Adv. Mater., 2005, 17, 217. 3. Wenping Hu, Hiroshi Nakashima, Kazuaki Furukawa, Yoshiaki Kashimura, Katsuhiro Ajito, Yunqi Liu, Daoben Zhu, Keiichi Torimitsu, J. Am. Chem. Soc., 2005, 127, 2804. 4. Kai Xiao, Yunqi Liu, Ping’an Hu, Gui Yu, Yanming Sun and Daoben Zhu, J. Am. Chem. Soc., 2005, 127, 8614. 5. Yanli Chen, Wei Su, Ming Bai, Jianzhuang Jiang, Xiyou Li, Yunqi Liu, Lingxuan Wang, and Shuangqing Wang, J. Am. Chem. Soc., 2005, 127, 15700. 6. Kai Xiao, Yunqi Liu, Ting Qi, Wei Zhang, Fang Wang, Jianghua Gao, Wenfeng Qiu, Yongqiang Ma, Guanglei Cui, Shiyan Chen, Xiaowei Zhan, Gui Yu, Jingui Qin, Wenping Hu, and Daoben Zhu, J. Am. Chem. Soc., 2005, 127, 13281. 7. Lei Fu, Yunqi Liu, Zhimin Liu, Buxing Han, Lingchao Cao, Dacheng Wei, Gui Yu and Daoben Zhu, Adv. Mater., 2006, 18, 181. 8. Wenping Hu, Jun Jiang, Hiroshi Nakashima, Yi Luo, Yoshiaki Kashimura, Keqiu Chen, Zhigang Shuai, Kazuaki Furukawa, Wei Lu, Yunqi Liu Daoben Zhu, Keiichi Torimitsu, Physical Review Letters, 2006, 96, 027801. 9. Yaling Liu, Hongxiang Li, Deyu Tu, Zhuoyu Ji, Congshun Wang, Qingxin Tang, Ming Liu, Wenping Hu, Yunqi Liu, and Daoben Zhu, J. Am. Chem. Soc., 2006, 128, 12917. 10. Ying Wang, Hongmei Wang, Yunqi Liu, Chong-an Di, Yanming Sun, Weiping Wu, Gui Yu, Deqing Zhang and Daoben Zhu, J. Am. Chem. Soc., 2006, 128, 13058.


O6.11
Energy Transfer Behaviors of semiconducting Polymers doped with Iridium-Complexes Chan Im¹, Jungyun Chang¹, Jongdeok An¹ and Young Kwan Kim²; ¹Dept. of Chemistry, Konkuk University, Seoul, South Korea; ²Dept. of Information Display, Hong-Ik University, Seoul, South Korea.

Electroluminescent devices based on organic materials are of considerable interest owing to their attractive optoelectronic properties and potential applications to flexible organic displays. In spite of some advantages, organic materials have a crucial fundamental limit correlated with singlet and triplet exciton formation probability. One of the advance of organic light-emitting diode (OLED) technology is the discovery of electrophosphorescence with which one can overcome the upper limit of the internal quantum efficiency. The design and synthesis of triplet emitting materials containing heavy-metal complexes, where strong spin-orbit coupling leads to singlet-triplet state mixing which removes the spin-forbidden nature of the radiative relaxation of the triplet state, are therefore particularly important in achieving high-efficiency electrophosphrescence in OLEDs. To understand the exact phosphorescent dopant-induced photoluminescence (PL) properties of photoactive thin films consisting of a pi-conjugated polymer matrix and a triplet dopant were prepared as thin films by the conventional spin coating method. As matrixes, diverse types of polymer including poly(9-vinylcarbazole) (PVK) and poly[9,9-bis(2-ethylhexyl)fluorene-2,7-diyl] (PF2/6) and as triplet emitters, some of typical iridium complexes including iridium(III)fac-tris(2-phenylpyridine) (Ir(ppy)3) or iridium(III)bis[(4,6- fluorophenyl)-pyridinato-N,C2’]picolinate (FIrpic) were chosen to study. Those doped films, as well as their pristine films, on quartz substrates were characterized by means of UV-Vis absorption, and PL and PL excitation (PLE) spectroscopy for a wide spectral range. It was found that the facility of exciton migration from matrix to dopant, and subsequent triplet emission, was significantly pronounced in the PVK-based blend systems. At the same time, PF2/6 blend systems showed an efficient PL quenching upon doping, although the PL maxima of the PVK and the PF2/6 as thin films were virtually identical. Additionally, we have performed series of cyclovoltammetric investigation to characterize their electrochemical properties. And we have also performed steady state photocurrent measurement to compare energy transfer behavior and electron transfer behavior within the films. Finally, we could show a competitive correlation between energy transfer and electron transfer between matrix and dopant molecules.


O6.12
Electronic Time-of-flight Measurement of Charge Carrier Mobility in a Polymer Field Effect Transistor. Debarshi Basu¹, Dharmendar Reddy¹, Lawrence Dunn¹, Ananth Dodabalapur¹, Martin Heeney² and Iain McCulloch²; ¹Microelectronics Research Center, The University of Texas at Austin, Austin, Texas; ²Merck Chemicals, Chilworth Science Park,, Southampton, United Kingdom.

An electronic method to characterize the drift velocity and mobility of charge carriers in organic thin film transistors (OTFTs) is presented. It combines the ease of electronic characterization with the insightful time-of-flight method to probe the transport along the channel of an OTFT. Charge carrier mobility of 0.52 cm²/Vs have been calculated using this method which are about 3 times higher than the field-effect transistor (FET) mobility. This technique can also be used to explore trap states and therefore obtain a comprehensive understanding of charge transport in these materials. This experiment is the first of its kind to be performed on a polymer transistor. Easy processibility, variety of available materials and ability to conform to curved surfaces make organic semiconductors ideal candidates for low cost, large area applications. This has prompted research leading to significant increase in their mobilities in recent years. FETs with an electrolytic dielectric that induces metallic conductivities in a thiophene based polymer, poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophenes) (pBTTT), have been reported to exhibit mobility of 3.5 cm²/Vs. The primary cause of such increase has been the reduction of traps and disorder by better chemical preparation of the polymer and effectively tuning the chemical properties of the dielectric surface. Insights gained from charge transport studies were, however, vital in guiding these efforts. The method presented here is a charge transport study on a polymer transistor with pBTTT as the active material that provides an alternate way to calculate the drift mobility of carriers. This technique involves the injection of a few extra carriers in the channel of a transistor that is already biased in steady state and conducting a constant current. The carriers are injected using an electronic impulse at the source, which then traverse the channel under the influence of previously applied source-drain bias, and are collected at the drain. The plot of the transient current generated by the movement of the extra carriers exhibits two distinct slopes when plotted in a log i-log t scale. This is characteristic of dispersive transport and the transit time is calculated from the intersection of the two linear regions by subtracting from this time the time at which the peak of the input waveform was obtained. By removing the effect of parasitic RC delays from the calculation we obtain the corrected transit time, which is then related to the drift mobility as μ = L²/τ.V_DS. A mobility of 0.52 (± 0.04) cm²/Vs at V_GS = -100 V, V_DS = -10 V is obtained from this method which is about three times higher than the FET mobility (0.18 cm²/Vs). This result supports the fact that there are high mobility carriers present in the channel. In summary, we have made the first measurements of drift velocity and mobility in a polymer transistor. The drift mobility obtained is higher than the field effect mobilities.


O6.13
Characterization of CuPc Semiconductor Thin Film Organic Field Effect Transistors (OFETs) Fabricated on Ultra-Thin High-k Gate Dielectrics. Cynthia F. Burham¹, Muhammad M Hussain² and Baxter Womack¹; ¹Electrical & Computer Engineering, The University of Texas at Austin, Austin, Texas; ²SEMATECH, Austin, Texas.

OFETs are promising, cost-effective alternatives for mass producible devices and large area applications such as radio frequency (RF) tags, sensors, and light emitting diode (LED) panels. Research has indicated that fabrication of Pentacene OFETs on high-k gate dielectrics may improve performance by lowering threshold voltage and enhancing mobility. These advances may make OFETs relevant to a wider range of applications. CuPc OFETs are excellent gas sensors, able to detect analyte concentrations in the low parts per million (PPM). Improved sensitivity may be possible using CuPc OFETs fabricated on high-k gate insulators which lower threshold voltage and improve mobility. This talk will present the fabrication and characterization of CuPc OFETs having extremely scaled HfO₂ gate insulators (sub 2nm EOT on planar Si FETs). The effects of both interface and film quality on CuPc OFET response will be analyzed.


O6.14
Robust Ultra-thin Fluorescent Freestanding LbL Conjugated Polymer Films Yen-Hsi Lin^1,2, Chaoyang Jiang¹, Jun Xu², Zhiqun Lin² and Vladimir V Tsukruk¹; ¹School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia; ²Department of Materials Science and Engineering, Iowa State University, Ames, Iowa.

The successful fabrication of purely polymeric free-standing structures with nanoscale thicknesses is technically challenging because of poor mechanical stability of conventional polymers. Conjugated polymers (CP) composed of rod-like chains with much stronger mechanical properties than usual flexible-chain polyelectrolytes are exploited here for LbL assembly. We report fabrication of freely suspended purely polymeric CP films but with a true nanoscale thickness (below 20 nm) which demonstrated excellent mechanical properties combined with high fluorescence. Furthermore, these films offer the possibility for impact in the areas of photovoltaic cells, light emitting diodes (LED), thin-film transistors, and bio-sensors due to their intriguing optoelectronic properties.


O6.15
Lower Hole-injection Barrier Between Pentacene and a 1-hexadecanethiol-modified Gold Substrate with a Lowered Work Function. Ki Pyo Hong, Jong Won Lee, Sang Yoon Yang, Kwonwoo Shin, Hayoung Jeon, Se Hyun Kim, Chanwoo Yang and Chan Eon Park; Chemical Engineering, POSTECH, Pohang, Kyung Buk, South Korea.

In organic devices such as organic field effect transistors (OFETs) and organic light emitting diodes (OLEDs), the metal-organic semiconductor interface plays a crucial role in determining the device performance. The key parameter at this interface is the injection barrier between the metal and the organic semiconductor, which is determined by the energy difference between the metal work function and the ionization energy of the organic material. We used ultra-violet photoemission spectroscopy (UPS) to study the injection barrier at the interface between pentacene and gold treated with 1-hexadecanethiol (HDT), which is known to decrease work function of metal. Through these UPS in-situ experiments, we found that the energy barrier between 1-hexadecanethiol (HDT)-modified gold and pentacene was 0.67 eV. This energy barrier was 0.03 eV smaller than that between bare gold and pentacene, despite the work function of HDT-modified gold being 0.8 eV lower than that of bare gold. This result does not follow the conventional idea that decreasing the work function of a metal increases the energy barrier. The observed behavior can be explained by two factors: 1/ the bare gold substrate exhibited a large vacuum level shift, whereas the HDT-modified gold did not; and 2/ pentacene on the HDT-modified gold had a lower ionization energy than pentacene on bare gold, which can be explained in terms of the polarization energy or the electronic splitting of the molecular orbitals related to the more crystalline structure of pentacene on the HDT-modified gold substrate, which was established by X-ray diffraction analysis. From this work, we found out that injection barrier between metal and pentacene is not determined only by work function of metal but also characteristics of metal surface and crystalinity of organic semiconductor which determines position of the highest occupied molecular orbital (HOMO) level.


O6.16
Polymer/Ta2O5 Hybride Gate Dielectrics for CuPc Thin-Film Transistors Jia Gao, Jianbin Xu, Xiaojiang Yu and Ning Ke; The Chinese University of Hong Kong, Hong Kong, Hong Kong.

Organic thin-film transistors (OTFTs) are of technical importance for applications of organic/photonic electronics. Modification of the inorganic high k gate dielectric with an ultrathin polymer buffer layer has been widely used for improving the device performance of OTFTs [1-3]. In this work, e-beam deposited Ta2O5 thin film was selected for the high k gate dielectric with the buffer layered modification by polymeric blend composed of PMMA (Polymethylmethacrylate) and PS (Polystylene). The transistor active layer of CuPc thin film was deposited after the buffer layered modification. It is found that the apparent mobility of the CuPc OTFT can be as high as 0.03 cm2/V-s. To understand the origin of the observed high mobility, the correlation between the morphology of CuPc thin film and the apparent device mobility has been investigated. Our results reveal that the average grain size (c.a. 60nm) of CuPc active layer with the buffer layered modification consisting of the polymeric blend is much larger than those without the polymeric blend treatment, which presumably facilitates the carrier transport and results in the high mobility. The resulting large grain size is tentatively assumed to result from the surface energy matching of the gate dielectric with CuPc whose surface energy is measured to be about 35 mJ/m2. This work is partially supported by the Research Grants Council of Hong Kong SAR, particularly, via Grant No. CUHK4172/06E.


O6.17
Characteristics of Amorphous Indium Zinc Tin Oxide (IZTO) Anode for Phosphorescent Organic Light Emitting Diodes. Han-Ki Kim¹, Jae-Wook Kang², Jang-Joo Kim² and Jung-Hyeok Bae¹; ¹Dept. of Information and Nano Materials Engineering, Kumoh National Institute of Technology, Gumi, Gyeongbuk, South Korea; ²OLED center, Soeul National University, Silimdong, Seoul, South Korea.

The preparation and characteristics of amorphous indium zinc tin oxide (IZTO) anode films grown using a rf sputtering at oxygen-free ambient for organic light-emitting diodes (OLEDs) is described. The electrical, optical, and surface properties of the amorphous IZTO anode were comparable to those of commercial ITO anode films even though it was prepared at room temperature. In addition, the work function of the amorphous IZTO anode (4.95 eV) is much higher than that of the commercial ITO anode (4.75eV). Furthermore, an Ir(ppy)3-doped phosphorescent OLED prepared on the amorphous IZTO anode film showed identical or better electrical and optical properties than that prepared on a commercial ITO anode film. Both the quantum efficiency (19%) and power efficiency (37.5 lm/W) of the OLED fabricated on the amorphous IZTO anode film was much higher than the quantum efficiency (12%)and power efficiency (22.1 lm/W) of an OLED with a commercial ITO anode.


O6.18
High-quality Thin-film Passivation by Catalyzer-enhanced Chemical Vapor Deposition for Organic Light-emitting Diodes. Han-Ki Kim¹, Jin-A Jeong¹, Myung Soo Kim², Jae-Wook Kang³ and Jang-Joo Kim³; ¹Information and Nano Materials Engineering, Kumoh National Institute of Technology, Gumi, Gyeongbuk, South Korea; ²Core Technolgoy Lab., Samsung SDI, Suwon, Gyeonggi, South Korea; ³OLED center, Seoul National University, Silimdong, Seoul, South Korea.

The thin-film passivation of organic light-emitting diodes (OLEDs) by a SiNx film grown by catalyzer-enhanced chemical vapor deposition was investigated. Using a tungsten catalyzer connected in series, a high-density SiNx passivation layer was deposited on OLEDs and bare polycarbonate (PC) substrates at a substrate temperature of 50 degrees C. Despite the low substrate temperature, the single SiNx passivation layer, grown on the PC substrate, exhibited a low water vapor transmission rate of 2~6*10^-2g/m;2<>2/day and a high transmittance of 87 %. In addition, current-voltage-luminescence results of an OLED passivated with a 150 nm-thick SiNx film compared to nonpassivated sample were identical indicating that the performance of an OLED is not critically affected by radiation from tungsten catalyzer during the SiNx deposition. Moreover, the lifetime to half initial luminance of an OLED passivated with the single 150 nm-thick SiNx layer was 2.5 times longer that that of a nonpassivated sample.


O6.19 TRANSFERRED TO O11.2

O6.20
Correlation Between Structure and Optical Properties of High Energy Gap poly(p-phenylenedifluorovinylenes. Maria Losurdo¹, Maria Giangregorio¹, Pio Capezzuto¹, Giovanni Bruno¹, Antonio Cardone², Carmela Martinelli², Gianluca Farinola², Francesco Babudri² and Francesco Naso²; ¹Chemistry, IMIP-CNR, Bari, Italy; ²Chemistry, ICCOM-CNR, University of Bari, Bari, Italy.

Poly(p-phenylenevinylene) (PPV) is one of the most widely investigated organic light emitting polymer, and literature has reported several derivatives of PPV with electron-withdrawing substituents on the arylene rings aimed at adjusting the HOMO and LUMO energies of the polymer, thereby generating a large variety of polymers emitting mainly in the red. Indeed, a few studies about electron-withdrawing groups on the vinylene unit of conjugated polymers have been reported In this contribution, we demonstrate the synthesis and properties of thin films of a new class of poly(p-phenylenedifluorovinylene) (PPDFV), which contain two fluoro atoms in the vinylene units. This class of fluoro-polymers is effective in reducing the barrier of electron injection. Specifically, we report on the correlation existing between the structure and optical functionality of the above polymeric films. Thin films of poly-methoxy-ethylexyloxy-phenylenedifluorovinylene (MEH-PPDFV) and of the ionic polymethoxy-propyloxysodiumsulfonate phenylenedifluorovinylene (MPS-PPDFV) have been deposited by casting on a variety of substrates including Si(001), ITO and Corning glass with a thickness ranging from few monolayers to approximately 1mm. In particular, we demonstrate that the above films have the largest HOMO-LUMO transition energy (above 3.5eV) ever reported for organic films and well suited for blue-light emission. We discuss the effect of the polymer ionicity, substrate and film thickness on the aggregation structure and its subsequent impact on the energy of fundamental electronic transition. The correlation between the structure and optical properties is established using spectroscopic ellipsometry (UVISEL- Jobin Yvon), which allows the identification of all the electronic transition characterizing the above polymeric films in the 0.75-6.5eV photon energy range, and measure the dispersion of the refractive index depending on the aggregation state. The thickness dependence also reveals a difference in the intermolecular interactions for the ionic MPS-PPDFV and non ionic MEH-PPDFV polymers also corroborated by a difference in the aggregation as detected by AFM. In particular, a blue-shift to value as high as 3.7eV is found for the HOMO-LUMO transition of the non ionic MEH-PPDFV, which smoothes with the increasing of thickness forming a lamellar structure with a surface roughness as low as 0.35nm well suited for device applications. Conversely, a red-shift of the electronic transitions are observed with the increase of thickness of MPS-PPDFV related to two-levels of chain organization consisting in clusters arranged in an hexagonal packing. This complex structural organization results in an increase of the surface roughness. Furthermore, the thermal stability of the above polymeric films has been investigated being stable at temperature as high as 140°C.


O6.21
High-performance Organic Light-emitting Diodes Prepared Using an Amorphous IZO Anode Film. Han-Ki Kim¹, Jong-Min Moon¹, Jung-Hyeok Bae¹, Jae-Wook Kang² and Jang-Joo Kim²; ¹Information and Nano Materials Engineering, Kumoh National Institute of Technology, Gumi, Gyeongbuk, South Korea; ²OLED center, Soeul National University, Silimdong, Seoul, South Korea.

The preparation and characteristics of amorphous indium zinc oxide (IZO) anode films for use in organic light-emitting diodes (OLEDs) is described. The electrical, optical, and surface properties of the amorphous IZO anode, which was prepared at room temperature (<50 ^oC) were comparable to those of commercial ITO anode films. Both fluorescent OLEDs and an Ir(ppy)3-doped phosphorescent OLEDs prepared on the amorphous IZO anode films showed identical or better electrical and optical properties than OLEDs prepared on a commercial ITO anode film. This indicates that amorphous IZO anode affords OLED performance that rivals or exceeds that of devices fabrication with commercial ITO anode.


O6.22
Novel Aromatic Diimides from the Diels-Alder Trapping of Photochemically Generated Bisdienes - Potential Materials for Photo- and Electroluminescent Devices. Michael A Meador¹, Daniel S. Tyson² and Faysal Ilhan²; ¹NASA Glenn Research Center, Cleveland, Ohio; ²Ohio Aerospace Insitute, Brook Park, Ohio.

Aromatic diimides have been utilized in molecular sensors [1], molecular electronics [2] and electroluminescent devices [3]. For the most part, this work has focused on perylene and naphthalene diimdes due, in part, to the ready availability of the corresponding dianhydride starting materials. The development of synthetic routes that allow access into other aromatic diimides will enable the development of new materials for electronics and photonics. A new method has been developed for the preparation of aromatic diimides. This approach involves the Diels-Alder trapping of o-xylylenols (photoenols) generated by photolysis of o-methylphenyl ketones. The chemistry is fairly versatile and has been used to prepare a variety of diimides, including those based upon anthracene, phenanthrene, benzo[e]pyrene and a novel Z-shaped perylene diiimde [4]. Some features of the new diimides include high fluorescence quantum yields and good photostability. The preparation of these compounds and selected examples of their photophysics will be discussed. [1] Holman, M.W.; Liu, R.; Zang, L.; Yan, P.; DiBenedetto, S.A.; Bowers, R.D.; Adams, D.M. J. Am. Chem. Soc. 2004, 126, 16126-133. [2] Xu, B.; Xiao, X.; Yang, X.; Zang, L. ; Tao, N. J. Am. Chem. Soc. 2005, 127, 2386-87. [3] Ego, C.; Marsitzky, D.; Becker, S.; Zhang, J.; Grimsdale, A.C.; Mullen, K.; MacKenzie, J.D.; Silva, C.; Friend, R.H. J. Am. Chem. Soc. 2003, 125, 437-443. [4] Ilhan, F.; Tyson, D.S.; Meador, M.A. Chem. Mater. 2004, 16, 2978-2980; Ilhan, F.; Tyson, D.S.; Stasko, D.J.; Kirschbaum, K.; Meador, M.A. J. Am. Chem. Soc. 2006, 128, 702-703; Ilhan, F.; Tyson, D.S.; Meador, M.A. Org. Lett. 2006, 8, 577-80.


O6.23
Abstract Withdrawn


O6.24
Efficient Top-emitting Polymeric White Light-emitting Diodes with Improved Charge-injection. Juo Hao Li, Jinsong Huang and Yang Yang; Materials Science & Engineering, UCLA, Los Angeles, California.

Efficient top-emitting polymeric white light-emitting diodes has been realized by improving charge-injection from both anode and cathode contacts. The device has a structure of anode/LEP/cathode, where light-emitting polymer (LEP) is poly(9,9-dioctylfluorene) doped with 0.2 wt% rubrene. It has been found that charge injection was significantly enhanced by adding an ultrathin electron-injection layer (EIL) and hole-injection layer (HIL) into cathode and anode, respectively. Mechanism study with photovoltaic measurement indicates that improved hole-injection is due to the reduction of barrier height, resulted from the addition of HIL. Peak power efficiency of 11.42 lm/watt was achieved at current density 1.2 mA/cm2 with the Commission International de L’Eclairage (CIE) coordinates of (x=0.35, y=0.43).


O6.25
Abstract Withdrawn


O6.26
Structural Investigation of Tetraceno[2,3-b]thiophene Thin Films for Transistor Applications. Quan Yuan¹, Stefan C. B. Mannsfeld², Minglee Tang², Michael F. Toney³, Jan Luning³ and Zhenan Bao²; ¹Materials Science and Engineering, Stanford University, Palo Alto, California; ²Chemical Engineering Department, Stanford University, Palo Alto, California; ³Stanford Synchrotron Radiation Laboratory, Stanford University, Palo Alto, California.

Understanding the structure-property relationship for organic semiconductors is crucial in molecular design and organic transistor process control. Charge carrier transport in organic field effect transistors predominantly occurs in a few semiconductor layers close to the interface with the dielectric layer. Therefore, a better understanding of the impact of the morphology in the first few monolayers above the dielectric layer on the charge transport is needed to improve the transistor performance. In this paper, the structure of 2nm and 20nm thick films of tetraceno[2,3-b]thiophene is discussed and compared to that of the similar widely used pentacene, and possible factors that affect the carrier mobility are discussed. To investigate the interface and thin film structure, grazing incidence X-ray diffraction (GIXD) using high-intensity synchrotron radiation source is utilized. It is found that films with a thickness of 20 nm or lower can be described by a single crystalline tetraceno[2,3-b]thiophene phase. Theoretical modeling is done to predict the molecular tilt angle and compare the resulting theoretical diffraction intensity to the experimental results. It is found that the very slight change in molecular structure (i.e. the thiophene substitution of a benzene ring) leads to measurable change in crystallinity, molecular packing and carrier mobility compared to the well-studied pentacene.


O6.27
Patterning of Organic Single Crystal Transistor Devices from Solution Stefan C. B. Mannsfeld, Armon Sharei, Jason Locklin and Zhenan Bao; Chemical Engineering, Stanford University, Stanford, California.

<p>For microelectronic applications such as sensor arrays or small-scale displays, low-cost production techniques for organic field effect transistor devices are desirable. We recently reported a materials-general method of fabricating large arrays of patterned organic single crystals by physical the vapour-phase transport deposition technique [1]. However, for some organic semiconductor materials, vapour-phase is not producing satisfactory results. Here, we present a patterning technique by which arrays of working field effect transistor devices based on organic single crystals (or crystalline agglomerates) can be produced by casting a molecular solution onto specially treated transistor device substrates. These substrates, comprising arrays of source-drain gold electrode pairs on a silicon oxide wafer, are chemically modified by treatment with octadecyltrichlorosilane (OTS) leading to preferential wetting of the gold electrode area by halogenated aromatic solvents. This contrast in wettability is employed to facilitate selective deposition of organic crystal which either already formed in solution or grow in-situ from solution droplets covering the electrodes. We discuss the patterning and resulting transistor performance for selected oligothiophene semiconducting molecules.

O6.28
Field Effect Transistors of Single Crystal Benzo-annulated Fused Oligothiophenes and Oligoselenophenes. Yukihiro Tominari¹, Koichi Yamada², Masakazu Yamagishi¹, Toshihiro Okamoto^3,4, Kenichi Kudoh^3,4, Atsushi Wakamiya^3,4, Shigehiro Yamaguchi^3,4 and Jun Takeya¹; ¹Osaka University, Toyonaka, Japan; ²CRIEPI, Komae, Tokyo, Japan; ³Nagoya University, Nagoya, Japan; ⁴SORST, JST, Kawaguchi, Japan.

In an effort to achieve higher performances in organic field-effect transistors, a progressive number of fascinating materials are being synthesized. So far, linearly fused acenes are the best known representatives of conventional compounds, which are challenged by newly synthesized compounds. High carrier mobility above 1 cm²/Vs has been indeed reproducibly achieved with high-quality polycrystalline thin films of pentacene parent molecules. Therefore, there have been intensive studies by now on synthetic approaches to their structural modification, reporting modulation of the electronic structures, modes of crystal packing, and thus carrier mobilities [1]. One drawback of the pentacene family is their inherent chemical instability, since these compounds have high oxidation potentials due to their quinoidal structures, so that they readily undergo photo or thermal oxidation. Because of serious concern on insufficient durability in the ambient atmosphere as the result, it is argued that additional encapsulation process is needed for the commercial use. Therefore, it is desired to explore new compounds featuring both high carrier mobility and high chemical stability, currently providing one of the major subjects in the field of material synthesis. Here, we introduce recently synthesized benzo-annulated fused oligothiophenes and oligoselenophenes as air-stable organic semiconductors for high-performance field-effect transistors. To evaluate electronic mobility intrinsic to the materials, the technique of single-crystal transistors are employed for two representative compounds of benzo-annulated pentathienoacene f-B5TB and its selenium analogue. The single crystal devices are prepared by the same technique that was applied to copper phthalocyanine and rubrene crystal transistors. The single crystals of f-B5TB and f-BT3STB are grown by horizontal physical vapor transport in a stream of argon gas. Selecting thin transparent crystals of transparent yellowish color with homogeneous surface morphology out of the products, we compose the FET structure incorporating a gate dielectric parylene thin film with the thickness of 0.7-1.8 μm. As the result of linear-regime measurement of the transfer characteristics, high mobility values of approximately 1.1 cm²/Vs and 0.5 cm²/Vs are achieved for the selenophene and thiophene compounds, respectively. Considerable Se- - -Se orbital interactions appear to be responsible for the higher mobility in the former compound. Moreover, both transistors exhibited no practical degradation in their performance, indicating their excellent air stability. Therefore, simultaneously bearing features of high mobility and high duration in ambient atmosphere, the materials are favored for practical applications in organic field-effect transistors. [1] M. M. Payne, S. R. Parkin, J. E. Anthony, C. C. Kuo, and T. N. Jackson, J. Am. Chem. Soc 127, 4986 (2005).


O6.29
Effect of Fabrication Temperatures on the Performance of Polypyrrole Coated Conductive Fabrics. Xiaoyin Cheng, Xiaoming Tao, Xiaoxiang Cheng, Hing Yee Joanna Tsang and Pu Xue; Institute of Textiles and Clothing, The Hong Kong Polytechnic University, Hong Kong, China.

This paper presents a comparison study of polymerisation temperatures on the performance of polypyrrole coated (PPy-coated) conductive fabrics. The pyrrole was coated on the fabrics by the chemical vapor deposition (CVD)process at room temperature (RT) and Low temperature (LT). The fabrics prepared at LT exhibit much higher strain sensitivity of over 400 at the strain of 50% with very large workable range of more than 50%. Furthermore, the LT CVD can produce the fabric sensor with higher electrical conductivity as well as better environmental stability. The surface morphology analysis by SEM and SPM confirms that the LT CVD can deposit a thinner, smoother, more uniform and ordered structure of PPy film on the fabric surface, which is beneficial to the sensing performance and environmental stability of the fabricated fabrics.


O6.30
Vertical-type Organic Hot-carrier Triodes Operated in Saturation Region with Ultra-high Gain. Chuan-Yi Yang¹, Tzu-Min Ou¹, Shiau-Shin Cheng¹, Meng-Chyi Wu¹, and Yi-Jen Chan³; ¹Institute of Electronics Engineering, National Tsing Hua University, Hsinchu, Taiwan; ²Research Center for Applied Sciences, Academia Sinica, Taipei, Taiwan; ³Electronics and Optoelectronics Research Laboratories, Industrial Technology Research Institute, Hsinchu, Taiwan.

In this paper, we investigate the promising vertical-type organic hot-carrier triodes based on small organic molecules. The devices show transistor-like characteristics which output current can be modulated by injecting various input currents on the thin metal base electrodes. The HCTs with an optimized 0.7 nm LiF hole injection enhancement layer and a 70 nm pentacene emitter can achieve an ultra high common-emitter current gain of about 50 in the saturation region at a low applied voltage VCE of -4 V and a base current density of 0.25 mA/cm^2. In addition, the HCTs exhibit a high output current density of 12.1 mA/cm^2 and a high on-off current ratio of about 10^3. It can not only operated in the current-injection mode but also bias-voltage mode.


O6.31
Origin of Green Emission in Extremely Pure Oligofluorene Films: Effect of Molecular Packing Jihoon Kang¹, Nayool Shin¹, Jungho Jo¹, Panagiotis E. Keivanidis², Frédéric Laquai², Gerhard Wegner² and Do Y. Yoon¹; ¹Department of Chemistry, Seoul National University, Seoul, South Korea; ²Max Planck Institute for Polymer Research, Mainz, Germany.

Time-resolved photoluminescence spectroscopy measurements of oligofluorenes with various side chains were studied. With extremely pure oligofluorenes, two kinds of red-shifted green emission were identified which have different origins; intermolecular interaction and on-chain chemical defect. Moreover, the unusual spectroscopic behavior of siloxane end-capped oligofluorenes, which exhibit high-order liquid crystalline phase, demonstrated that the molecular packing plays a serious role in the low energy emissions of the solid films of oligofluorenes, and therefore, polyfluorenes.


O6.32
Abstract Withdrawn


O6.33
Directly Imprinted DFB Polymer Sensor Laser Devices. Martin Gaal¹, Michael Teuchtmann², Veronika Rinnerbauer², Christine Hasenfuss², Holger Schmidt³, Kurt Hingerl² and Emil J.W. List¹; ¹Christian Doppler Laboratory Advanced Functional Materials, Institute of Solid State Physics, Graz University of Technology, Graz, Austria and Institute of Nanostructured Materials and Photonics, JOANNEUM RESEARCH, Weiz, Austria; ²Christian Doppler Laboratory Surface Optics, Institute of Semiconductor and Solid State Physics, Johannes Kepler University Linz, Linz, Austria; ³Institute of Applied Physics, Friedrich Schiller University Jena, Jena, Germany.

Conjugated polymers have attracted remarkable interest since they inhere convenient photo-physical properties and simple device fabrication. Their tune-ability over the whole visible spectrum combined with broadband emission and strong absorption make them to ideal candidates for solid state laser devices. Furthermore, with increasing demand for security applications, improvements regarding the sensitivity of chemical and biological sensors will greatly benefit from an ascending emphasis on sensor devices based on conjugated polymers. For this purpose enhancing the sensitivity of conjugated polymers on environmental factors, such as oxygen, water, specific organic vapors, etc. is an important step towards novel sensor devices. This sensitivity enhancement can be achieved by using conjugated polymer laser device, which bear the advantage of amplified spontaneous emission and laser emission being orders of magnitude more responsive to analytes than the regular photoluminescence from films. We report on the detection enhancement of an optical sensing device by means of amplified spontaneous emission as well as laser emission from the conjugated polymer poly[(2-methoxy-5(2-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEH-PPV). Upon photo-pumping of spin-caste films with the striped, focused 532 nm pulsed output of a Nd:YAG laser, amplified spontaneous emission was achieved. Periodic distributed feedback (DFB) structures were fabricated directly in the active polymer layer using a cheap soft-lithographic technique of liquid imprinting. Starting with a single master, a multiplicity of stamps can be fabricated. Furthermore, the liquid imprinting step can be performed several times using the very same stamp. This makes liquid imprinting to a simple, inexpensive, versatile and repeatable process, which enables to structure a wide range of materials and makes this method a promising candidate for mass production of conjugated polymer laser sensor devices. When the structured 390 nm distributed feedback grating was photo-pumped, laser emission was observed perpendicular to the substrate. For both, ASE as well as lasing, we monitored the thresholds under the influence of argon and ambient atmosphere, resulting in a change in the output emission intensity. In our investigations a change in output intensity divided by initial output intensity of about 50% for ASE and ca. 90% for laser emission above threshold was obtained.


O6.34
Abstract Withdrawn


O6.35
Extremely High Efficiency Orange-Red Organic Electrophosphorescent Devices Using Novel Electron Transport Materials Containing Dipyridylphenyl Groups. Junji Kido^1,2, Eisuke Gonmori¹, Nobuhiro Ide², Daisaku Tanaka^1,2, Ken-ichi Nakayama^1,2 and Yong-Jin Pu¹; ¹Polymer Science and Engineering, Yamagata University, Yonezawa, Yamagata, Japan; ²Optoelectronic Industry and Technology Development Association, Bunkyo-ku, Tokyo, Japan.

We have synthesized novel electron transport materials containing dipyridylphenyl groups. These materials have low LUMO level of about 3.2 eV, which can be expected to decrease electron injection barrier from cathode. In addition, these materials have wide HOMO-LUMO energy gap of about 3.5 eV, which leads to enough high triplet energy level to confine the triplet energy of phosphorescent metal complexes. We fabricated and evaluated the orange-red electrophosphorescent devices with a structure of ITO / p-doped polymer buffer layer / wide-gap arylamine hole-transport layer / CBP doped with PQ2Ir(dpm) / dipyridylphenyl compound / LiF / Al. The device exhibited low drive voltages and an extremely high power efficiency of 49 lm/W and an external quantum efficiency of 20 percent at 100cd/m2. Even at 1000cd/m2, a luminous efficiency of 30 lm/W was obtained. To calculate these efficiency values, angular distribution of emission from the device was characterized and considered. These values are the highest so far reported for orange-red electrophosphorescent devices.


O6.36
Abstract Withdrawn


O6.37
Perfluorinated Oxadiazole-based Oligomers for N-type Semiconductor Devices. Howard E. Katz, Amy Sarjeant and Chad A. Landis; Materials Science and Engineering, Johns Hopkins University, Baltimore, Maryland.

Developments in the field of organic semiconductors have progressed at an astounding rate over the past couple of decades. Nearly all of this research is based on p-type materials and in order to further expand the knowledge and possibilities of this field, a greater number of n-type semiconducting materials with mobilities on par with p-type materials are necessary. Previous examples of n-type materials show that the additions of highly electron-withdrawing groups and the addition of perfluoro groups are necessary for an effective n-type material. Our strategy involves the addition of perfluoro groups to oxadiazole-based oligomers. Oxadiazoles are known to have a higher electron affinity than thiophene and have already seen use in polymers as electron transport materials for organic LEDs. Our first derivatives are oligomers consisting bis-oxadiazoles capped with either trifluoromethylphenyl or perfluorophenyl groups. Single-crystal XRD and electrochemistry was carried out of these first derivatives, which show that the derivatives have reduction potentials at or less than -1.5 V. X-ray diffraction shows that the trifluoromethylphenyl derivative packs in a slipped-stack fashion. Initial attempts at device preparations from these new derivatives are also underway.


O6.38
High Work Function Materials for Source/Drain Electrodes in Printed Polymer TFTs. Veronica Sholin¹, Robert A. Street², Fred Endicott² and Ana Claudia Arias²; ¹Physics, University of California Santa Cruz, Santa Cruz, California; ²Palo Alto Research Center, Palo Alto, California.

Part of the challenge of fully printing polymer-based TFT arrays is finding suitable inks for the source and drain electrodes. The source and drain are in direct contact with the semiconducting polymer and a high work function ink is required for charge injection and minimal contact resistance at that interface. In addition to this, the ink must be homogeneous, compatible with the printer nozzle, and highly conductive. Gold nanoparticle inks satisfy these requirements; however, lower cost alternates are preferred. In this study we consider the use of Pedot:PSS (a high work-function polymer) in combination with Ag nanoparticles in bilayer structures and blends as an alternative material for source and drain of polymer-based TFTs. We also tested the use of a Pedot:PSS solution modified by the addition of conductivity enhancers such as Ethylene Glycol and poly(vinyl) pyrrolidone. These materials were tested in a TFT structure with (poly[5,5 -bis(3-dodecyl-2-thienyl)-2,2 -bithiophene] (PQT-12) as the semiconducting layer. TFTs with Ag:Pedot bilayers and the modified Pedot as source and drain showed very low contact resistance and transistor characteristics comparable to those of Au electrode devices. TFT mobilities of 4x10-3cm2/Vs, ON/OFF ratios of 106, and subtreshold slopes of 2.7V/decade were achieved at drain voltages of 30V. Although yielding good transistor characteristics (with mobilities of 10-2cm2/Vs and ON/OFF ratios of 107), full sintering of the Ag nanoparticles in the blends was hindered by the PSS in the Pedot:PSS solution. We were not able to achieve the high conductivity of pure Ag nanoparticles films when using the blend. Film morphology and printing conditions will also be discussed.


O6.39
Permeation Rate Measurements of Flexible Thin-film Encapsulation by Calcuim Corrosion Test. Namsu Kim¹, Seunghyup Yoo², William John Potscavage², Benoit Domercq², Bernard Kippelen² and Samuel Graham¹; ¹School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia; ²School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, Georgia.

Successful commercialization of flexible organic electronic devices is largely dependent on proper encapsulation that protects them from permeation of oxygen and water vapor. At present, low permeation encapsulation materials consist of multilayer films of organic/inorganic materials which can be deposited by chemical vapor deposition or atomic layer deposition methods. However, problems still exist when these materials are used in applications which require flexibility due to damage which can occur in the brittle inorganic film layers. Therefore, understanding the limits of these materials under mechanical deformation is important for the development of organic-flexible electronics We present our study on the performance of thin-film encapsulation (TFE) materials prepared by several low-temperature deposition techniques (PECVD, physical vapor deposition) on flexible stainless steel substrates. Both monolithic (SiOx, SiNx) and multilayer encapsulation materials (SiOx/parylene and SiNx/CNx:H) are investigated. Water vapor and oxygen permeation rate is determined by calcium corrosion test at accelerated test conditions in an environmental chamber in which change in resistance of calcium thin-film sheet encapsulated with TFE is monitored in situ. Most of all, we focus on the effect of the number of multilayer and the bending on the barrier properties of TFE. Effects of the radius of curvature for bending are systematically investigated. Additionally, we present mechanical properties (Young’s modulus and hardness) of thin film by nanoindentation for further analytical study of the stress and deformation in the inorganic/organic multilayer stack. Relationships between the amount of degradation in the encapsulation after bending and the mechanical properties of the films will be presented.


O6.40
Characterization of the Mesophase of a High Performance Semiconducting Polymer. Andrew J. Moad, Dean M. DeLongchamp, R. Joseph Kline, David J. Gundlach, Daniel A. Fischer and Lee J. Richter; NIST, Gaithersburg, Maryland.

Solution processable polymer semiconductors have potential applications in flexible electronics, displays, and solar cells. Poly(2,5-bis(3-alkylthiophen-2yl)thieno[3,2-b]thiophene) or pBTTT is a semiconducting polymer with exceptional hole mobility in thin film transistors upon annealing into a mesophase identified by differential scanning calorimetry of powder to exist in the region of ~ 140 C to 240 C. We have applied a variety of linear spectroscopic techniques: NEXAFS, spectroscopic ellipsometry (SE), and IR absorption to the characterization of pBTTT-C14 as a function of temperature, to identify the specific structural motifs of both the mesophase and the high performance, thermally processed film. Previous x-ray scattering studies have established that the material forms pi-stacked lamella, typical of polythiophenes. Upon cooling from the mesophase, the films exhibit exception order, characterized by molecular terraces as observed in AFM, with the long chain axis lying completely in the surface plane (SE) and a high degree of pi stacking order of the conjugated backbone. The side chains are interdigitated and exhibit essentially all trans order (IR), which may be a driving factor in the growth of large crystals. Upon re-heating, the side chains (IR) and conjugation length (SE) monotonically disorder with temperature until ~ 160 C, when the mesophase is entered. The mesophase is characterized by highly disordered side chains, near ideal in-plane order of the polymer chains, and moderate torsional disorder of the backbone. Upon re-cooling to ~140 C, side-chain order is reestablished upon entry into the ordered phase. The hysteresis of the side chains mimics the DSC of powders, however the thermochromism (back-bone torsional order) does not exhibit significant hysteresis. As-cast films exhibit greater disorder than annealed in all degrees of freedom (long axis order, pi-stacking, side chain order), and reveal a low temperature (~45 C) side chain disorder transition that is not present after thermal cycling. Entry into the mesophase is necessary to remove the long chain axis disorder. The structural transitions of the annealed thin film appear reversible, so long as the anneal temperature does not exceed the stability range of the mesophase and maybe critical to film reorganization in high carrier mobility semiconducting polymers, thereby informing future synthesis strategies. The spectroscopic data will be correlated with mobility measurements following a similar thermal schedule.


O6.41
Organic Molecular Monolayers for STM-induced Light Emission Measurements. Alan Shu-Chung Wan¹, James P. Long² and Antti J. Makinen¹; ¹Optical Sciences Division Code 5611, Naval Research Laboratory, Washington, District of Columbia; ²Chemistry Divison Code 6176, Naval Research Laboratory, Washington, District of Columbia.

The drive for developing nanoscale optoelectronic devices has lead researchers to explore thin films of organic molecules, or even single organic molecules as electroluminescent sources. By using the combination of photoemission spectroscopy and scanning tunneling spectroscopy (STS) we have investigated the electronic structure of an organic semiconductor N,N -ditridecyl-3,4,9,10-perylenetetracarboxylic diimide (PTCDI-C13), which has been successfully employed in organic solar cells1 and organic thin-film transistors.2-3 Measurements of scanning tunneling microscopy induced light emission (STM-LE) provide an excellent additional means to characterize the optoelectronic properties of such systems with high spatial resolution. In order to reduce the quenching of the molecular light emission by the conducting substrate, we deposited PTCDI-C13 films on an insulating spacer layer comprising of a dodecanethiol self-assembled monolayer (SAM) grown on a Au(111) substrate. Interestingly, PTCDI-C13 monolayers deposited on the SAM are found to exhibit the same herringbone structure as PTCDI-C13 films grown on a bare Au(111) crystal surface. We will discuss STS and STM-LE from PTCDI-C13 films grown on dodecanethiol SAMs and Au(111) and determine the exciton binding energy based on the measured optical and transport gaps. References: 1. A. K. Pandey, S. Dabos-Seignon, and J. Nunzi. Appl. Phys. Lett. 89, 113506 (2006) 2. S. Tatemichi, M. Ichikawa, T. Koyama, and Y. Taniguchi. Appl. Phys. Lett. 89, 112108 (2006) 3. T. Sakanoue, R. Yamada, and H. Tada. Proc. of SPIE 5940, 594DDY-1 (2005)


O6.42
The Influence of Inserting Novel Metal for Transparent PLEDs Chao-Wen Teng¹, Yen-Hsun Lu¹, Kou-Chen Liu¹, Lai-Cheng Chen² and Sung-Cheng Hu³; ¹Graduate Institute of Electro-Optical, Chung Gung University, Tao-Yuan, Taiwan; ²DELTA OPTOELECTRONICS, INC, Hsinchu, Taiwan; ³Chung Shan Institute of Science and Technology, Tao-Yuan, Taiwan.

In this work, the different metals were inserted below the ITO cathode to fabricate transparent PLEDs. The well performance of ITO can be easily achieved using sputtering deposition at low temperature. However, the ITO cathode sputtering usually accompanies with bombard damage that degrades the performance of devices. Moreover, the high work function of ITO cathode also reduces the electron injection ability. Therefore, a thin metal/LiF layer is inserted to diminish the sputtering damage and modify the electron injection barrier. The literatures indicated that using low work function metals associate with LiF layer will sufficiently increase the electron injection. However, these metals are sensitive with environment, especially during ITO cathode sputtering. Therefore, the stable metals, such as Al, Ag, and Au were attempted for a buffer layer. The reaction between Al and LiF will enhance the electron injection that is well known. Base on our investigation, inserting Ag layer has the similar phenomenon, although the enthalpy formation barrier of this reaction is larger than Al/LiF. It is suggested that the sputtering process of ITO will transfer the kinetic energy to the heat which help to overcome the formation barrier. However, the reaction between Au/LiF is not significant due to Au is much stable. Hence, the devices show the turn on voltage of 4.6, 4.8, 6.6, and 7V for devices with Al, Ag, ITO, and Au. The different work function metals also influenced the leakage current which perform the value of 3.6E-7, 3.73E-7, 4.1E-7, and 9.32E-7 A/cm2 for devices with Al, Ag, ITO, and Au, respectively. The higher work function metal cause the larger leakage current that can be attributed to the lower barrier height for hole injection at reverse bias. The maximum luminance of devices is 1200, 5300, 4870, and 2600 cd/m2 for Al, Ag, ITO, and Au. The luminance reduced with increasing work function except the device with Al. The high work function metal leads to the large electron injection barrier and reduce the luminance due to the carrier unbalance. The device with Al shows the lowest luminance that may be explained by the Li ion diffusion. Al easily reacts with LiF to liberate Li ions, and the sputtering-generation heat will further drive Li ion into emitting layer to form the quench centers. This conjecture was also examined by XPS measurement that shows the device with Al metal has the higher Li concentration in emitting layer than others. It is believed the quench centers will reduce the emission intensity. The XPS result also shows the device with thin metal layer has lower concentration of In, Sn and O which are the components of ITO in emitting layer than device without metal layer. This result implied that a thin metal sufficiently diminishes the sputtering bombard. In conclusion, choosing the proper novel metal, such as Ag will not only perform the lower operation voltage and lower leakage current, but also reduce the sputtering bombard damage.


O6.43
Surface Enhanced Fluorescence in Light Emitting Conjugated Polymer Films A. L. Holt and S. A. Carter; Physics, UC Santa Cruz, Santa Cruz, California.

Surface enhanced techniques that utilize localized surface plasmons inherent in nano-sized metal structures such as surface enhanced Raman (SERS), fluorescence (SEF) and absorption (SEA) spectroscopies have been used to probe many small molecules and their interactions; however, with conjugated polymers, this type of research has been limited. We report here on reproducible surface induced photoluminescence enhancement in light emitting polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) using thermally evaporated metal nano-islands and a spun-cast dielectric spacer layer. Maximum relative enhancements were observed with dielectric layer thickness near 50 nm, 15 nm polymer film thicknesses and large overlap between the absorption spectra of MEH-PPV and the extinction spectra of the nano-islands. We also observed photoluminescence quenching for films without the dielectric spacer layer and for MEH-PPV, found the effective distance of the localized surface plasmons to be 20 nm. We discuss the relevancy of the enhancement and quenching results as related to the polymer and nano-island film formation as well as to the choice of aligning the nano-island extinction spectra with the absorption or the emission spectra of the polymer molecule. We also discuss the significance of careful spacing between conjugated polymer and nano-structured metal which is associated with increases in florescence efficiency and absorption cross-section for improving OLED and solar cell efficiencies as well as for improving conjugated polymer optical sensing technologies.


O6.44
High-k Dielectrics for Organic MISFETs David Martin Taylor¹, Janet Lancaster¹ and Henrique Leonel Gomes²; ¹School of Electronics, University of Wales, Bangor, Gwynedd, United Kingdom; ²CEOT, Universidade do Algarve, 8005-139 Faro, Portugal.

It has been argued [1] that low-k dielectrics are preferred for organic MISFETs in view of the reduction in mobility observed when the dielectric constant of the gate insulator increases. It is suggested that high-k dielectrics give rise to charge scattering in the induced channel by dipoles at the insulator-semiconductor interface. However, lower threshold voltages and steeper sub-threshold slopes have been observed [2] in higher capacitance devices using TiO2 and Al2O3. The low mobility observed in these F8T2 devices was attributed to bulk semiconductor traps rather than to interface traps. In the present contribution, we present the results of an admittance study of MIS capacitors formed from poly(3-hexylthiophene) spin-coated onto the composite dielectric Aluminium-Titanium-Oxide (ATO) formed by atomic layer deposition and which has a dielectric constant of ~24. During voltage sweeps in negative gate bias, capacitance-voltage measurements made in the dark reveal a large anticlockwise hysteresis consistent with hole trapping in shallow interface traps. Sweeping the voltage to positive bias returns the threshold voltage approximately to zero i.e. emptying of hole traps and no electron trapping. On the other hand, under illumination with photons of energy greater than the semiconductor bandgap, an anticlockwise hysteresis loop is again observed but this time shifted to positive bias consistent with trapping of electrons in shallow interface states. Furthermore, the increase observed in the minimum capacitance when the device is driven into depletion is evidence for the formation of an n-type inversion layer at the semiconductor-insulator interface [3]. Further work is in progress (a) to develop effective measures for passivating the interface traps and (b) to fabricate and characterise MISFETs formed on this dielectric. [1] J Veres, S Ogier, G Lloyd and D de Leeuw, 2004, Chem Mater 16, 4543-4555. [2] J Swenson, J Kanicki, G Wang, A Heeger and S Martin, 2003, Proc SPIE, 5217, 159. [3] D M Taylor, J A Drysdale, I Torres and O Fernández, 2006, Appl Phys Lett, (in press).


O6.45
Bio-Sensing Using Organic Light Emitting Diode Sandeep Devabhaktuni and Shalini Prasad; Department of Electrical and Computer Engineering, Medical, Micro devices and Nanotechnology Laboratory, Portland State University, Portland, Oregon.

We demonstrate the development and functioning of an organic light diode (OLED) for bio-molecule detection. This has been achieved by incorporating the techniques of organic light emitting diodes used for improved visual displays with bio-chemical functionalizing techniques. Bio-sensing using OLED’S is expected to demonstrate improved sensitivity, selectivity and speed of detection as compared to other optical bio-sensing methods that have been developed using the principles of micro and nanofabrication. The advantage this technique has over other methods such as laser spectroscopy, fluorescent tagging etc; it has very high efficiency due to improved electron-hole recombination, longer output displays due to reduced optical quenching and the ability to produce luminescence in the whole visible region. The OLED structure comprising of a sandwich of ITO—AlQ3—Aluminum (cathode) is fabricated using a multi-layer deposition technique using thermal vapor deposition. Here the binding between OLED and bio-molecule is achieved using percolation and immobilization techniques. The change in the intensity of the light is the key to the identification of the bio-molecule. The addition of bio-molecule of varying concentrations is expected to produce a serial alteration to the light intensity that can be measured in a real time manner.


O6.46
Highly Efficient Excimer-based Phosphorescent White Organic Light-emitting Diodes. Evan L. Williams, Kirsi Haavisto, Jian Li and Ghassan E. Jabbour; School of Materials and Flexible Display Center, ASU, Tempe, Arizona.

Recently, organic light-emitting diodes (OLEDs) have attracted wider attention as potential solid state lighting sources. In particular, white OLEDs promise to be an alternative lighting source with better efficiency than at least the incandescent lamp. Although great strides have been made, the multilayer device structure using small molecules remains complicated, and with relatively low efficiency. Relying on an excimer based system (single dopant) to create white light offers a simplified fabrication process and potentially lower production cost. By utilizing (4’,6’-difluorophenylpyridinato-N,C2’) platinum(II) (2,4-pentanedionato-O,O’) [FPt] as the emission center, incorporating a novel carbazolyl-pyridine based host, and modifying the device architecture, we will show that a nearly 100% internal quantum efficiency is possible for the first time with white OLEDs. External quantum (power) efficiencies of 15.9% (12.6 lm/W) have been realized at 500 cd/m². This talk will focus on the improved efficiency as determined by charge transport characteristics and the interface between transport and emissive layers. The relationship between monomer and excimer emission will also be addressed.


O6.47
Abstract Withdrawn

O6.48
Two solution processes to improve the alignment of P3HT and their OTFT properties. Satoyuki Nomura¹, Aram Amassian¹, George Malliaras¹ and Detlef-M Smilgies²; ¹Materials Science & Engineering, Cornell University, Ithaca, New York; ²Cornell High Energy Synchrotron Source, Cornell University, Ithaca, New York.

It is well known that the charge transport properties in organic thin films are greatly affected by the film morphology or alignment of the molecules. The morphology or alignment of organic molecules varies according to the film processing conditions. Therefore, it is very important to understand the effect of process conditions on the morphology or alignment of the organic molecules. In this study, we describe two solution-based processes that improve the alignment of conjugated polymer films. It was shown that the film of regioregular polythiophenes (P3HT) which were fabricated by normal spin coating process had low alignment by grazing incidence small angle x-ray scattering (GISAXS). On the other hand, it was shown that films of P3HT which were fabricated by two solution-based processes had exceptional alignment. Their performance in transistors was measured using a top contact field effect transistor configuration. The field-effect mobility for normal spin coated film of P3HT was about 1x10^-3 cm^2/Vs. It was shown that new solution-based processes could improve the mobility. We measured mobility dependence on film deposition direction but there were no significant difference between film deposition direction and the mobility. We also discuss the mobility based on top gate configuration.


O6.49
Study of PECVD Silicon Nitride and Silicon Oxide Gate Dielectrics for Organic Thin-Film Transistor Circuit Integration Flora M. Li¹, Yiliang Wu², Beng S Ong², Yuri Vygranenko¹ and Arokia Nathan^3,1; ¹Electrical & Computer Engineering, University of Waterloo, Waterloo, Ontario, Canada; ²Xerox Research Centre of Canada, Mississauga, Ontario, Canada; ³London Centre for Nanotechnology, Imperial College London, London, United Kingdom.

Research in organic thin-film transistors (OTFTs) has unleashed fascinating opportunities for organic electronics in areas requiring low-temperature processing, large area coverage, mechanical flexibility, and overall low cost. Prospective applications for OTFTs include AMOLED displays, large-area flexible displays, electronic papers, RFID tags, and low-cost and low-end printable electronic devices. For active-matrix display applications, silicon nitride (SiN_x) has been a prevalent choice of gate dielectric and passivation in a-Si TFT technology. Attractive attributes of SiN_x include low temperature deposition, large-area capability, and good dielectric strength. Low temperature gate dielectrics that are compatible with plastic substrates are demanded for fabrication of flexible electronics. Hence, there is a strong motivation to investigate SiN_x gate dielectric for OTFT circuit integration, to facilitate the development of large-area flexible electronics. Plasma-enhanced chemical vapour deposited (PECVD) SiN_x films of varying compositions, ranging from N-rich to Si-rich, were explored to determine an optimal choice for OTFTs. Bottom-gate bottom-contact OTFTs featuring solution-processed poly[5,5’-bis(3-dodecyl-2-thienyl)-2,2’-bithiophene)] (PQT-12) semiconductor, Cr-Au source/drain contacts, and SiN_x gate dielectrics were studied. By executing proper surface modification steps on the dielectric and contacts, these PQT-12 OTFTs exhibited on-off current ratio of 10⁶-10⁸ and effective field-effect mobility of 0.03-0.1 cm²/V-s. A dependence of the OTFT parameters on SiN_x film composition was evident, where an overall improvement in effective mobility, on/off current ratio and gate leakage current was observed as the Si-content in SiN_x film increases. These results confirmed that the SiN_x film composition influences the dielectric surface properties, which in turn governs quality of the semiconductor-dielectric interface and molecular ordering of the overlying organic semiconductor layer; altogether, these factors present a strong bearing on the OTFT performance. Thus, careful optimization of the gate dielectric composition and proper control of the dielectric surface properties are crucial for attaining higher performance OTFTs. PQT-12 OTFTs on PECVD silicon dioxide (SiO₂) gate dielectric were also considered. These devices demonstrated on/off current ratio of 10⁷ and effective field-effect mobility of up to 0.5 cm²/V-s. Overall, the results collected from this study revealed that PECVD SiN_x and SiO₂ are promising gate dielectric candidates for OTFT integration in large area flexible electronics. The processing details and optimization of the dielectric surface modification scheme will be elaborated in greater detail in the paper, along with a more in-depth comparison of PQT-12 OTFTs on various gate dielectrics. The development of PQT-12 OTFT pixel circuits with PECVD SiN_x dielectric for active matrix display backplanes will be addressed.


O6.50
Fiber Based Organic Light Emitting Diodes and Photovoltaic Cells Brendan O'Connor¹, Kevin Pipe¹ and Max Shtein²; ¹Mechanical Engineering, University of Michigan, Ann Arbor, Michigan; ²Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan.

We demonstrate fiber-shaped organic light emitting devices (OLEDs) and photovoltaic (PV) cells. The devices consist of archetypal molecular organic semiconducting compounds (e.g. NPD, Alq3, CuPc, C60) and metallic (non-ITO) electrodes deposited as thin films on silica fibers using vacuum thermal evaporation. We characterize their optoelectronic performance and present new fabrication and optimization methods for this device geometry. Our results indicate that the fiber based OLED spectrum is invariant with radial and axial emission angle, in contrast to the strong directional dependence of planar, top-emitting OLEDs. Fiber OLEDs also show enhanced external quantum efficiency. Organic fiber PV cells exhibit power conversion efficiency comparable to planar cells, and simulations indicate that bundled fibers with external coatings can further improve overall power conversion efficiency without relying on ITO as an electrode material. The broadband light in-coupling efficiency is also independent of the incident angle. These results suggest that fiber based devices can potentially increase energy conversion efficiency, while broadening the range of fabrication approaches, thus providing a viable and convenient platform for integration of devices into woven fabrics and fabric-reinforced composites.


O6.51
Electronic Properties of Doped Polyaniline and Doped Polyaniline/Crystalline Silicon pn Heterostructure Solar Cells: Observation of an Inverse Meyer-Neldel Rule. Weining Wang and Eric Schiff; Syracuse University, Syracuse, New York.

We have studied the transport properties of doped, hole transporting polyaniline films with a wide range of polyaniline conductivities, along with the properties of the heterostructures of these films when applied to crystalline silicon. Polyaniline dispersions with different conductivities were realized by simple solvent dilution of the original dispersions optimized for high conductivities. The range of conductivities of polyaniline films was 10^-5- 10² S/cm. The heterostructure results appear to be strong evidence favoring a "Fermi glass" model for transport, meaning that the hole transport occurs at an energy (the "transport edge") that is well below the Fermi level of the film. Recent work has shown true metallic behavior in polyaniline when the Fermi energy is sufficiently low, with a metallic conductivity of about 10³ S/cm [1]. We draw the conclusion of Fermi glass behavior in our films from measurements of open-circuit voltages under intense illumination, which are consistent with the built-in potential difference between the polyaniline film and the silicon substrate. Temperature dependence studies of the films showed an "inverse Meyer-Neldel behavior." Individual films show simply-activated behavior in plots of conductivity vs. reciprocal temperature. The lines for the various samples all focus at a negative Meyer-Neldel temperature: T_MN≈-185 K. Overhof and Beyer [2] predicted long ago that negative Meyer-Neldel temperatures should be observed when Fermi levels fall deeply into bandtails; their prediction appears to correspond well with the present measurements, which involve films with conductivities up to 10⁷ smaller than the optimized value. We do not have a satisfactory understanding of why relatively modest dilutions yield such a large range of conductivities. Interestingly, the conductivity at T_MN is: σ₀₀≈2×10² S/cm. This value is remarkably close to the minimum metallic conductivity predicted by Mott for bandedge transport and Anderson localization [3], and lies just slightly below the true metallic conductivities achieved by Lee, et al. [1]. This research was supported by the Thin Film Photovoltaics Partnership of the National Renewable Energy Laboratory (NDJ-2-30630-24). 1. Kwanghee Lee, Shinuk Cho, Sung Heum Park, A. J. Heeger, Chan-Woo Lee and Suck-Hyun Lee, Nature 441, 65-68 (2006) 2. H. Overhof and W. Beyer, Phil. Mag. B 47, 377 (1983). 3. N. F. Mott and E. A. Davis, Electronic Processes in Non-Crystalline Materials, 2nd Ed. (Clarendon Press, Oxford, 1979).


O6.52
Electrical Conductivity of Single Molecules in Water via Electrochemical Tunneling Spectroscopy. Krzysztof Slowinski, Emil Wierzbinski, William Hammond and Justin Arndt; Chemistry & Biochemistry, California State University, Long Beach, Long Beach, California.

The electrical conductance of single n-alkanethiol and alkanedithiol molecules was measured via in situ distance tunneling spectroscopy (in-situ STS) in aqueous 0.1 M KOH solution. The statistical analysis of the conductance values show that the alkanedithiol molecule trapped in the STM break junction can adopt two distinct geometries that result in “lower” and “higher” conductivity values. In contrast, n-alkanethiol molecules trapped in the junction show only a single conductivity value characteristic for a particular molecule. Furthermore, the “lower” conductivity value determined for alkanedithiol is virtually identical to the electrical conductivity of the n-alkanethiol containing the same number of atoms in the backbone. Moreover when the STM tip is polarized to electrochemical potential preventing a chemical reaction between terminal the -SH group and Au, only “lower” conductivity values are observed for alkaneditiols. In a separate experiment in-situ STS was used to investigate the conductivity of ds-DNA molecules in water. The influence of the DNA structure, in particular the presence of a single base mismatch will be reported.


SESSION O7: Processing and Device Fabrication
Chair: Ana Claudia Arias
Thursday Morning, April 12, 2007
Room 2002 (Moscone West)
8:30 AM *O7.1
Low Cost Fully Printed P-OLED Displays on Flexible Substrates. Sue Carter^1,2, J. J. Breeden², J. P. Chen², E. Jones², M. Kreger², Y. Nakazawa², A. Palmer², V. Van Vo², Y. Yoshioka² and J. D. Mackenzie²; ¹University of California - Santa Cruz, Santa Cruz, California; ²Add-vision, Scotts Valley, California.

Add-Vision, Inc. (AVI) has developed a low-cost print technology for P-OLED displays on flexible substrates that meets several essentials for a new technology including: (1) Functionality advantages over incumbent technology, including low DC voltage and wide color gamut; (2) Low barrier to entry through utilization of inexpensive, existing print-based tools; (3) Performance meeting initial market applications. The AVI process is based on large-area printing of a combination of doped emissive and air-stable cathode inks on flexible substrates utilizing truly low-cost tools to create PLED flexible displays for near term commercial and military applications. The AVI approach to P-OLED seeks to leverage the existing equipment and know-how of the graphic arts, printed circuit board and flex circuit industries. This underpins AVI’s effort to realize large-area flexible display fabrication in high throughput at very low cost. AVI’s recent progress has been driven by a combination of advancements. The combination of air-stable high work-function cathode materials which promote high quantum efficiency and low voltage injection without damaging underlying materials; doped LEP layers with appropriate solution properties, improved print morphology, uniform emission and good stability; and optimized processing have led to consistent performance improvements. Maximum quantum efficiency, brightness, and lifetimes have increased substantially, approaching our development goal of >5 Cd/A efficiency, 1000 hours at 100 Cd/m2 on flexible substrates. This lifetime, coupled with low operating voltages ranging from 10 to 20V DC and corresponding improvements in power efficiency, positions printable P-OLED favorably with respect to inorganic thick-film EL systems, the incumbent technology, in many early-entry backlighting, signage and low-information content applications.


9:00 AM O7.2
High Switching Speed Polymer Transistors via Self-aligned Inkjet Printing. Yong-Young Noh and Henning Sirringhaus; Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom.

From the first demonstration, performance of organic field effect transistors (OFETs) has been greatly improved via a process optimization and development of new materials. In spite of the significant progress, OFETs have been expected to apply limited applications particularly, electronic devices for low speed circuits since they have a limitation on the transport speed of charge carriers. To extent this limited applications via increasing switching speed of transistors, scaling down of device is considered as the most challenging way because of some technical problems. This talk will give information on our approach how to realize high switching speed polymer transistors by inkjet printing. High performance polymer transistors were fabricated via a spin-coating of polymer semiconductors and dielectrics on 100 nanometer channel length realized via a self-aligned printing of various conductive inks [1]. For full scaling down of transistors, thin polymer gate dielectrics were developed and applied in top gate configuration. All printed FETs showed a perfect transistor working below 5 V of gate and drain-source voltage with the mobility range of 10^-3 ~ 0.1 cm^2/Vs depending on which polymer semiconductors were used. The detail properties of thin polymer dielectrics will be discussed. Top gate structure was completed by the inkjet printing of gate electrodes. To minimize overlap capacitance between gate to source and drain, a self-aligned gate architecture was applied. The transistors with the self-aligned gate showed 10 times lower parasitic capacitance than those with a simply printed gate. The characteristics of transistors will be also discussed based on how to reduce contact resistance in polymer transistors with several hundreds nano-size channel length. [1] C. W. Sele, T. Werne, R. H. Friend, H. Sirringhaus, Adv. Mater. 2005, 17, 997.


9:15 AM O7.3
Light Emission from Ambipolar Transistors of Tetracene Single Crystals. Taishi Takenobu^1,2, Tetsuo Takahashi¹, Yukitaka Matsuoka^1,2, Kazuhiro Watanabe¹, Jun Takeya³ and Yoshihiro Iwasa^1,2; ¹Institute for Materials Research, Tohoku University, Sendai, Miyagi, Japan; ²CREST, Japan Science and Technology Agency, Kawaguchi, Saitama, Japan; ³Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka, Japan.

Interface issues in organic field effect transistors (OFET) are known to be crucial to control the device performance. The control of various interfaces inherent in the OFET structures involves rich device physics which may dramatically improve the device performance. Particularly, ambipolar operation and light emission from OFETs, which are both attracting considerable attentions, are achieved by the state-of the-art interface control. Polycrystalline thin film is a standard form of OFETs, whereas, single crystal, which is basically free from grain boundaries in sharp contrast to thin films, is an important and useful tool for investigating the intrinsic aspects of operation mechanisms in OFETs. For instance, the highest mobility was achieved in rubrene single crystal FETs [1], and the measurement of Hall effect, which is crucial for the establishment of device physics of organics, was first made on rubrene single crystals [2]. Here we report our attempt to control the carrier sign, density and injection in polyacene OFETs, and observation of light emission from the ambipolar transistor of tetracene single crystals. Polyacene single crystals, grown by a physical vapor deposition, were laminated on the SiO2/Si substrate, the surface of which was coated by polymethylmethacrylate (PMMA). We have demonstrated that the ambipolar operation of rubrene single crystal FETs, which is usually p-type, using Ag electodes. This result indicates that the electron can be injected by application of high enough source-drain voltage, and the electron conduction is achieved on a PMMA insulator [3]. The current-voltage characteristic is controlled by the metal work function, which offers the possibility of controlling the Schottky barrier height by the choice of the metal. In the process of the study of metal-polyacene contacts, we have realized an ambipolar transistor and a Schottky diode in an identical single crystal device with asymmetric electrodes. These data provide direct evidence of the weak Fermi level pinning and formation of depletion layer on metal-polyacene contacts. Furthermore, we succeeded in observation of light emission from tetracene single crystal FETs with asymmetric electrodes. Due to the ambipolar operation of the tetracene FET, we were able to tune the position and emission intensity of the recombination zone by changing the gate voltages. Single crystal OFETs might be advantageous for improving the light emitting properties of OFETs, because high current density is easily obtained due to their higher mobility than that in thin films. References [1] E. Menard et al., Adv. Mater. 16, 2097 (2004). [2] J. Takeya et al., Jpn. J. Appl. Phys. 44, L1393 (2005), V. Podzorovet al., Phys. Rev. Lett. 95, 226601 (2005). [3] T. Takahashi et al., Appl. Phys. Lett. 88, 033505 (2006).


9:30 AM O7.4
Evaluation of the mid-range order degree in molecular crystals. Michele Cerminara, Giorgio Macchi, Franco Meinardi and Riccardo Tubino; Dipartimento di Scienza dei Materiali, Università degli Studi di Milano-Bicocca, Milano, Italy.

Organic semiconductors have been widely studied in the last decades with the ambition to couple the properties of common plastics with those of inorganic semiconductors, aiming to obtain “plastic electronics”. Many prototype devices (such as electro luminescent devices, transistors, photovoltaic cells and lasers) were proposed based on these materials, but at the moment only LED can be produced with specifics that can be competitive with those of common inorganic semiconductors. The limiting factor often resides in the poor charge transport of organic materials in the solid state: indeed, even though conjugated molecules can be chemically tailored to accomplish with the requirements needed to obtain good charge injection and good intramolecular delocalisation of the charge, solid state phenomena can occur that reduce the charge mobility within the bulk of the material. The highest values of charge mobilities in organic semiconductors are reported for high quality single crystals. In these systems the excited states are delocalised over a large part of the crystals thanks to the translational invariance which is assured by the ordered structure, then charge mobility is quite higher than in amorphous solids. The limiting factor is represented by the presence of disorder (due to structural imperfection, grain borders or to the presence of impurities) that results in a strong quenching of the mobility. The formation of delocalised excited states in molecular crystals has an effect not only on the mobility of the material, but also on its optical properties. Indeed absorption and emission spectra show the fingerprints of the formation of delocalized states in the form of Frenkel excitons inside the crystal. If the luminescence is allowed by the symmetry of the molecule and by the relative arrangement of the molecular dipoles within the crystal, it has peculiar spectroscopic features, i.e. the emitting state has a superradiant behaviour. Superradiance is the emission by an ensemble of atoms or molecules in which there is a phase correlation that results in constructive interaction of the emitters forming the ensemble. The superradiant behaviour is strongly correlated to the number of molecules forming the emitting state, i.e. to the size of the region of the crystal over which the exciton is delocalised. The presence of disorder results in a reduction of this size, therefore superradiant behaviour is strongly affected by the presence of disorder and can be used to evaluate it. We will show that coupling measurements of CW and time-resolved photoluminescence it is possible to analyse in detail the superradiant behaviour of various molecular crystals (oligothiophenes, oligophenylenevynilenes and porphyrins) and to evaluate the coherence length over which the excitation is delocalised. In this way it is possible to investigate the mid-range order of samples as a function of the growth parameters.


9:45 AM O7.5
Self-Aligned Soluble-Pentacene Crystals via Inkjet Printing for High-Performance Organic Field-Effect Transistors Kilwon Cho, Jung Ah Lim, Wi Hyoung Lee, Yeong Don Park, Hwa Sung Lee and Jong Hwan Park; Chemical Engineering, Pohang University of Science and Technology, Pohang, South Korea.

Inkjet printing is attractive technique for direct-writing patterns for organic electronics. In particular, uniform deposition and desired molecular ordering of organic semiconductors become essential challenge for ink-jet printing of organic semiconductors. In this study, we have fabricated the highly ordered soluble-pentacene crystals using ink-jet printing by controlling evaporation behavior in a printed droplet. In general, ring-like deposit morphologies of 6,13-bis((triisopropylsilylethynyl) pentacene (TIPS_PEN) was produced from homo-chlorobenzene solvent. However, self-aligned TIPS_PEN crystals with highly ordered molecular structure were obtained successfully by using mixed-solvent with high boiling point and low surface tension through control of evaporation-induced flow in a droplet. Formation of self-assembled crystals from the mixed solvent may be due to the induced Marangoni flow (surface-tension-driven flow) during drying process in the droplet. The field-effect transistors fabricated from these printed crystals shows significantly improved performance with an effective field-effect mobility of 0.1 cm2/Vs comparable to vacuum deposited devices. This study may offer an excellent way to control the molecular ordering of organic semiconductors for the direct-write fabrication of high-performance organic electronics. Acknowledgement. This work was supported by a grant (F0004022-2006-22) from the Information Display R&D Center under the 21st Century Frontier R&D Program, ERC program (R11-2003-006-03005-0) of the MOST/KOSEF, and the BK21 Program of the Ministry of Education and Human Resources Development of Korea.


10:30 AM *O7.6
Field Effect Transistors based on Self-Organised Molecular Nanostructures. Fabio Biscarini, Chiara Dionigi, Pablo Stoliar, Cristiano Albonetti, Jean-Francois Moulin, Silvia Milita and Massimiliano Cavallini; Nanotechnology of Multifunctional Materials, CNR - ISMN, Bologna, Italy.

The response of organic field effect transistors depend on a complex interplay between intermolecular and interfacial interactions, molecular order and structural traps, as well as on the organisation of the organic semiconductor at mesoscopic length scales in the device. Our aim is to control the supramolecular organisation of the organic semiconductor across length scales, as well as the shape, dimensionality and orientation of the semiconductor domains in the channel. We exploit self-organisation of soluble organic semiconductors at all length scales in confined environments created ad hoc in the device. We realize the confined deposition by stamps with microfabricated features which lead to mesoscopic menisci of the solution; micro- and mesoscopic channels by microfluidics; nanofabricated templates; or by deposition of colloidal latex beads decorated with organic semiconductors which form close-packed structures with nano-sized cavities. We present different working field effect transistors where drain current flows through a precisely defined array of self-organised nanostructures. In one case, molecularly ordered stripes of ter-tiophene-bis-fluorene (T3F2), two-monolayer high and 200 nm wide are fabricated across the channel of the transistor. In another case, a crystalline network of tetrahexylsexythiophene (H4T6) is formed by deposition of water-soluble colloidal beads decorated with H4T6 in the device channel with a micro-syringe or a microfluidics. The size of the confinement governs the molecular order, crystal phase distribution and the transistor response. The charge mobility measured on these transistors is a few orders of magnitude larger than the ones measured in their counterpart made with high-vacuum-sublimed or spin-coated thin films. This work is part of collaborations with Max Planck Institute-Mainz (K. Muellen), Universitè de Mons-Hainaut (R. Lazzaroni), Risoe-Roskilde (M. N. Nielsen), CNR-ICAS Milan (S. Destri and W. Porzio), University of Bologna (A. Brillante), supported by the EU-Integrated Project NAIMO (No NMP4-CT-2004-500355).


11:00 AM O7.7
Cavity Effects on Light out-coupling Efficiency of Organic Light Emitting Devices Jaewon Lee and Franky So; Dept of Materials Science and Engineering, University of Florida, Gainesville, Florida.

In this paper, we will demonstrate that due to the strong internal cavity effects, the out-coupling efficiency of an organic light emitting device (OLED) without any light extraction schemes can be over 50%. Previous studies using classical ray tracing have suggested the maximum external quantum efficiency (QE) of an OLED was estimated to be 22% and this value has been used to estimate the device internal quantum efficiencies. However, in most OLED devices, an inherent cavity is formed between the metal cathode and the ITO electrode and cavity effects need to be taken into account when estimating the extraction efficiencies of OLED devices. To study the cavity effects in OLEDs, we tuned the location of the recombination zone of the OLED devices by adjusting the cavity length. To measure the light out-coupling efficiencies, we used a silicon photodiode in direct contact with the glass substrate with a thin layer of refractive index matching gel. The devices used in this study has the following structure: indium-tin-oxide (ITO) as the transparent anode, copper phthalocyanine (CuPc) as the hole injection layer, N,N'-bis-(1-naphthl)-diphenyl-1,1'-biphenyl-4,4'-diamine (NPD) as the hole transporting layer, tris (8-quinolinolato) aluminum (Alq3) as the emitting layer, 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) as the hole blocking layer, Alq3 as the buffer layer to tune the cavity length, and lithium fluoride (LiF) and aluminum as the cathode. Thickness of all layers was kept constant except for the Alq3 buffer layer, which was varied from 20nm to 140nm. We observed the coupling efficiency is about 50% with a 20nm thick buffer layer and this value is more than doubled that estimated based on the ray tracing calculations. As the thickness of Alq3 buffer layer was increased, coupling out factor decreases to ~16%. This shows that the light coupling efficiency is strongly dependent on the device architecture and estimates of internal quantum efficiency using classical ray tracing might be overestimated. In addition, both our optical simulation results and experimental data show blue light tends to be strongly trapped in the substrate while red light tends to have higher coupling efficiencies.


11:15 AM O7.8
Low-voltage Organic Transistors on a Polymer Substrate with an Aluminum Foil Gate Fabricated by a Laminating and Electropolishing Process. Chanwoo Yang¹, Kwonwoo Shin¹, Sehyun Kim¹, Chan Eon Park¹ and Hoichang Yang²; ¹Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang, Gyungbuk, South Korea; ²Rensselaer Nanotechnology Center, Rensselaer Polytechnic Institute, Troy, New York.

We report flexible and low-voltage pentacene organic field-effect transistors (OFETs) constructed with an aluminum foil gate electrode, which was fabricated by the simple and low-cost roll-to-roll lamination process. Electropolishing the surface of laminated aluminum foil and spin-coating it with an additional thin polymer film resulted in a gate dielectric surface with a root-mean-square roughness of about 0.85 nm. These pentacene OFETs with a poly(α-methylstyrene)/anodized Al2O3 dual-layered gate dielectric exhibit a mobility of 0.52 cm^2/V s, an on-off ratio of 10^5, a subthreshold swing of 317 mV/decade, and little hysteresis when operating at -5 V.


11:30 AM O7.9
Electric-field Effects of Very High Mobility Organic Single-crystal Transistors. Jun Takeya¹, Yukihiro Tominari¹, Masakazu Yamagishi¹, Takao Nishikawa², Takeo Kawase² and Satoshi Ogawa³; ¹Osaka University, Toyonaka, Japan; ²SEIKO EPSON Corporation, Fujimi, Nagano, Japan; ³Dept. of Chemical Engineering, Iwate University, Morioka, Japan.

Developing high-mobility organic field-effect transistors (OFETs) is one of the key technologies that can lead to an extensive market of organic electronics, providing easy-to-fabricate switching components in fundamental logic circuits, for example. In this presentation, we report our attempt to approach the maximum field-effect mobility intrinsic to organic materials, minimizing complications due to the device fabrication processes with the use of single crystal OFETs. We grow rubrene single crystals to thin platelets with the thickness less than 1 μm by physical vapor deposition techniques, purifying the materials by repeating the sublimation process more than twice. The crystal OFETs are prepared by laminating the thin rubrene crystals on insulating layers of cross-linked poly(vinilphenol) (PVP), SiO₂ covered with organosilane self-assembled monolayers (SAMs), and diphenylanthracene (DPA) thin crystals grown in the same method. With the application of weak gate fields, the mobility of the devices is evaluated typically in the range of 10-18 cm²/Vs for the statistics of 20 good-looking devices from these dielectric materials, and the contact-free intrinsic mobility turned out to be 30-40 cm²/Vs as the result of the four-terminal measurements. The PVP devices show somewhat larger deviation among the samples, which can be due to their less homogeneous surfaces. Only a few of the samples exhibited saturation in output characteristics with the saturation mobility exceeding 30 cm²/Vs, while either short-channel like behavior or saturation with poorer mobility appear for others. Transfer characteristics of the above devices have steeper slope at low gate voltages and the response to the gate voltage becomes less pronounced with further hole accumulation. The “convex” curvature, which favors the low-power operation, is contrasting to typical “concave” curvature of thin-film devices with the presence of subthreshold regions. A simple consideration based on Poisson’s equation indicates distribution of carriers more than 10 molecular layers deep in the crystal at the weak gate field, whereas the accumulated carriers are confined to a few monolayers under higher gate field, as the result of competition between thermal diffusion of the carriers and electric force by the gate field. With the in-crystal channel less affected by random potentials at the interface, the holes induced in the inner crystals can be highly mobile, approximated as the intrinsic carrier transport in the semiconductor. The high mobility of the single-crystal OFETs is resulted from least amount of effective interface levels that terminate the (weak) gate fields, so that it penetrates deeper into the crystals. We finally note that the decline of the mobility at higher fields is least serious for the devices with DPA crystal gate dielectrics, where scattering by the random potentials appears to be minimized at the crystal-to-crystal interfaces.


11:45 AM O7.10
Surface-directed Phase Separation of Conjugated Polymer Blends for Efficient Light-emitting Diodes Keng-Hoong Yim¹, Zijian Zheng², Wilhelm T. S. Huck², Richard H. Friend¹ and Ji-Seon Kim¹; ¹Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom; ²Melville Laboratory for Polymer Synthesis, University of Cambridge, Cambridge, United Kingdom.

In solution-processed conjugated polymers, de-mixing of two spin-coated polymers causes phase separation, and the interpenetrating network of the two polymers provides randomly distributed interface structures. Here we demonstrate the precise control of such phase separation via substrate surface energy modification. We rely on self-organisation to form lateral photonic microstructures, utilising the fact that phase separation in polymer blend films is strongly dependent upon substrate surface energy. We use micro-contacting printing technique to periodically deposit an appropriate choice of covalently bound self-assembled monolayer (SAM) to create sufficient surface energy contrast with the underlying substrate. It is then possible to induce phase separated structures that replicate the pre-determined pattern. Here, we investigate the phase separation of poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT) and poly(9,9-di-n-octylfluorene-alt-(1,4-phenylene-((4-sec-butylphenyl)imino)-1,4-phenylene) (TFB). With appropriate choice of polymer molecular weight and blend ratio, the phase separation in the blend film replicates the micron-scale pattern closely, with more polar F8BT being deposited on hydrophilic poly(styrene sulphonate)-doped poly(3,4-ethylene dioxythiophene) (PEDOT:PSS) layer, while TFB on the hydrophobic areas defined by SAM of 7-octenyltrichlorosilane. Comparing to ordinary blend films with natural phase separation, light-emitting diodes fabricated with this patterned blend film exhibit significantly improved initial electroluminescence (EL) efficiency with more gentle decay at high voltages. We consider that the well-defined and localised EL emission from the patterned area is responsible for the observed increase in initial EL efficiency, as a result of spatial confinement of charge carriers in the TFB-rich domains that improves recombination efficiency. This observation also suggests the absence of continuous TFB wetting layer that is usually present in F8BT:TFB blend films, further indicating the effectiveness of surface-directed phase separation. In addition, the presence of micron-scale phase separated structures in the patterned blend film leads to the observed improvement of light out-coupling in the forward direction. On the other hand, slower decay in EL efficiency is attributed to smoother film surface that reduces leakage current at high voltages. We have shown the applicability of surface-directed phase separation of conjugated polymer blends to fabricate functional devices. These observations provide a new way of controlling polymer-polymer interfaces to further improve the performance of conjugated polymer-based devices.


SESSION O8: Synthesis of Materials II
Chair: Dean Delongchamp
Thursday Afternoon, April 12, 2007
Room 2002 (Moscone West)
1:30 PM *O8.1
New Organic Materials for High Performance Transistors Zhenan Bao, Stanford University, Stanford, California.

Organic semiconducting materials are now being considered as the active materials in displays, electronic circuits, solar cells, chemical and biological sensors, actuators, lasers, memory elements, and fuel cells. The flexibility of their molecular design and synthesis makes it possible to fine-tune the physical properties and material structure of organic solids to meet the requirements of technologically significant applications. In contrast to inorganic materials, active organic thin films can be deposited at much lower substrate temperatures (less than 120 C) in low vacuum or atmospheric pressure environments. It has been demonstrated that low-cost deposition techniques such as solution spin-coating, casting, and even printing can be used for deposition of solution soluble organic materials. These processing advantages, together with the natural abundance of organic solids, make semiconducting organics attractive for large-area and low cost applications. The performance of OTFTs depends on the construction of each of the active layers, which are the organic semiconductor layer, insulating (dielectric) layer and the electrodes. The deposition method, condition, sequence, post-deposition treatment, and surface treatment significantly impact OTFT performance. Therefore, it is important to fully understand various factors that affect the thin film growth processes. Specifically, one needs to pay attention to how the molecular structure of the organic semiconductor and thin film morphology affect the performance of OTFT devices, namely, the field effect mobility and on/off ratio. In this talk, I will discuss our recent work in new dielectric materials and organic semiconductors.


2:00 PM O8.2
Effect of Confined Charge Carriers and Excitons in Organic Electrophosphorescent Devices : Mechanism of Light Emission and route to Efficient White Devices Byung Doo Chin¹, Nam Su Kang^1,2, Byeong-Kwon Ju², Jae-Woong Yu¹ and Jai Kyeong Kim¹; ¹Optoelectronic Materials Research Center, Korea Institute of Science and Technology, Seoul, South Korea; ²Department of Electrical Engineering, Korea University, Seoul, South Korea.

In this study, study, we have fabricated the phosphorescent organic light-emitting devices (OLEDs) using various types of charge transport and blocking material with different charge/exciton blocking characteristics, and studied the link between charge confinement and device performance. The light emitting efficiency, spectrum, and the lifetime of the devices, whose emission characteristics are strongly dominated not only by the energy transfer but also by the charge trapping, were explained by differences in the energy levels of the host, dopant, and nearby transport layers. It was illustrated in detail that such energy level differences determine the charge carrier trapping behavior and distribution of the exciton formation zone inside the light emission, charge transport, and blocking layers. On the basis of our finding on device performance and photocurrent measurement data by time-of-flight (TOF), we suggest a detailed emission mechanism, along with a physical interpretation and practical design scheme for improving the efficiency and lifetime of devices. By use of the device structures for the charge-confined light emission layer, green device efficiencies more than 60cd/A (25~30cd/A at conventional device structure) was obtained as well as improved lifetime up to 3000hr at 5000nit initial brightness. Furthermore, information on the physics of OLEDs presented here was applied for a design of highly efficient electrophosphorescent white OLED devices with more charge-balanced structures by enhancing the confinement of triplet excitons and substantial application of singlet excitons for blue emission. Supplement improvement of external light emission efficiency of white OLED device was discussed using the microcavity structures such as dielectric mirrors and nanostructured sub-layers.


2:15 PM O8.3
Charge Injection, Transport and Degradation of Fluroene-based Copolymers. H. H. Fong, Alexios Papadimitratos and George G Malliaras; Materials Science and Engineering, Cornell University, Ithaca, New York.

Fluorene-based copolymer is considered to be one of the most promising hole transporting and blue light-emitting conjugated polymers used in polymeric light-emitting diodes (PLEDs) due to its high-lying highest occupied molecular orbital (HOMO) of arylamine moiety and high carrier mobilities. It is therefore desirable to investigate the charge injection and transport in these copolymers and also their degradations. Our work attempts to provide a comprehensive understanding on charge transport and injection of fluorine-arylamine copolymers. Time-of-flight (TOF) technique has been employed to evaluate the charge drift mobility under a temperature range between 200 - 400 K at the thick film regime (1-10 micron). Meanwhile, contact ohmicity is studied by Dark Current Space Charge Limited Conduction (DISCLC) technique. Charge injection efficiencies from different electrical contacts are also investigated, together with the examination of device degradation at elevated temperatures. Charge injection barriers are further independently investigated by combining photoemission and electroabsorption spectroscopies. Results show that the copolymers exhibit non-dispersive charge transport behavior and possess superior mobilities of up to 0.01cm^2V^(-1)s^(-1) while single-carrier devices from various electrical contacts such as PEDOT:PSS are varied from bulk limited to injection-limited (10^(-4)), depending on the chemical structure of amine component in the fluorene-triarylamine copolymers. Contact failure with different extents appears when the devices are electrically stressed, showing that the formation of non-injecting region is responsible to the degradation. Results will shed light on the enhancement of device efficiency and stability in the future polymer electronic devices.


2:30 PM O8.4
Direct Evidence for Injection-induced Dedoping of a Conducting Polymer During Device Operation. Perq-Jon Chia^1,2, Lay-Lay Chua¹, Sankaran Sivaramakrishnan¹, Yee-Chia Yeo² and Peter Ho¹; ¹Department of Physics, National University of Singapore, Singapore, Singapore; ²Department of Electrical and Computer Engineering, National University of Singapore, Singapore, Singapore.

Conducting polymers, particularly p-doped poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonic acid) (PEDT:PSSH), the most thermally and electrochemically stable example, are important as hole-injecting layers, electrodes and interconnects in organic electronics, as well as in the active layer electrochromic devices, super-capacitors, actuators and biosensors. We report here direct evidence for an irreversible injection-driven dedoping when PEDT:PSS is electrically driven in the solid state. This means that the doping level of PEDT shifts towards zero in an ultrathin layer at the cathode contact, when the electric field exceeds a threshold, that is very close to that present in state-of-the-art high-injection-current devices, including both pulsed organic LEDs and organic TFTs. Using a combination of impedance spectroscopy and micro-Raman spectroscopy, we determined that a layer of ca. 30 nm becomes nearly completely dedoped, and electrically insulating at the cathode contact. This occurs because holes are extracted more quickly at the cathode contact than they are injected at the anode contact, according to our charge-modulation spectroscopy results. This results in a local dedoping which becomes locked in by an associated electrochemical process involving the counterion. This and not Joule heating is the cause of the irreversible “conductance switching” that has been reported in PEDT and other conducting polymers. The redox state of the conducting polymer can therefore be variable and altered by injection during normal device operation. Such an electrochemical pathway must depend on the availability of a counter oxidation of the PSSH matrix for charge compensation. By replacing PSSH with an ionic polyelectrolyte, we found it thus possible to improve the electric field stability of PEDT by one order of magnitude, which is useful for organic electronics and other applications.


3:15 PM *O8.5
Approaches to Optimizing the Performance of Polythiophene Transistors Yiliang Wu, Ping Liu, Yuning Li and Beng Ong; Xerox Research Centre of Canada, Mississauga, Ontario, Canada.

Organic thin-film transistors (OTFTs) have received extensive interest in recent years for their potential as low-cost alternatives to silicon-based technologies for large-area (e.g. active matrix display), and low-end (e.g. radio-frequency identification tags) flexible electronics. The economic advantages of OTFTs stem from low-cost fabrication using common solution-based deposition techniques such as spin coating and inkjet printing. Among organic semiconductors, regioregular polythiophenes are a class of promising solution-processable semiconductors for OTFTs. Last two decades have seen significant progress in mobility of polythiophene transistors. In this talk, we will use our polythiophenes as examples to discuss the approaches to optimizing transistor performance, including molecule design, synthesis methods, device structure design, and interface optimization.


3:45 PM O8.6
Solution-Deposited n-channel Organic Transistors and their use in Complementary Integrated Circuits Byungwook Yoo¹, Brooks A Jones², Debarshi Basu¹, Antonio Facchetti², Michael R Wasielewski², Tobin J Marks² and Ananth Dodabalapur¹; ¹Electrical and Computer Engineering, The University of Texas at Austin, Austin, Texas; ²Department of Chemistry, Northwestern University, Evanston, Illinois.

Printed organic electronic circuits are attractive for low-cost, mechanically flexible microelectronics applications, such as sensors, displays and radio frequency identification tags. Small-molecule and polymeric organic semiconductor-based field-effect transistors hold promise for applications where inexpensive solution-based deposition techniques are more crucial than the fast circuit speeds associated with conventional inorganic semiconductors. In order for such low-cost, flexible devices to become a reality, the semiconductors should be solution processable to be compatible with the printed electronics. Air-stable high-mobility n-channel organic field-effect transistors (OFETs) are also required for the fabrication of organic complementary metal-oxide-semiconductor (CMOS) circuits which exhibit higher noise margins and lower power dissipation. In this work, the small molecule semiconductor N,N’-bis(n-octyl)-dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDI-8CN2) was employed to fabricate solution-deposited n-channel OFETs. A saturation region mobility of 2.4×10^-2 cm²/Vs and threshold voltage of -1.1 V were obtained in vacuum at V_DS of 40 V. To optimize the circuit fabrication, the different process parameters and surface treatments for the solution deposited transistor were discussed. Utilizing with these results, an organic complementary ring oscillator and D flip-flop using PDI-8CN₂ solution as the n-channel semiconductor and poly-3-hexylthiophene (P3HT) as the p-channel semiconductor have been fabricated. The ring oscillator operated at an oscillation frequency of 3.2 kHz in vacuum and 2.2 kHz in ambient. The D flip-flops operated with clock frequencies in excess of 1 kHz. These results represent the first organic complementary circuits based on solution-deposited semiconductors.


4:00 PM O8.7
Organic Crystal Gate Insulators for High-mobility Organic Single-crystal Transistors. Masakazu Yamagishi, Yukihiro Tominari and Jun Takeya; Osaka University, Toyonaka, Japan.

Towards application of organic field-effect transistors (OFETs) to wide variety of device components including logic circuit elements, maximizing carrier mobility is the principal subject in the material development. So far, various organic materials have been introduced for the active semiconductors as well as for the gate insulators [1,2]. Recently, in an effort to reach the material limit in the carrier mobility, we have been working on the OFETs with thin platelets of small-molecular organic semiconductor crystals, whose surfaces are molecularly flat in an area over 10 x 10 μm². The single-crystal OFETs with SiO₂ and polymer gate insulators indeed have realized pronounced device performances superior to those of amorphous silicon semiconductors by an order [3,4]. To make even better use of the molecularly flat surface and the perfect order of the organic semiconductor crystals, we have incorporated insulating organic crystals with equally flat surface as gate dielectric layers in the present OFET devices. We first grow thin crystals of rubrene for the active layers and 9,10-diphenylanthracene (DPA) for the gate insulators by physical vapor transport. The rubrene crystals of the same batch showed high performance as active semiconductors with average mobility of ~ 20 cm²/Vs in the OFETs with SiO₂ gate dielectrics. The DPA crystals, on the other hand, turned out to be a good gate insulator: the platelet crystals with the thickness about 1 μm show negligible leakage current with voltage up to 100 V. Our ac impedance measurements give the relative dielectric constant of ~ 4 for the DPA crystals, which is close to the value in literature. We attached a rubrene crystal and a DPA crystal in sequence on a substrate with source and drain electrodes at the surface, and evaporated a gold gate electrode on the DPA to fabricate the OFET with the crystal-to-crystal interface. Employing the four-terminal conductivity measurements with sweeping gate voltage, we measured transfer characteristics of the rubrene/DPA single-crystal transistors, excluding influences of contact resistances. Very high carrier mobility of ~ 35 cm²/Vs is evaluated from the slope of the transfer curve. Furthermore, at the highest gate voltage applicable to the DPA crystals, the sheet conductance of the rubrene crystal reaches up to 6 cm²/Vs, which is the largest among those ever reported for organic transistors. The results can be attributed to the “molecularly flat” surface conductivity channels realized in the crystal-to-crystal OFETs. [1] For a recent review, A. Facchetti, M.-H. Yoon, and T. J. Marks, Advanced Materials 17, 1705 (2005), for example. [2] H. Klauk et al., J. Appl. Phys. 92, 5259 (2002). [3] V. Podzorov et al., Appl. Phys. Lett. 83, 3504 (2003). [4] J. Takeya et al., Appl. Phys. Lett. 85, 5078 (2004).


4:15 PM O8.8
Comparative Studies on the Stability of Organic and Inorganic Gate Dielectrics for Pentacene Thin-film Transistors. Do Kyung Hwang, Jeong Min Choi and Seongil Im; Institute of Physics and Applied Physics, Yonsei University, Seoul, Sudaemoon-ku, South Korea.

Organic thin-film transistors (OTFTs) have been extensively investigated due to their potential for future organic electronic applications such as drivers for flat-panel displays, low-end smart cards, and electronic identification tags. In particular, the pentacene-based TFTs have attracted dominant attention, since they often exhibit a considerably high hole mobility comparable to or better than the electron mobility of amorphous silicon TFTs, and already demonstrated their potentials toward organic electronic applications. Now it is about time to solve further practical problems such as electrical reliabilities of OTFTs with pentacene active layer. The issue of electrical stability is very important to any applications and they are mainly about the behavior of gate-bias-induced hysteresis and time-dependent drain-current changes under a constant gate bias or bias stress. Recently, several research groups investigated the hysteresis or bias-stress-induced instability of pentacene TFTs with SiO2 and polymer single gate dielectric films. However, the origins of the unreliable behavior found in the two types of TFTs are still not clear at all. We fabricated pentacene TFTs with poly-4-vinyl phenol (PVP), SiO2, and PVP/ high-k yttrium oxide (YOx) dielectrics to compare their own electric instabilities and to further examine their hysteresis mechanisms. The pentacene TFTs with the PVP cured for 15 min exhibited a large hysteresis and an abnormal drain-current increase under a gate bias stress over time while the other TFT with SiO2 displayed a small hysteresis but its drain-current decreases with time. It is interesting to note that the hysteresis directions observed from the two TFTs with immaturely-cured PVP and SiO2 are opposite to each other. The origins of hysteresis and stress-induced ID changes are attributed to the remnant dipoles due to slow polarization for PVP-type pentacene TFTs and the trapping/de-trapping of electrons at the channel/dielectric interface for the SiO2-type. Based on our understanding of the origins of these instabilities, we fabricated a reliable, hysteresis-free pentacene TFTs with PVP by extending PVP curing time to 1 hour. In the case of PVP/YOx double dielectric layer, the tendency of the device stability is similar to that of the TFTs with the single PVP layer cured 15 min. More details on the comparison of device stability are to be presented in the coming conference.


4:30 PM O8.9
Bipolar Copolymers for Electroluminescent Devices: The Effects of the Molecular Structure and Film Morphology on the Device Performance. Biwu Ma^1,2, Lan Deng¹, Bumjoon J. Kim¹, Mark E. Thompson³ and Jean M.J. Frechet^1,2; ¹Chemistry, University of California, Berkeley, Berkeley, California; ²Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California; ³Chemistry, University of Southern California, Los Angeles, California.

Efficient solution-based polymer light emitting diodes (PLEDs) have been achieved by doping phosphorescent complexes in conjugated or nonconjugated polymer hosts. In these devices, the transport and recombination of the oppositely charged carriers (holes and electrons) in the active polymer layer play significant roles in determining performance. We have now developed bipolar transport polymers for use as host materials for electroluminescent devices by incorporating electron transporting and hole transporting functionalities into copolymers. Our target is to explore the effect of molecular structure and film morphology of these copolymers on device performance. Therefore, two different copolymers, one diblock copolymer and one random copolymer with the same overall composition have been prepared. The diblock copolymers show pronounced phase segregation forming different nano-morphologies as observed by modern microscopic techniques. These morphologies are not observed for the random copolymer under the same thin film preparation conditions. The phase-segregated morphology of the diblock copolymer film dramatically affects both charge transport and charge recombination. Facilitating hole-electron recombination in the diblock copolymer leads to a higher quantum efficiency than measured for a random copolymer with the same composition. Efforts to optimize film morphology in order to improve performance in a single layer device will also be presented.


4:45 PM O8.10
Temperature Dependence of Hall Effects Using Polycrystalline Pentacene Thin-film Transistors on Plastic Films. Yasushi Takamatsu, Tsuyoshi Sekitani and Takao Someya; School of engineering, The University of Tokyo, Tokyo, Japan.

We performed the Hall effect measurements using high-quality polycrystalline pentacene thin-film transistors (TFTs) on plastic films in a wide temperature range from 200 to 380 K. The temperature dependence of charge density evaluated from Hall effect measurements is compared with that from the CV measurements. In low temperatures less than 260 K, there is a large discrepancy of charge density between Hall effect and the CV measurements, while a discrepancy is moderate above 260 K. This result indicates that the conduction carriers are dominated by hopping between polycrystalline grain boundaries below 260 K and tend to be delocalized above 260 K. Polycrystalline pentacene TFTs for the Hall effect measurement (van der Pauw method) are manufactured by a vacuum evaporation process. First, the gate electrode was formed by thermal evaporation of 50-nm-Au through a shadow mask on a 75-μm-thick polyimide film. Then, a 500-nm-thick polyimide gate dielectric layer was prepared by spin coating and a 50-nm-thick pentacene was deposited through a thermal evaporation. Finally the 60-nm-thick Au was evaporated to form four electrodes using a shadow mask. The total size of the pentacene layers is 600 x 600 μm^2 and the size of each electrodes on the corner of pentacene is 50 x 50 μm^2. For reference, pentacene TFTs with the channel length of 100 μm and width of 1 mm are also fabricated, which show the field-effect mobility of 0.4 cm^2/Vs and an on/off current ratio of above 10^6 The sample was set in the superconducting magnet, which can apply the magnetic fields up to 9 T in a helium gas environment. These TFTs were annealed before measurements at 140 oC for 12 hours in order to suppress the drifts of transistor performance under bias stresses. It enabled us to achieve the excellent stabilities on these TFTs. After annealing process, the change of current was less than 0.05% after continuous voltage applications of V_G=-40 V for 2 hours. We measured the Hall voltage (V_H) with applications of gate voltage bias of -40 V and the current of 0.15 μA, in a wide temperature range from 200 to 380 K. We evaluate the inverse of Hall constant, 1/R_H =IB/V_H, which indicates the charge density in band model. In high temperature regime (>260 K), 1/R_H was increased with increasing temperatures. 1/R_H is three times larger than Q evaluated from the CV measurements. In low temperature regime (<260K), the discrepancy between 1/R_H and Q is increased as temperature decreases. It indicates that conduction carriers are strongly localized below 260 K. This work was supported by Special Coordination Funds for Promoting and technology, JST-CREST, and TOKUTEI (15073204).


SESSION O9: Poster Session: Materials, Devices and Characterization III
Thursday Evening, April 12, 2007
8:00 PM
Salon Level (Marriott)

O9.1
Charge Carrier Concentration Dependence of the Hole and Electron Mobility in Blue Polymer LEDs. S. L. M. van Mensfoort^1,2, M. Bartyzel², S. I. E. Vulto², J. Billen¹, R. A. J. Janssen¹ and R. Coehoorn^1,2; ¹Molecular Materials and Nanosystems, Department of Applied Physics, Eindhoven University of Technology, Eindhoven, Netherlands; ²Philips Research Laboratories, Eindhoven, Netherlands.

The mobility models commonly used to describe charge transport in organic light-emitting diodes (OLEDs) are highly under debate. Recently, it was found that for OLEDs based on poly(p-phenylene vinylene) (PPV) hole transport should be described by taking into account that the mobility of the charge carriers depends on their local concentration. Such a dependence is expected to occur in the case of a sufficiently strong energetic site disorder, described by a Gaussian density of states [1]. It is not yet clear to which extent this model is applicable to the hole mobility in other organic materials used in OLEDs, to the electron mobility, and to diffusion coefficients. Furthermore, it is not yet clear how these effects of disorder affect the recombination profile and luminous efficacy. In this contribution we present the results of an experimental and modelling study of OLEDs based on a blue-emitting polyfluorene (PF) polymer with a small concentration of hole-transporting (HT) units. Blue-emitting organic materials are key in most common white OLEDs - as a matrix material or in combination with phosphorescent down-conversion layers. Whereas in the case of PPV-derivatives charges are believed to be delocalised over several polymer chain segments, charges on the HT units in the polymer used are expected to be highly localised. Therefore, the polymer studied should be even more suitable than PPV-derivatives to critically assess the appropriateness of the model presented in ref. 1. Indeed, we find from the analysis of the current density in hole-only devices with a wide range of active layer thicknesses that also for this material the concentration dependent mobility model describes the current-voltage (I-V) curves significantly better than the more conventional mobility model, which assumes a mobility with a Poole-Frenkel type field dependence. The effect was neglected in a preliminary analysis of the hole mobility in these materials [2]. Furthermore, we demonstrate the important role of taking hole diffusion into account, at least for small voltages (up to ~ 3 V), by giving an analysis of impedance measurements. For double carrier devices, the effect of the carrier concentration dependence of the mobility and the effect of diffusion on the exciton generation rate is demonstrated by giving results obtained from a drift-diffusion device model. A comparison is made with the experimentally determined I-V curves and position dependence of the recombination rate across the organic layer. [1] W.F. Pasveer et al., Phys. Rev. Lett. 94 (2005), 206601. [2] R. Coehoorn et al., Proc. SPIE 6192 (2006), 61920O.


O9.2
Abstract Withdrawn


O9.3
pH Dependence on the Over-oxidation Mechanism in PEDOT:PSS Films. Payman Tehrani, Anna Kanciurzewska, Xavier Crispin, Nathaniel D. Robinson, Mats Fahlman and Magnus Berggren; Dept. of science and technology, Linköping University, Norrköping, Sweden.

Here, we present results from studies performed on the electrochemical over-oxidation (ECO) of thin films of poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate) (PEDOT:PSS) and how this effect varies depending on the pH of the electrolyte. The over-oxidation process, which results in a permanently non-conducting and electrochemically inactive polymer film, is complex and the entire mechanism is still somewhat unknown. We found that by increasing the pH of the electrolyte the over-oxidation potential is shifted to lower potentials vs. the Ag/AgCl reference electrode. Combining the results from such electrochemical studies with spectroscopic measurements, we recognize reaction routes and end products that are similar to those observed in other polythiophene derivatives. Over-oxidation is believed to be one of the main degradation mechanisms of the anodically addressed electrode in electronic and electrochemical organic-based devices in general but the mechanism also enables us to subtractively pattern PEDOT:PSS thin films at very high throughput speeds in printing tools.


O9.4
Abstract Withdrawn


O9.5
Simultaneous Single Molecule Raman and Fluorescence Spectroscopy of Conjugated Polymers. Manfred J Walter^1,2, J. M. Lupton^1,2, K. Becker¹, J. Feldmann¹, G. Gaefke³ and S. Hoeger³; ¹Photonics and Optoelectronics Group, Physics Department and CeNS, Ludwig-Maximilians-Universität, Munich, Germany; ²Department of Physics, University of Utah, Salt Lake City, Utah; ³Kekulé-Institut für Organische Chemie und Biochemie, Universität Bonn, Bonn, Germany.

A particularly appealing advantage of organic electronics is the opportunity for molecular scale engineering of material characteristics. To overcome extrinsic restrictions, molecular level characterization techniques offer an ideal approach to learn more about the structure-property relationships of conjugated polymers. For the first time, we combine two such powerful means - single molecule fluorescence spectroscopy and single molecule surface enhanced resonance Raman spectroscopy (SERRS) - to a simultaneous, low-temperature spectroscopic tool whose strength we demonstrate with the example of excited state relaxation mechanisms. On the one hand, we are able to resolve different chromophores on a single conjugated polymer chain by making use of the extraordinary fingerprinting abilities of both single molecule techniques. While resonance Raman spectra probe the ground state vibrational modes of the absorbing chromophore, fluorescence spectra display the respective modes of the emitting one. Comparing both simultaneously acquired spectra from a single molecule allows us to determine whether or not an energy transfer (ET) process occurred as different chromophores on a single chain show slightly shifted vibronic fingerprints. Additionally, the data suggest that exciton self-trapping in the excited state occurs spatially randomly within the entire chromophore. On the other hand, the resonance condition of Raman allows us to monitor the absorbing chromophore by the scattering intensity, while the fluorescence spectra display the molecules’ emission characteristics at the same time. We use the well known phenomenon of spontaneous spectral diffusion to vary the resonance condition of the chromophore and the exciting laser. Determining whether or not a correlation between the SERRS intensity and the fluorescence spectral position exists allows us to discriminate between situations where absorption and emission occur from the same chromophore or where an ET event shifts the emission. The simultaneous single molecule SERRS and fluorescence spectroscopy technique turns out to be a powerful optical nanoscale characterization method for conjugated polymers. We demonstrate its usefulness by showing that we can track inter- and intrachromophoric excited state relaxation mechanisms, namely excitation energy transfer and exciton self-trapping.


O9.6
Molecular-weight Dependence of Interchain Polaron Delocalization in High-mobility Conjugated Polymers. Jui-Fen Chang¹, Jenny Clark¹, Ni Zhao¹, Henning Sirringhaus¹, Dag W. Breiby², Jens W. Andreasen², Martin M. Nielsen², Mark Giles³, Martin Heeney³ and Iain McCulloch³; ¹Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom; ²Danish Polymer Centre, Risø National Laboratory, Roskilde, Denmark; ³Merck Chemicals, Southampton, United Kingdom.

Interchain interactions have a profound effect on charge transport properties of conjugated polymer thin films. We present here a detailed study of interchain interaction effects on charge transport in poly-3-hexylthiophene (P3HT) as a function of molecular weight (MW) and thin film processing conditions. In highly crystalline, anisotropic films the field-effect mobility increases with MW and saturates at values around 0.1 cm²/Vs in the high MW regime. From comparative studies of transistor characteristics, film morphology, and spectroscopic properties, we observe experimental evidence for reduced exciton bandwidth and increased polaron delocalization with increasing conjugation length and crystalline quality. It is shown that the crystalline quality of P3HT nanoribbons which depends on the chain folding kinetics and polymer chain lengths plays a decisive role to determine the MW dependence of mobility. This result provides a microscopic understanding of the factors that limit the field-effect mobilities around 0.1 cm²/Vs in high-MW P3HT. We further investigate the effects of polaron activation and disorder on charge transport in different MW polymers by experimental and theoretical analyses of charge-induced optical absorption spectra and low temperature data. A dynamic vibronic model is applied to fit the charge-transfer transition bands to extract the characteristic parameters associated with the degree of polaron delocalization. The crossover between localized and delocalized polarons is demonstrated from low- to high-MW polymers. The estimated polaron activation energy is then incorporated into transport models to extract the distribution of tail states and mobility prefactor from temperature dependent transport data. Our study concludes that the polaron effect is comparable with disorder effect in low-MW, low-mobility devices (μ<10^-3 cm²/Vs), whereas disorder becomes the dominant mechanism limiting charge transport in high-mobility devices (μ>10^-2 cm²/Vs).


O9.7
Micromolding in Capillaries and Transfer Printing of Metal Electrodes: A Soft Lithographic Approach for the Fabrication of Source/Drain Electrodes in Organic Field Effect Transistors. Alexander Bluemel¹, Andreas Klug¹, Sabrina Eder^1,2, Ullrich Scherf³ and Emil J.W. List^1,2,4; ¹Institute of Solid State Physics, Graz University of Technology, Graz, Austria; ²Christian Doppler Laboratory Advanced Functional Materials, Institute of Solid State Physics, Graz University of Technology, Graz, Austria and Institute of Nanostructured Materials and Photonics, JOANNEUM RESEARCH, Weiz, Austria; ³Makromolekulare Chemie, Fachbereich Chemie, Bergische Universität Wuppertal, Wuppertal, Germany; ⁴NanoTecCenter Forschungsgesellschaft mbH, Weiz, Austria.

Soft lithography is a non-photolithographic approach for the micron-sized structuring of materials. In recent years a great number of publications in various fields of application such as microelectronics (micro-wires, organic light-emitting devices (OLEDs), organic field-effect transistors (OFETs) or biology (patterned cell growth) underline its flexibility and applicability to a great variety of research areas. We report the use of Micromolding in Capillaries (MIMIC) for the fabrication of source and drain electrodes used in well-performing bottom-gate/bottom-contact OFETs. This technique combines the advantage of solution-processability of the silver nano-dispersion with high lateral resolution for highly accurate micron-sized patterns. In MIMIC the stamp is used to form a system of capillaries between the substrate and the stamp. Applying a droplet of the desired material at one end of the capillary makes the liquid enter the microchannel due to capillary action. In comparison we have used Micro-Transfer Printing (µTP), the micron-sized counterpart to conventional letterpress printing. Here the stamp is set onto an inking pad, so that the protruding surfaces of the stamp come into contact with the silver nano-dispersion, and the material is transferred in the liquid state directly to the substrate. In all devices we used regioregular poly(3-hexylthiophene) (P3HT) as active channel material, a well-established conjugated polymer. The devices were performing well and showed hardly any hysteresis in the output and transfer characteristics.


O9.8
Structural Analysis of Oligothiophenes and Some of their Derivatives: a Combined X-Ray Single Crystal Data and DFT Calculations Mamoun M. Bader^1,2 and Phuong T. T. Pham²; ¹Chemistry and Earth Sciences, Qatar University, Doha, Qatar; ²Chemistry, Pennsylvania State University, Dunmore, Pennsylvania.

We report here the results a combined experimental and theoretical investigation on the geometric structures of a series of oligothiophene molecules. The impact of structural modifications on the calculated and observed structures is analyzed. In the case of x-ray data, some molecules showed rather unusual syn-conformations while others formed pi satcking. Structural features leading to these data are discussed. Comparison is made between our findings and the known structures of oligothiophenes (from the Cambridge Database) are made.


O9.9
Inkjet-Printed Low-Voltage Organic Thin-Film Transistors : Towards Low-Cost Flexible Electronics Mickael Barret, Mustapha Chouiki, Sebastien Sanaur and Philippe Collot; Packaging and Flexible Substrates, Ecole Nationale Supérieure des Mines de Saint-Etienne, Site Georges Charpak, Gardanne, France.

Low-cost processing is a major challenge of solution-processable Organic Thin-Film Transistors (OTFTs). Another important issue is to combine flexibility and low-operating voltage of such devices for their integration in cheap disposable electronic products. In this paper, we demonstrate how to realize OTFTs on flexible substrate by low-cost technique with low-operating voltages : we show that inkjet printing is a suitable low-cost technique to dispense polymers and inorganic nanoparticules, as described in a previous paper [1], for the direct-writing of OTFTs. For this, we first evaluate the performances of several inkjetted couples of semiconductor/electrodes on doped silicon wafer with a thermal oxide layer. Devices are made of conducting polymers or inorganic materials as source/drain electrodes, and poly(alkylthiophene) polymers as active materials. The strategy consists of printing the same semiconductor/electrodes couple on cheap (<20$/m2) flexible 50-µm-thick PET foil, coated with a thin layer of aluminium (60 nm), and then, with an ultrathin SiO2 layer (4 nm): this assembly constitutes the gate structure of our devices. We will discuss electronic properties of our OTFTs upon piezoelectric drop-on-demand inkjet parameters (driving signal form, solution viscosity, …) and ink properties (solvent, concentration, …). The effect of surface modification of the gate oxide with silane self-assembled monolayers (SAMs) will also be presented. As expected from the ultrathin oxide layer, the OTFTs display low-operating voltages within 3V. Also, the surface treatment of the oxide layer by octadecyltrichlorosilane (OTS) improves significantly electrical characteristics of the OTFTs. In particular, these ones exhibit low threshold voltages (|VT|<1V). In summary, we report one of the first demonstration of an inkjet printing process for low-cost production of OTFTs on flexible foils. Futhermore, thanks to a well-chosen cheap multiple-layer substrate, we obtain low-operating voltages (|V| <3V), a requirement for their integration in disposable products. This paves the way towards using inkjet printing as a key technology for such applications in plastic electronics. Keywords : Inkjet Printing, Organic Thin-Film Transistors, Low-Operating Voltage, Polythiophenes. [1] S. Sanaur, A. Whalley, B. Alameddine, M. Carnes, C. Nuckolls, Organic Electronics 7, 423-427 (2006)


O9.10
Can We Use Scanned Probe Microscopy to Measure Local Charge Carrier Mobility in Organic Semiconductors? Showkat Yazdanian, Seppe Kuehn and John Marohn; Cornell University, Ithaca, New York.

Considerable insight into charge injection and trapping processes in π-conjugated systems has been gained by making scanned probe measurements of local electrostatic potential and capacitance. Estimates of carrier mobility, however, have thus far been determined solely via bulk techniques. It would be exciting if a local measurement of carrier mobility or diffusion constant could also be made. It is known that low-spring-constant cantilevers can be used to observe electric field fluctuations arising from thermal dielectric fluctuations in polymers [1]. We suggest that ultrasensitive cantilevers can likewise measure the local charge diffusion constant via the effect of the associated electric field fluctuations on cantilever frequency and ringdown time. Analytical scaling laws and numerical simulations of the electric field power spectrum resulting from the thermal motion of holes in a N,N'-diphenyl-N-N'-bis(3-methylphenyl)-(1,1'-biphenyl)-4,4'-diamine (TPD) / polystyrene field effect transistor suggest that the local hole diffusion constant can be inferred in this system from measurements of cantilever frequency and ringdown time as a function of tip height and charge density. We suggest a route to directly testing the Einstein relation by comparing the locally measured charge diffusion constant to the bulk field effect transistor mobility. 1. S. Kuehn, R. Loring and J. Marohn. Phys. Rev. Lett. 96, 156103 (2006).


O9.11
Efficient Colour Tunable Light Emitting Diodes based on Heteroleptic Iridium(III) Complexes with Phenylpyridine and Hydroxyquinoline Ligands. Sabrina Eder^1,3, Stefan Kappaun², Stefan Sax¹, Fabian Niedermair², Kerstin Waich⁴, Christian Slugovc² and Emil J.W. List^1,3,5; ¹Institute of Solid State Physics, Graz University of Technology, Graz, Austria; ²Institute for Chemistry and Technology of Organic Materials, Graz University of Technology, Graz, Austria; ³Christian Doppler Laboratory Advanced Functional Materials, Institute of Solid State Physics, Graz University of Technology, Graz, Austria and Institute of Nanostructured Materials and Photonics, JOANNEUM RESEARCH, Weiz, Austria; ⁴Institute of Analytical Chemistry and Radiochemistry, Graz University of Technology, Graz, Austria; ⁵NanoTecCenter Forschungsgesellschaft mbH, Weiz, Austria.

Cyclometalated Iridium(III) complexes are very promising candidates for possible applications in the emerging field of organic active materials used in light-emitting diodes (OLEDs), polymer lasers, photovoltaic devices, field effect transistors or sensing purposes. One of the main goals concerning applications in e.g. flat panel displays is the possibility of tuning the emission colour to cover the entire visible spectrum. Although there are numerous reports on colour tuning of homoleptic Iridium(III) complexes by modifications of the cyclometalating ligands, the corresponding synthetic procedures require cumbersome preparation steps or harsh reaction conditions. Alternatively, the synthesis of heteroleptic Iridium(III) complexes has received considerable attention due to a facilitated synthesis and the possibility of tuning the emission colour via the ancillary ligand. However, this is considered to be one of the most promising approaches for colour-tuneable Iridium(III) complexes, examples of emission characteristics controlled by the ancillary ligands are still rare. In this contribution we report our fast, versatile and cheap approach for the preparation of heteroleptic Iridium(III) complexes bearing phenylpyridine and different hydroxyquinoline ligands with emission colours dominated and controlled by the nature of the hydroxyquinoline ligand. The present work provides a powerful tool for tuning the emission characteristics as well as absorption properties that allow an easy finding of appropriate host materials and paves the way to tailor-made phosphorescent materials for practical applications.


O9.12
Synthesis of Conjugated Oligomers and Polymers Functionalized with Small Biomolecules for Advanced Sensing Applications. Gianluca M. Farinola¹, Francesco Babudri^1,2, Gabriele Giancane⁴, Omar Hassan Omar², Patrizia Iliade¹, Francesco Naso^1,2, Francesco Palmisano^1,3, Maria C. Tanese¹, Luisa Torsi^1,3, Ludovico Valli⁴ and Pier Giorgio Zambonin^1,3; ¹Chemistry, University of Bari, Bari, Italy; ²CNR ICCOM Bari- Dipartimento di Chimica University of Bari, Bari, Italy; ³Centro di Eccellenza TIRES, University of Bari, Bari, Italy; ⁴Innovation Engineering, University of Lecce, Lecce, Italy.

Conjugated oligomers and polymers bearing chiral small biomolecules as substituents appear interesting candidates as active materials in enantioselective sensors, owing to the recognition capability of the substituents combined with the semiconducting features of the conjugated backbone. In the frame of our studies dealing with novel organometallic strategies for the synthesis of polyconjugated materials [1] we developed a protocol, based on a modified Cassar-Heck-Sonogashira reaction, for the synthesis of poly(aryleneethynylene)s (PAEs) functionalized with biomolecules, such as glucose [2] or amino acids [3]. Quartz crystal microbalance gravimetric sensors based on these substituted PAEs demonstrated their ability to discriminate menthol enantiomers in vapour phase [4]. Due to the inherently poor PAE conductivity, no current modulation was observed in an Organic Thin Film Transistor (OTFT) device configuration. With the aim to synthesize materials suitable for OTFT sensors, we moved to phenylene-thienylene systems. Indeed, good performances were obtained in a resistive NO2 sensor [5] and in OTFT devices [6] based on Langmuir-Schaefer (LS) thin films of a regioregular alkoxy-substituted poly(phenylenethienylene) synthesized via a straightforward organometallic protocol [7]. Hence, a versatile strategy, based on the Suzuki-Miyaura cross-coupling reaction has been developed to obtain oligo(phenylenethienylene)s functionalized with protected amino acid or glucose units in different positions starting, in some cases, from the same building blocks employed for the synthesis of PAEs. Good field effect properties were observed in bio-substituted oligo(phenylenethienylene) thin films, which were used as OTFT sensors for chiral discrimination. References: [1] F. Babudri, G.M. Farinola, F. Naso, J. Mat. Chem. 2004, 14, 11. [2] F. Babudri, D. Colangiuli, P.A. Di Lorenzo, G.M. Farinola, O.Hassan Omar, F. Naso, Chem. Commun. 2003, 130. [3] F. Babudri, D. Colangiuli, L. Di Bari, G.M. Farinola, O. Hassan Omar, F. Naso, G. Pescitelli, Macromolecules 2006, 39, 5206. [4] M.C. Tanese, L. Torsi, N. Cioffi, L.A. Zotti, D. Colangiuli, G.M. Farinola, F. Babudri, F. Naso, M.M. Giangregorio, L. Sabbatini, P.G. Zambonin., Sens. Actuators B 2004, 100, 17. [5] F. Naso, F. Babudri, D. Colangiuli, G.M. Farinola, F. Quaranta, R. Rella, R. Tafuro, L. Valli, J. Am. Chem. Soc. 2003, 125, 9055. [6] 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, Chem. Mater. 2006, 18, 778. [7] F. Babudri, D. Colangiuli, G. M. Farinola F. Naso Eur. J. Org. Chem. 2002, 2785.


O9.13
Hexathiapentacene: Structure, Molecular Packing, and Field-Effect Transistors. Alejandro L. Briseno^1,2, Miao Qian³, Fred Wudl³, Zhenan Bao², Hong Meng⁴ and Younan Xia¹; ¹Chemistry, University of Washington, Seattle, Washington; ²Chemical Engineering, Stanford University, Stanford, California; ³Chemistry, UCLA, Los Angeles, California; ⁴Central Research and Development, Experimental Station, E. I. DuPont, Wilmington, Delaware.

The past decade witnessed a tremendous development in exploiting organic molecules for use in electronic and optoelectronic devices. Oligo-acenes, tetrathiafulvalene (TTF) and their derivatives have been extensively studied as fundamental building blocks for organic electronics. As an example of oligo-acenes, pentacene has led organic semiconductors with the highest field effect mobility in thin film transistors. The advent of molecular design and synthesis has made it possible to tune the physical properties and molecular structures of organic materials to meet the technological requirements for fabricating practical devices. Aromatic molecules have recently shown that self-assembly through strong pi-pi interactions can lead to the formation of one-dimensional (1D) nanostructures for applications in field-effect transistors. Inspired from studies on TTF derivatives, we became interested in introducing S-S interactions to oligo-acenes to provide an alternative charge transport pathway other than pi-pi interactions, which are well known in herringbone packing for oligo-acenes. In this presentation we report the electrical characteristics of hexathiapentacene (HTP) and emphasize on the unusual chemical structure and molecular packing. Although HTP was first described over 30 years ago, only minimal characterization was reported. This work appears to be the first to determine its molecular structure/packing mode and to study its application in organic transistors. We also discuss preliminary results on the synthesis of 1-D nanostructures and charge-transport through single-crystal wires.


O9.14
A Microscopic Description of Structural Defects in Pentacene Thin Films. Stijn Verlaak¹, Cedric Rolin^1,2 and Paul Heremans¹; ¹IMEC, Leuven, Belgium; ²PCPM, UCL, Louvain-La-Neuve, Belgium.

The growth of a pentacene monolayer is modelled microscopically by adding one molecule at a time to a predefined crystalline aggregate. For each added molecule, the potential energy versus molecular orientation in the crystal is calculated using the MM3 molecular mechanics force field. This delivers a potential energy surface that reveals stable molecular orientations at its local minima. Four different predefined aggregates are used to start the calculation. Each case can initiate a different type of defect: 1. Adsorption of a pentacene molecule at a step-free (110) surface. The most stable configuration deviates from an ideal-crystal configuration. 2. Adsorption of a molecule at a step. This molecule will relax into the ideal-crystal configuration, yet a defect can be initiated by thermal activation. 3. Adsorption of a molecule sandwiched between two steps. The ideal-crystal configuration is most stable, yet a molecule can get trapped at a kinetic defect configuration, depending on flux and temperature. 4. Adsorption of a molecule at a grain boundary. Interstitial molecules intrinsicically form defects that generate film stresses. Upon addition of further molecules onto an already disordered aggregate, the defect can follow different paths: if structural relaxation is faster than the average time to add another molecule to the site, the defect can completely relax to a perfect crystalline order. Alternatively, if relaxation times are too long, the defect can propagate. Most defects relax to the ideal crystal structure after they were formed. Structural defect densities less than 1e16 cm-3 can be estimated in the bulk of the film. Grain boundaries are the most pronounced and stable defects and are a source for compressive stresses that may lead to dislocations. Ultimately, it is to be expected that grain boundaries will dominate charge transport in carefully prepared pentacene films. The present study leads to a conceptually simple frame for understanding the details of film growth, and is a first step to unravel the presently unclear link between growth thermodynamics and electronic properties and charge transport in organic small molecular films.


O9.15
Organic Materials for Low Cost, Large Area Photoconductors Alyson Niemeyer^1,2, Brian Crone², Ian H. Campbell² and Franky So¹; ¹Dept of Materials Science and Engineering, University of Florida, Gainesville, Florida; ²Los Alamos National Laboratory, Los Alamos, New Mexico.

There is a great need for low cost, large area (>10 cm2) photodetectors for use with scintillators in radiation detection for special nuclear material and radiological isotope monitoring. Photodetectors for homeland security applications must be low cost and should have large area, low capacitance, negligible dark current, and reasonable response time. In this work we investigate organic photoconductors with interdigitated planar electrodes. Unlike silicon, organic materials can have the low dark currents necessary for these low capacitance, large area structures. We present the effects of polymer film thickness, polymer ratio, interdigitated diode finger spacing, and applied voltage on diode quantum efficiencies. Polymer film thickness was found to have no significant effect on quantum efficiency. The optimum polymer (MEH-PPV) to exciton dissociator (PCBM) ratio was found to be 1:4. This optimum ratio arises from an interplay of maximizing absorption and hole transport through increased MEH-PPV concentration, and maximizing exciton dissociation and electron transport through increasing PCBM concentration. Devices with interdigitated diode finger spacings of 5, 10, and 100 μm were measured. Electrode spacing had little effect on the quantum efficiency at a given electric field. Quantum efficiency increased at higher electric fields, as charge collection improved. Implications of the measured results on optimization of organic photoconductors will be discussed.


O9.16
Enhanced Electron Injection in Organic Transport Layers by Successful Doping with LiF. Jong Hyuk Yoon, Kaushik Roy Choudhury and Franky So; Dept of Materials Science and Engineering, University of Florida, Gainesville, Florida.

Organic electroluminescent devices are attracting a great deal of research interest for large-area display and lighting applications. In organic light emitting diodes (OLEDs), electron injection contact is problematic because reactive low work function metals are required. Among the small molecular electroluminescent materials, tris-(8-hydroxyquinoline) aluminum (Alq3) is one of the most widely used electron transporting materials, and is often considered a model system for studying OLEDs. In the present study we demonstrate efficient n-type doping of Alq3 using LiF as an n-dopant to facilitate electron injection and transport. We systematically studied the effect of dopant concentration on charge injection and carrier transport in this system and substantial enhancement in conductivity was observed. Typical single-carrier devices consisted of the thin-film geometry ITO/Alq3: x % LiF/Al where the active organic transporting layer was fabricated by co-evaporation of Alq3 and LiF. The doping ratio, LiF/Alq3, was varied from 1 to 15 wt%. Current voltage measurements on the hermetically sealed devices exhibited increased device current with the increase of LiF doping. In all cases the current of the doped samples is significantly higher than the undoped samples. The current density, in all cases, shows a transition from a linear ohmic region to a space-charge limited (SCLC) regime indicating that the traps are being filled by the dopants. Interestingly, even within the SCLC regime a dependence of the current on the doping level is distinctly observed. Small-molecule OLED devices were fabricated with Alq3:LiF as an electron injection layer and Al as a cathode and their performance compared with devices with undoped Alq3 electron injection layer with LiF/Al cathode. Dramatic enhancement of luminance, accompanied by significant lowering of the driving voltage led to a doubling of the luminous efficiency in the LiF-doped-Alq3 devices compared to their undoped counterparts. The experiments clearly indicate a method of controlled n-type doping of electron injection layers to lower the injection-barrier and increase conductivity, leading to balanced injection and transport of charge carriers.


O9.17
Click Dielectrics: Polymer-nanoparticle Composites for Flexible Electronics Applications Meghann Anne White¹, Ashok J. Maliakal², Nicholas J. Turro^1,3 and Jeffrey T. Koberstein³; ¹chemistry, Columbia University, New York, New York; ²Bell Laboratories, Lucent Technologies, Murray Hill, New Jersey; ³Chemical Engineering, Columbia University, New York, New York.

Improved high K dielectric materials are important for the development of devices for use in flexible electronics. One method of creating high K materials is through the use inorganic/organic core-shell nanoparticles. Novel surface ligands have been designed and synthesized for high K TiO2 nanoparticles, which allows for chemical modification at the surface using “click” chemistry, specifically Cu-catalyzed azide-alkyne cycloaddition. The “click” functional ligands prevent nanoparticle agglomeration and allow us to covalently bind various polymers to the surface of the TiO2. The resulting polymer-nanoparticle composites are transparent and easily spin cast onto substrates. The dielectric properties of the resulting nanoparticle-polymer composites have been studied for potential as high K dielectric materials for flexible electronics applications.


O9.18
High-performance Thin Film Transistors From Semiconducting Liquid-crystal Phases by Solution Processes. Fapei Zhang, Masahiro Funahashi and Nobuyuki Tamaoki; National Institute of Advanced Industrial Science and Technology (AIST), Ibaraki, Japan.

Recently the potential of liquid crystals (LCs) in organic electronics is being recognized in that they are solution processable and can self-organize into large-area ordered mesephases. The high mobility more than 0.1 cm2V-1s-1, independent of electric field and temperature, has been achieved on several type of LC semiconductors from time-of-flight measurement, and the large-area LC thin film transistors were fabricated with carrier mobility of 0.02 cm2V-1s-1. Recently we have developed several asymmetrically substituted oligothiophene derivatives, which show LC mesophases at room temperature and good ambipolar carrier transport. Here we report the fabrication and characterization of thin-film transistors from one of such materials: 5-propyl-5``-(4-pentylphenyl)-2,2`:5`,2`-terthiophene (3-TTPPh-5) by solution process. The LC thin films were prepared on doped Si wafers covered with thermally oxidized SiO2 layer. From AFM and polarized microscopy, the film consists of large-size (ca. 10-30 μm) smectic multi-domains with the ordered lamellar structure. X-ray diffraction (XRD) shows that such lamellar structures are highly oriented with each of smectic molecular layers parallel to the substrate surface. Moreover side-by-side close packing of LC molecules within the smectic layer is indicated by in-plane XRD after annealing of as-prepared films. For the top-contact thin film transistors (TFT) of 3-TTPPh-5, a high hole mobility of 3-4 x10-2 cm2V-1s-1 has been achieved in ambient air, the highest value in LC-phase TFTs to date. Such high mobility is attributed to the formation of highly ordered LC domains with large-scale (comparable to channel length of the TFTs) in the films, Furthermore, the investigation was also made about work function of the electrode and surface chemistry of dielectric modified by different types of SAM molecules, as well as the influence on carrier transport properties of LC TFTs. References: 1. M. Funahashi and J. Hanna, Appl. Phys. Lett. 76, 2574 (2000) 2. M. Funahashi and J. Hanna, Adv. Mater. 17, 595 (2005)


O9.19
Organically Modified Silicon-based Stamping Materials for High Resolution Soft Lithography. Kyung M. Choi, Bell Labs, Lucent Technologies, Murray Hill, New Jersey.

We introduce here a novel molecular design of functional silicon elastomers as advanced stamp materials in soft lithography. In nanofabrication, 'soft lithography' is alternative to conventional UV photolithography; this technology allows us to fabricate small patterns with low cost and easy processability. However, commercially available silicon elastomers often results in collapse and mergence due to its low mechanical strength, especially in the nano-scale regime (< 100 nm). Since the resolution of soft lithography techniques is rely on the elastomeric elements, these limitations have motivated us to develop a new stiff, photocured silicon elastomers, which satisfy a diverse set of demands, such as enhanced physical stiffness, good elastomeric properties, low linear polymerization shrinkage, photocurability, and freedom from stress. The new silicon elastomer results in enhanced performances in nanofabrication, which is beneficial to advanced nanotechnology. The relation between silicon rubber structures and resulting functions will be also discussed for high resolution nanofabrications.


O9.20
Electrolyte-Gated Field-Effect Transistor of Rubrene Single Crystal Hidekazu Shimotani^1,2, Haruhiko Asanuma¹, Jun Takeya³ and Yoshihiro Iwasa^1,2; ¹Institute for Materials Research, Tohoku University, Sendai, Miyagi, Japan; ²JST-CREST, Kawaguchi, Saitama, Japan; ³Department of Chemistry, Osaka University, Toyonaka, Osaka, Japan.

It is important to study electronic properties of organic materials under carrier doping for their any applications to electronics. Organic field-effect transistor (FET) is one of the useful methods from the viewpoint, because it enables controllable, reversible, nondestructive,and in situ doping on the surface of various organic materials. However, carrier density controlled by FETs is so small that wide range of electronic properties has not been studied. Therefore, it is needed to increase carrier density on the surface of organic materials accumulated by FET. It is demonstrated by Takeya et al.^[1] and Panzer et al.^[2] that electrolyte can serve as a gate electrode in rubrene single crystal FETs, and it is expected that the electrolyte-gated FETs accumulate high carrier density on the crystal surface due to their large gate capacitances. The purpose of this investigation is to accumulate carriers on the surface of organic materials in high density by electrolyte-gated FET, to estimate its carrier density, and to study its working mechanism. The device was consist of a rubrene crystal with a Au source and a drain electrode immersed in electrolyte [LiClO₄/Poly(ethylene oxide)] and a Pt-coil gate electrode also immersed in the electrolyte. High density carrier accumulation (∼0.33 carrier/molecule) in a rubrene single crystal FET was achieved by applying small gate voltage (-1.2 V) through the electrolyte.^[3] The carrier density was estimated from the gate capacitance (15 μF/cm²) measured independently on a gold thin film. Interestingly, the transfer characteristics (drain current vs. gate voltage) showed a peak, which has not been observed in SiO₂-gate FETs. The origin of the peak will be discussed in the presentation. References: [1] J. Takeya et al., Appl. Phys. Lett., 88, 112102 (2006). [2] M. J. Panzer et al., Appl. Phys. Lett., 88, 203504 (2006). [3] H. Shimotani et al., Appl. Phys. Lett., in press


O9.21
Improved of the Performance in n-channel Organic Thin Film Transistors by Nanoscale Interface Modification. Chih Wei Chu, Academia Sinica, Taipei, Taiwan.

We demenstrated that the electrical properties of n-channel thin films transistor can be enhanced by inserting a nanoscale interfacial layer between organic semiconductor and Source/Drain (S/D) electrodes. Devices with the modified S/D electrodes showed a reduction of the contact resistance not only with respect to Al, but also versus Ca or LiF/Al. The improvement is attributed to the reduction in the energy barrier of electron injection and the prevention of unfavorable chemical reaction between the organic layer and the metal electrode. High field-effect mobility of 0.045 cm2/V-s and on/off current ratios of 105 were obtained in the [6,6]-phenyl C60 butyric acid methyl ester (PCBM) based organic thin film transistors (OTFTs) using the modified S/D electrodes at a gate bias of -40 V.


O9.22
Fast Photoresponse of Functionalized Pentacene Thin Films Jonathan Day¹, Oksana Ostroverkhova¹, John Anthony² and Robert J. Twieg³; ¹Physics, Oregon State University, Corvallis, Oregon; ²Chemistry, University of Kentucky, Lexington, Kentucky; ³Chemistry, Kent State University, Kent, Ohio.

Organic semiconductors are of interest due to their potential applications in molecular electronics and photonics. In order to develop organic semiconductor devices, it is important to understand the mechanisms of (photo)conductivity in organic materials. However, using thin-film device structures and traditional dc methods (such as field-effect transistor and space-charge-limited current geometries) to characterize intrinsic electronic properties of materials is complicated, in part due to the influence of defects on the overall electronic response of the device. In contrast, monitoring of the charge carrier dynamics on short time scales after excitation allows one to probe charge transport properties, prior to trapping at defects. In this way, one can study both intrinsic charge transport and the process of charge trapping. Extension to longer time scales permits the exploration of charge detrapping processes and equilibrium charge transport mechanisms, relevant for devices operating in the dc regime such as thin-film transistors. In our studies, we explored photoconductivity of high-performance functionalized pentacene (FPc) thin films on time scales from picoseconds to many seconds after photoexcitation. The FPc thin films were deposited from solution on glass substrates with patterned interdigitated aluminum electrodes. In studies of fast transient photoconductivity, the samples were excited with laser pulses of ~100 fs duration at a wavelength of 400 nm, and the photocurrent due to transport of photoexcited charge carriers was monitored using a 50 GHz digital sampling oscilloscope. The photoconductivity at longer (milliseconds through seconds) time scales was investigated using continuous wave (cw) illumination and a source-delay-measure unit. Both experiments were performed under conditions of varied electric field strength, fluence and temperature. In all samples, we observed fast charge carrier photogeneration (<70 ps), limited by time resolution of our setup) followed by decay of the photocurrent over the period of several nanoseconds due to charge trapping and recombination, linear dependence of the peak photoconductivity on the fluence and quadratic dependence of the peak photoconductivity on the applied electric field. We also investigated the influence of charge traps on the photoconductive response of FPc thin films, depending on the nature and properties of the traps. A small amount of various dopants was introduced into the FPc matrix, and the changes in the amplitude and decay dynamics of the transient photoconductivity were monitored. By choosing different dopants, such as molecules similar to the host FPc molecules (such as a different FPc derivative) or highly polar organic dyes, we created different traps (shallow structural traps or deep energetic traps, respectively) in the FPc matrix. A comprehensive study of the trapping and detrapping dynamics depending on the properties of the trap molecule will be presented.


O9.23
Optimizing the Growth of Rubrene Thin Films for Organic Thin Film Transistors. Parul Dhagat¹, Hanna Heikkinen^1,2, Jizheng Wang¹ and Ghassan E. Jabbour¹; ¹Flexible Display Center and School of Materials, Arizona State University, Tempe, Arizona; ²Optoelectronics and Measurement Techniques, University of Oulu, Oulu, Finland.

We investigate the growth of rubrene thin films for use in organic thin film transistors. The growth and morphology of these films is controlled by three deposition parameters: substrate temperature, deposition rate and substrate treatment. A systematic analysis of the thin film structure is performed using atomic force microscopy and x-ray diffraction. There is a direct correlation between the structure and the electrical performance of a transistor made using a given thin film. For transistors fabricated from poly-crystalline thin films and tested in air we obtain mobility of 10^-2 cm²V^-1sec^-1, on/off ratio of 10⁴ and low off current ~ 10^-9 A. To the best of our knowledge this is the first time that mobilities such as these, are reported for pristine thin films of rubrene grown in a vacuum chamber at a pressure of 10^-6 Torr and without any buffer layer (e.g. pentacene or such).


O9.24
Mobility Improvement of Pentacene Field-effect Transistor Using Oxygen Plasma Treatment and Ppost-aging Process. Kwonwoo Shin, Sang Yoon Yang, Chanwoo Yang, Hayoung Jeon and Chan Eon Park; Chemical engineering, Pohang University of Science and Technology, Pohang, Gyungbuk, South Korea.

In this study, we investigated the method to fabricate the pentacene transistors with both the large pentacene grains and the good interfacial properties between pentacene films and gate dielectrics by combining the oxygen (O₂) plasma treatment of a polymer gate dielectric and the post-aging process of pentacene transistors. In this system, the gate dielectrics of pentacene transistors with top contact geometry have polymer and inorganic dual layer gate dielectrics. The upper poly(methylmethacrylate) (PMMA) layer plays the role of controlling the surface functional groups of the dielectric layer through the O₂ plasma treatment and the post-aging process, and SiO₂ layer existing under the PMMA can minimize the damage to the gate dielectric layer by plasma treatment, which reduces the leakage current and secures uniform electric properties. When a polymer is treated with O₂ plasma, the chains on the surface are broken by the high energy of plasma and the polar and hydrophilic functional groups such as -OH and -COOH are introduced, resulting in the increase of surface energy. When the PMMA/SiO₂ gate dielectric layer was treated with O₂ plasma, the surface energy increased from 47 to 59mJ/m2. After then, by putting the O₂ plasma-treated gate dielectric layer in a desicator at room temperature, we found that the surface energy decreased to 50mJ/m², which is resulted from the rearrangement of polar functional groups on plasma treated PMMA layer. The decrease of surface energy was mainly attributed to the decrease of the polar term rather than the increase of the dispersion term. Such aging phenomena can also occur when pentacene is deposited on the O₂ plasma-treated PMMA layer because the polar groups can diffuse into PMMA layer when they are covered with the relatively non-polar pentacene molecules. The pentacene films immediately deposited on the gate dielectrics after O₂ plasma treatment show a good wetting on the gate dielectrics and two-dimensional growth. These properties can enhance the coalescence between grains, enlarge the gain size and increase the crystallinity of pentacene active layer. However, the polar functional groups increase the interface trap states in the band gap, resulting in large activation energy for charge transport and lower carrier mobility. The post-aging of fabricated transistors decreased the interface polarity, which in turn reduced trap states, by the rearrangement of interface polar functional groups. However, the morphology of pentacene film did not change during the rearrangement process. The resulting transistors have not only larger grain size of 2~3 μm but also a good interfacial property. The mobility was increased from 0.47 to 0.73cm²/Vs after 2 days of aging, showing around 40% higher than that without plasma treatment.


O9.25
Threshold Voltage Shift of Organic TFTs during Pulse Operation Hsiao Wen Zan, Shih-Chin Kao, Ting-Shuan Cheng and Huang-Wei Pan; Department of Photonics, Display Institute, HsinChu, Taiwan.

One of the important stability issues in organic thin-film transistors is the shift in the threshold voltage when applying a bias to the gate electrode. The applied negative gate bias for a p-type semiconductor causes the threshold voltage shift to be more negative. Also, positive threshold voltage shift can be observed when the positive gate bias stress is applied. The threshold voltage will recover to its original value if the device is left unbiased. The physical origin of this instability is the buildup of charges at the semiconductor/insulator interface, which then trapped by defect states. The shift in threshold voltage but not in mobility is often believed to be caused by deep traps with long discharge time constants. While many studies on DC bias stress effects focus on the trap creation mechanism in OTFTs, the discharge behavior is not well understood. In this research, the trapping and discharge behavior was studied by applying pulse (AC) bias stress on the gate electrode. Pentacene organic TFTs with top-contact structure were used. The thermally grown SiO2 film was served as the gate dielectric. To modify the dielectric surface polarity, SAM surface treatment was applied on some devices prior to the pentacene deposition. The experimental results revealed that, for conventional devices without SAM treatment, the discharge behavior influenced threshold voltage shift pronouncedly. When the frequency increased or the pulse duration decreased, the smaller pulse width caused smaller threshold voltage shift. However, when high-polarity SAM material was coated on the dielectric surface, the threshold voltage shift became almost not dependent on the pulse width variation. To analyze this phenomenon, we firstly extracted the mean activation energy for trap creation (EA) from DC bias stress measurement according to the method proposed by H. L. Gomes et al.. Similar EA (~0.55eV) values were obtained for all the devices. Then, if the effective accumulation time constant was considered according to the study on AC bias stress effect in a-Si TFTs, the OTFTs with high-polarity SAM layer exhibited much smaller accumulation time constant than the conventional ones. This was not reasonable since the high-polarity SAM layer was often considered as a kind of dipole material that exhibited large time constant when converting the polarity. The discharge mechanism was therefore considered to be the dominant factor in AC bias stress. When the SAM layer acted as a kind of dipole layer between the dielectric and the organic active layer, it responded to the change of bias condition slowly. Therefore, the dipole characteristics helped to keep the trapped charge during the unbiased duration, suspended the charge de-trapping mechanism. As a result, the threshold voltage shift in OTFTs with high-polarity SAM treatment was not influenced by the bias pulse width. While in conventional OTFTs, threshold voltage shift is strongly dependent on the gate-bias pulse width


O9.26
Contact Behaviors of Molecularly Doped Polymers Used in Light-emitting and Transistor Based Applications. H. H. Fong and George G Malliaras; Materials Science and Engineering, Cornell University, Ithaca, New York.

Charge injection is critical for both light-emitting and transistor based devices. Non-ohmicity of contacts limits the injected current, resulting in high operating voltage devices and degradation. This work aims to understand the contact behaviors of common organic electronic polymers used in light-emitting devices and transistor devices. Molecularly-doped polymer light emitting diodes and transistor devices are promising for electronic applications because of simplicity in manufacture. Doping provides flexibility of mobility tuning on both hole and electron transports as well as charge injection. However, the mechanism of charge injection becomes complicated. In our studies, various families including fluorene, carbazole and polythiophene based polymers are considered as model compounds to investigate their charge injection behaviors.


O9.27
Nondispersive Bipolar Carrier Transports in tris (8-hydroxyquinoline) aluminum (Alq3) and the Mobility Influence Due to Molecular Doping. H. H. Fong¹ and Shu K. So²; ¹Materials Science and Engineering, Cornell University, Ithaca, New York; ²Department of Physics, Hong Kong Baptist University, Kowloon, Hong Kong.

The hole and electron transporting properties of tris (8-hydroxyquinoline) aluminum (Alq3) were investigated by time-of-flight (TOF) technique between 278-373 K, under an applied field tange of 0.3-1.3 MV/cm. At room temperature, the hole mobility has a value between 10^-9 to 10^-8 cm^2V^-1s^-1 while the electron mobility has a value between 10-7 to 10-5 cm2V-1s-1. The hole mobility is at least two orders of magnitude less than electron under identical preparation and measurement conditions. Generally, all hole and electron TOF transients of Alq3 exhibit a non-dispersive behavior, with a clear plateau region and a dispersion tail. Two disorder transport models, namely the Gaussian Disorder Model (GDM) and the Correlated Disorder Model (CDM), were applied to analyze the temperature and field dependent hole mobility data. The GDM, however, is found to be invalid because it fails to produce a meaningful positional disorder parameter. The CDM gives a better fit to the data, yet the model is still not satisfactory. Possible explanations are discussed. The effects of various ambient gases (nitrogen, oxygen, water) to the mobility of Alq were also examined. Importantly, exposing pristine Alq3 to moisture of dosages in the range 1-100 Torr.s causes a gradual reduction in mobility by a factor of 4 while the effect of oxygen on mobility can be observed only at much higher dosages (>100 Torr.s). It is proposed that water diffuses into Alq3 during exposure and subsequently acts as charge traps for externally injected free electrons. Moreover, mobility influences in Alq3 due to doping of carrier transporters (e.g. phenanthroline, triphenylamine, polycyclic aromatic hydrocarbon compounds) and fluorescent dyes (e.g. Coumarin) are also considered. Results provide valuable fundamental parameters for evaluating the device characteristics of Alq3 based organic light emitting diodes. Reference: [1] H.H. Fong, S.K. So, J. Appl. Phys. (2006) (accepted). [2] H.H. Fong, S.K. So, J. Appl.Phys. 98, 023711 (2005). [3] S.C. Tse, H.H. Fong, S.K. So, J. Appl. Phys. 94, 2033 (2003). [4] H.H. Fong, K.C. Lun, S.K. So, Jpn. J. Appl. Phys. 41, L1122 (2002).


O9.28
Passivation of Pentacene Field-effect Transistors with AlOx Films Grown by Atomic Layer Deposition. Hayoung Jeon¹, Kwonwoo Shin¹, Sang Yoon Yang¹, Se Hyun Kim¹, Sang-Hee Ko Park² and Chan Eon Park¹; ¹Polymer Research Institute, Pohang University of Science and Technology, Pohang, South Korea; ²Basic Research Laboratory, Electronic and Telecommunications Research Institute, Daejeon, South Korea.

Pentacene field-effect transistors (FETs) have been considered to be the most feasible devices for achieving the flexible electronics because of its high field effect mobility comparable to that of amorphous silicon-based FETs. However, pentacene FETs tend to easily degrade when they are directly exposed to ambient air and this drawback gives rise to the issues for environmental stability and lifetime in pentacene FETs. It is known that degradation of pentacene devices mainly occurs due to oxygen- and moisture-associated species. To prevent the diffusion of O₂ and H₂O into pentacene FETs effectively, the quality of passivation layer should be guaranteed, i.e., the passivation layer should be formed with good step coverage and free from pin-holes. In addition to the film quality of passivation layer, it is also important for the passivation process to be compatible with the organic semiconductor layer. Therefore, in this study, we have introduced the atomic layer deposition (ALD) method to passivate pentacene FETs, which can form a dense inorganic film at low temperature without causing damage to pentacene active layer in FET. A 50 nm-thick aluminum oxide (AlOx) was deposited on pentacene FET with top-contact geometry in ALD reactor at 90°C with nitrogen as a carrier gas. Trimethylaluminum and H2O were used as precursors for aluminum and oxygen, respectively. Although the mobility of the device just after passivation decreased by around 8% as compared to the device before passivation, no significant degradation have been observed even after two months in ambient air. Owing to excellent barrier properties of AlOx films, long-term stability of pentacene FETs can be achieved, in other words, more than 90 % of initial mobility and on-current were maintained after two months. On the other hand, pentacene FET without passivation showed the sharp decrease of mobility by a quarter of its initial value after two months, indicating severe degradation. Also, we have investigated the temperature dependence of mobility and the variation of electrical parameters in pentacene FETs by storing the devices under O₂ and H₂O exposure condition in order to find out the major origin for degradation of pentacene FETs in ambient air. In case of pentacene FETs exposed to pure oxygen gas for 6h, no remarkable change was observed in activation energy. In contrast, pentacene FETs exposed to humid condition (RH~70%) for 6h exhibit a sharp increase of activation energy. This suggests that the trap distribution become broader and deeper in the band gap because of H₂O in air. The results of these experiments indicate that the major factor critically affecting the degradation of device performances was H₂O in air, the adsorption of which causes charge trapping in both pentacene bulk and pentacene/dielectric interface.


O9.29
Correlation Between the Morphology and the Mobility in Pentacene FET with Alkanethiol Treated Gold Electrodes. Soumya Dutta, Pablo Stoliar and Fabio Biscarini; CNR- Institute for the Study of Nanostructured Materials, Bologna, Italy.

The apparent dependence of field-effect mobility on the channel length is very common issue for organic field-effect transistors especially in coplanar geometry. This dependence becomes more prominent in smaller channel length (L). The effect was attributed to the presence of parasitic contact resistance appeared at the metal-organic interfaces. The similar feature was also explained on the basis of different mobilities of organic material in the channel region and the region atop the metal. We report the dimensional effect in case of OFET based on pentacene. Our results reveal the excellent correlation between the morphology and the mobility, resulting apparent decrease in mobility with decreasing L predominantly in the low L (L < 2 micrometer) regime. However, this proposition cannot explain the continuous increase in mobility with respect to L, which is also falacious feature to determine the exact mobility. The dimensional effect in case of the device structure that is subjected to a modification in injection interface using self-assembly monolayer of alkanethiol with different chain lengths (n, where n = 0 corresponds to gold-pentacene interface) yields more insight. The device corresponding to n = 4 shows the mobility variation almost similar to the unmodified electrode (n = 0). However, for higher chain length (n >= 7), the mobility becomes almost independent of L at the higher L (L > 2 micrometer) regime, whereas decreases with the decreasing L in the lower L (L < 2 micrometer) regime. The correlation between the morphological aspect and the mobility variation with respect to L becomes more accurate over the entire range of L in case of higher chain length (n >= 7). These results can be attributed to the clean metal-organic interface due to the modification with self-assembled monolayer of alkanethiol.


O9.30
Stable and Efficient White Light-Emitting Diode through Miscibility Control Hang Ken Lee¹, Tae-Ho Kim² and O Ok Park¹; ¹Korea Advanced Institute of Science and Technology, Daejeon, South Korea; ²Beckman Institute, Urbana, Illinois.

Organic and polymer light-emitting diodes(OLEDs/PLEDs) that emit white light are attracting increasing interest for their potential applications as full color displays, backlights for liquid-crystal displays, and even next-generation paper-thin light source. Various strategies have been utilized to fabricate polymeric white-light-emitting diodes. The firstly proposed method was multilayer structure device, but nowadays single layer structure is preferred due to its low production cost and easy processability[1]. In single layer structure, emissive layer consists of several polymers, electron and hole transport material [for balance of charge carriers] as well as luminous polymers. However, most polymers are not miscible each other due to the low entropy of mixing, phase separation occurred when the thin film from solution was fabricated[2]. This phase separation causes some problems such as color change by applied voltage and low efficiency. we suggested a new solution for this problem. By introducing copolymer in emissive layer to enhance miscibility, we obtained voltage-invariant white emission from polymeric LEDs. The efficient energy transfer and charge trapping were observed in this system, so the relative intensity could be fine-tuned by varying the contents of three materials. The near pure white-light emission observed at wide range of voltage from 10V to 16V with high brightness[10295 cd/m2] and high luminance efficiency[8.34cd/A]. These white-light-emitting PLEDs were single-layer devices fabricated by spin-casting from solution. The simple device structure and the promise of low-cost manufacturing make this approach attractive for solid-state-lighting applications. [1] T. H. Kim, H. K. Lee, O. O. Park, Adv. Func. Mater. 16, 611(2006) [2] T. Kietzke, D. Neher, K. Landfester, R. Montenegro, R. Guntner and U. Scherf, Nature 2, 408 (2003)


O9.31
New Photoreactive Dielectric Polymers for Organic Field Effect Transistor Ju Hee Kim, Kyung Hwan Kim, Min Ju Cho and Dong Hoon Choi; Department of Chemistry, Korea Univ., Seoul, South Korea.

Organic field effect transistor (OFET) materials based on extended linear p-conjugated systems have been very intriguing and significant development has been achieved in these materials over the last several years. Besides semiconducting active materials, high-performance OFET gate dielectrics are of intense current interest. The materials should satisfy the requirements of excellent insulating properties, efficient low-temperature solution fabrication, and compatibility with diverse organic semiconductors. In this study, we have prepared 1st-generation p-conjugated dendrimers bearing thiophene-based peripheral groups, which exhibit 2-dimensional planar geometry as a active materials and some polyacene molecules are also employed for good comparison. We also demonstrate a new low temperature approach to achieve high-quality cross-linked polymer dielectrics from photoreactive polymer precursors. The photoreactive polymers were designed to contain either a photopolymerizable or a photocrosslinkable moiety. The photoreacted dielectric layers can be fabricated without any additive except a trace amount of catalyst. The polymer can be spin-coated simply and insolubility after UV exposure can allow us to deposit a semiconductor layer by using a solution process. We fabricate metal-insulator-metal (MIM) device to investigate film capacitances and dielectric constants as a function of frequency (100 - 106 Hz) and capacitance-voltage (C-V) and capacitance-frequency (C-f) measurements were performed. We also studied the relationship between the dielectric constant and the thickness of the layer. Resulting from those experimental data, the effects of properties of new materials were studied on OFET devices. These results demonstrate that implementation of new solution processable polymer dielectrics offers better OFET device performance.


O9.32
Polymer Field-effect Transistors beyond 1 MHz. Veit Wagner, Paul Woebkenberg, Arne Hoppe and Jörg Seekamp; School of Engineering and Science, International University Bremen, Bremen, Germany.

Switching speeds of organic field effect transistors (OFETs) below 1 kHz are usually sufficient for simple tasks. However, many applications require for higher switching speeds or higher current levels. Such improvements could be obtained by using materials with higher mobilities, improved interfaces (gate insulator, contacts) or by smaller channel lengths. In this paper we exploit the last concept of smaller channel lengths. Usually switching speed is defined either by the channel transit time t or by the frequency where the AC current amplification factor drops to one. In both cases the gradual channel approximation predicts a frequency of f ~ μV/(2πL²), with μ the mobility, V the applied voltage and L the channel length of the transistor. A switching speed of 1.6 MHz can be expected if we assume a mobility of 0.02 cm2/(Vs) at a voltage of 5 V for a transistor channel length of L = 1 μm. However, actually measured switching speeds are below this value mainly because of gate-source and drain-source parallel capacitances, contact resistances and not ideal material characteristics. We have manufactured series of transistors with channel lengths down to L = 0.2 μm using an optimised transistor layout to reduce parallel capacitances and to allow for high currents levels up to 10 mA at low voltages. For the transistor structure source and drain gold electrodes are patterned on a 50 nm-SiO2 gate insulator located on a heavily doped Silicon wafer as substrate. The organic semiconducting layer is deposited onto this structure afterwards. As semiconductor the P3HT polymer is deposited wet-chemically from chloroform solution. While our transistor design is suitable to fabricate top gate configurations we will focus in this paper on the bottom gate arrangement, i.e. using the substrate as common gate electrode. With this bottom contact arrangement we demonstrate switching frequencies beyond 1 MHz at RT and in air ambient. Furthermore we found, that optimal frequency response is not observed for the smallest channel lengths but for channel length between 0.5 and 1.0 mm. In this context a model describing the influence of contact resistance on the switching frequency is given.


O9.33
Analysis of Self-Assembly Effects with Organic Molecuels on Performance of Low Voltage OTFTs containing the Self-grown Al2O3 Gate Fielectric Chung Kun Song and Kang Dae Kim; Electronics Eng., Dong-A University, Busan, South Korea.

We have proposed a low voltage OTFTs with self-grown metal oxide (Al2O3) gate dielectric. In this paper we investigated the effects of self-assembly with the various organic molecules on the Al2O3 gate in the low voltage OTFTs. We prepared four kinds of molecules such as (Benzyloxy)alkyltrichlorosilane (BTS) with eleven alkyl chains (BTS-11) as well as twelve alkyl chains (BTS-22), (Benzyloxy)alkyltrimethoxysilane (BSM), and also octadecyltrichlorosilane (OTS). As shown in Table, where NOX stands for the natural SiO2 and MTOX for the metal oxide Al2O3, the OTFTs produced the different performance depending on the self-assembled molecules. BTS-22 show the better insulating property than BTS-11 as expected with the smaller off-state current. However, MTOX/BTS and MTOX/BSM exhibited the worse performance than MTOX without SAM, which is different from our expectation. Interestingly, MTOX/OTS enhanced mobility with 0.45 cm2/V.sec and also sub-threshold slope with 0.18 V/dec but the on/off current ratio was not increased due to the large off-state current. We estimate the dipole potential across OTS layer as the reason of the large off-state current. We will discuss the effects of SAM on Al2O3 and propose a SAM molecule for the self-grown Al2O3 in presentation.


O9.34
Novel Organic Oxide/Al Composite Cathode in Small Molecular Organic Light-emitting Diodes. Tzung-Fang Guo^1,3, Fuh-Shun Yang¹, Zen-Jay Tsai¹, Ten-Chin Wen², Ching-In Wu⁴ and Chia-Tin Chung⁴; ¹Institute of Electro-Optical Science and Engineering, National Cheng Kung University, Tainan, Taiwan; ²Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan; ³Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan, Taiwan; ⁴Chi Mei Optoelectronics Corporation, Tainan, Taiwan.

An ultra-thin, salt-free, neutral, and thermally-deposited organic-oxide polymer layer incorporated with a supplementary organic buffer film and Al electrode is reported as a novel composite cathode structure for the fabrication of high-performance small molecular organic light-emitting diodes (OLEDs). The formation of specific organic oxide/Al complex at the cathode interface, as characterized by the X-ray photoelectron spectroscopy, facilitates the injection of electrons through the Al cathode and prevents the metal-induced quenching sites of luminescence in light-emissive layers. The devices, incorporating the rubrene/poly(ethylene glycol) dimethyl ether (PEGDE)/Al composite cathode, presents the markedly improved performance. The luminous efficiencies of tris-(8-hydroxyquinoline) aluminum (Alq3)-based OLEDs biased at ~ 100 mA/cm2 with the cathodes of rubrene(50 Å)/PEGDE(15 Å)/Al and rubrene(50 Å)/PEGDE(15 Å)/LiF(5 Å)/Al cathode devices are 4.8 cd/A and 5.1 cd/A, and those of devices with Al and LiF(5 Å)/Al cathodes are 1.3 cd/A and 3.8 cd/A, respectively.


O9.35
High-performance N-type Pentacene-based Organic Field-Effect Transistors. Tzung-Fang Guo^1,3, Zen-Jay Tsai¹, Shi-Yu Chen¹ and Ten-Chin Wen²; ¹Institute of Electro-Optical Science and Engineering, National Cheng Kung University, Tainan, Taiwan; ²Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan; ³Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan, Taiwan.

This study addresses the functionalities of the polymer gate dielectric on determining the accumulation and transport of the positive or negative charge carriers in the studies of organic field-effect transistors (OFETs). Incorporating a thick cross-linked poly(4-vinyl phenol) (PVP) layer and another thin hydroxyl-group-rich polymer interfacial layer as a composite polymer gate dielectric, OFETs of the pentacene (been extensively reported as the organic semiconductor for the p-type OFETs) as the active layer performed the n-channel conduction carrying a saturated, apparent pinch-off drain-source current with the high electron mobility (~ 0.15 cm2V-1s-1). Presumably, the hydroxyl groups on the polymer gate dielectric are favorable to the formation of n-channel conduction in the pentacene active layer, which are of the different functionality to those on SiO2 gate dielectric. (It was reported by Chua et. al. that the n-channel conduction of OFETs was in fact interfered by the trapping of electrons at the hydroxyl groups on the surface of SiO2 gate dielectric layer.) The increased capacitance was observed in the quasi-static capacitance-voltage measurement for devices of the metal-insulator-semiconductor configuration biased at the n-type accumulation regime.


O9.36
Abstract Withdrawn


O9.37
Measurement of Local Photocurrents in Polymer:fullerene Solar Cells. Klara Maturova, Martijn Kemerink and Rene A.J. Janssen; Molecular Materials and Nanosystems, Eindhoven University of Technology, Eindhoven, Netherlands.

Operation of bulk heterojunction solar cells is commonly believed to rely on separation of photo-generated charges at the donor-acceptor interface. So far there is however no direct prove that the photocurrent is indeed predominantly generated at the interface between the phase-separated acceptor- and donor-rich phases. Using Scanning Tunneling Spectroscopy (STS) we were able to directly map out the local photocurrent. These experiments show that for the investigated cells the photocurrent is maximal throughout the entire mixed, donor-rich phase, rather than predominantly at the interface. In order to further understand the charge separation and transfer in these solar cells, we combined STM, STS and KP-AFM (Kelvin Probe AFM). Experiments were performed under N2 atmosphere in dark and under illumination. The investigated solar cells consist of an active layer of a semiconducting polymer (MDMO-PPV, donor) blended with a fullerene derivative (PCBM, acceptor) in weight ratio 1:4 that is spin cast from either chlorobenzene solution or toluene on a bottom electrode of PEDOT:PSS and ITO on glass. The length scale of phase separation in the active layers is larger for toluene (~200 nm) than for chlorobenzene cast samples (~15 nm) For both solvents, the PCBM-rich phase in almost pure, whereas the PPV-rich phase contains about 50% PCBM. The current-voltage curves obtained by STM do show a strong effect of illumination, including the occurrence of a so-called built-in voltage, showing that the setup indeed acts as a nm-sized solar cell. The photocurrent map of the toluene sample shows a substantial, homogeneous photocurrent in the entire mixed area, while the PCBM clusters have a far lower photocurrent. Moreover, we found strong indications that the hole mobility is significantly enhanced just around the PCBM clusters. This we tentatively attribute to a PCBM-depletion of the mixed phase by the nearby PCBM-rich cluster. In KP-AFM measurements of the surface potential (SP) of the same samples, we observed three different effects: a dark contrast between the two phases, a reversible shift of SP towards higher values under illumination and a quasi-permanent shift of the dark SP after first illumination. These effects are explained by band bending due to pile up of interfacial, photo-created or trapped charges, respectively. Since KP-AFM is only sensitive to the local presence of charges, but not to the place from which these charges originate, this technique is complementary to STM-based spectroscopy. In particular, the presence of trapped charges, that is found to depend on the degree of oxidation of the sample, is not observed in STM experiments.


O9.38
High-mobility Organic Single-crystal Transistors with Secondary Gates on the Source and Drain Electrodes. Kouji Hara, Yukihiro Tominari and Jun Takeya; Osaka University, Toyonaka, Japan.

To achieve the maximum device performance of organic field-effect transistors (OFETs), a still challenging subject is to achieve efficient carrier injection at the electrodes. The contact performance is the most seriously concerned for short-channel devices with the channel length typically less than sub-micrometers, though they are highly attractive because of their high-frequency response and capability of high-density integration in principle. Since, however, still little is elucidated systematically of microscopic mechanisms of the carrier injection at the organic/metal connection, it is useful to control injection rate at the contacts in the same devices. In this work, we develop a device structure with “split gates” on the source and drain electrodes buried in the gate-insulating layers, so that the carrier density in the organic semiconductors can be varied in the vicinity of the electrodes independently of the primary gate electric fields applied to the central channel in the semiconductors. To fabricate the bottom contact OFETs, we first deposit the split-gate electrodes of gold on 500-nm thick SiO₂ / doped silicon substrates by evaporation. Crosslinked polyvinylphenol (PVP) dielectric is prepared to the thickness of 2-3 μm for the second gate dielectric layer, and source and drain electrodes are formed to the size slightly smaller than the split-gate electrodes, so that the split gate extends to the semiconductor channel in the vicinity of the edges of the source and drain electrodes. Finally, a thin platelet of rubrene crystal, which was independently grown by physical vapor transport technique, is laminated by natural electrostatic force. With the application of gate voltages to the secondary split electrodes, carriers are accumulated locally near the source and drain electrodes to reduce the contact resistances. We measured transfer characteristics, sweeping the primary gate voltage to the doped silicon layer, with and without application of the secondary split-gate volatges. As our usual rubrene single-crystal devices with PVP gate insulators, mobility values of the present samples are estimated typically to be ~ 20 cm²/Vs without the application of the split-gate voltage in case relatively high-density carriers are accumulated in the central channel of the rubrene crystal. With the application of high enough secondary gate voltage at the split gate electrodes, however, the mobility is enhanced up to ~ 30 cm²/Vs. The result indicates that the influence of the carrier injection at the metal electrodes / organic semiconductor interface is relevant even for very high-mobility single-crystal transistors and that the performance of the device can be further improved if higher carrier injection rate is achieved. The observation also demonstrates that the split-gate configuration is useful to separate the influence of the carrier injection barriers from the intrinsic performance of the organic semiconductor materials.


O9.39
Deposition and Patterning of Highly Pure and Robust Poly(3,4-ethylenedioxythiophene) (PEDOT) Thin Film on SiO2 Surface for an OTFT Application. Ilsun Pang, Sungsoo Kim and Jaegab Lee; Kookmin Univ., Seoul, South Korea.

Vapor phase polymerization of 3,4-ethylenedioxythiophene on a SiO2 surface can offer an easy and convenient way to deposit pure and conductive PEDOT thin films. However, the vapor phase deposition method generally shows a very poor adhesion and difficulty of patterning of the PEDOT thin film onto the oxide dielectric substrate. To significantly improve the patternability and adhesion of the PEDOT thin film to the SiO2 surface, the substrate was pre-patterned with n-octadecyltrichlorosilane (OTS) molecules by using μ-contact printing method. And, then the negative patterns were backfilled with four different amino-functionalized silane self-assembled monolayers, e.g.,(3-aminopropyl)trimethoxysilane(APS), N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (EDAS), (3-trimethoxysilylpropyl)diethylenetriamine(DETS), and N-(trimethoxysilylpropyl)ethylenediamine, triacetic acid (EDTAS). Amine group of SAMs was absorbed oxidant FeCl3 using as catalyst of PEDOT film formation. The 3,4-ethylenedioxythiophene(EDOT) monomer can be polymerized only in the presence of oxidant such as FeCl3. Therefore, evaporated EDOT monomer was reacted to FeCl3 on amine group of SAMs patterned regions. Conductivity of PEDOT in thin films be up to 68 S/cm for a film thickness of about 98nm. Quality of the patterned aminosilane monolayers and PEDOT thin films were investigated with contact angle analyzer, ellipsometer, XPS, optical and atomic force microscopes. Finally, the electrical characteristics of the PEDOT thin films were measured and compared with each other to evaluate their applicability to the organic thin film transistor(OTFT) device. This work was supported by the ERC(CMPS, Center for Materials and Processes of Self-Assembly) program of MOST/KOSEF(R11-2005-048-00000-0)


O9.40
Synthesis and Characterization of Blue-Light Emitting Amorphous Dendrimer Composed of Carbazolyl Moieties Tae Wan Lee, Min Ju Cho, Jung-Il Jin and Dong Hoon Choi; Chemistry, Korea University, Seoul, South Korea.

In recent years, organic light-emitting devices (OLEDs) have attracted much attention because of their applications in full-color flat-panel displays. Since the initial work on molecular and polymeric OLEDs, were performed, respectively by Tang et al. and Burroughes et al., there have been extensive studies on layered organic electroluminescent (EL) devices to improve device performance. High EL efficiency and good durability are particularly important for practical applications. Dendrimers are highly ordered, well-defined branched polymers, with segments between branch points (spacers) comprising relatively short chains. Their treelike structure is formed by a stepwise, iterative reaction sequence of the functional groups in each generation. When the repeated process leads to complete shells for each generation, monodisperse giant molecules are prepared with a defined number of end groups, and their size and architecture. The structure can be controlled by a synthetic method precisely. In this work, we demonstrated the properties of new blue light emitting dendrimers containing carbazole moieties. 1st-generation dendrimers were successfully synthesized to investigate their photophysical properties. Organic Light-emitting devices were fabricated in an ITO(indum-tin-oxide)/PEDOT/dendrimer/Li:Al alloy configuration. The properties of carbazole-based dendrimer were evaluated by the photoluminescence (PL), electroluminescence (EL), and current-voltage characteristics, respectively. The EL and PL spectral analysis in the dendrimer devices reveals that the dendrimer formation of isolated carbazole units reduces the interchain molecular excimer emission from the aggregated carbazole moieties.


O9.41
Multi-functional Organic Interlayer in Flexible Organic Thin Film Transistor. Younggug Seol¹, Nae-Eung Lee¹, Sangho Park² and Jin-Young Bae²; ¹Materials Science and Engineering, Sungkyunkwan University, Suwon, South Korea; ²Polymer Science and Engineering, Sungkyunkwan University, Suwon, South Korea.

Flexible organic thin film transistors with an inverted-staggered structure were fabricated using pentacene as a semiconducting layer and electroplated nickel (Ni) as a gate electrode. The channel length varied from 10 to 110 μm, and the channel width was 800 μm. In order to improve adhesion between the semiconducting layer and the gold (Au) as a source/drain (S/D) electrode, we applied a new organic material as an interlayer. The newly synthesized oligo(3-methylsulfanylthiophene) was thermally evaporated as the interlayer on the pentacene layer using a shadow mask. The fabricated OTFT exhibited the best electrical characteristics with the field-effect channel mobility of 0.16 cm²/V-s and the current on/off ratio of 10⁴ using 60-nm interlayer. The cyclic bending tests with the bending radius of 3 mm showed that the field-effect channel mobility, the current on/off ratio, and threshold voltage were not changed significantly even after 150,000 repetitive bending in tension. The interlayer contributed to improvement in adhesion between Au source and drain layer and the pentacene layer. In addition to adhesion improvement, the interlayer reduced the contact resistance between the organic semiconductor layer and Au source and drain layer. Therefore, the interlayer can function as a multi-functional layer for performance improvement of the flexible OTFT devices.


O9.42
HfO2/PVP Gate Dielectric Stack Structure for Pentacene FETs Sang Seol Lee¹, In Sung Park², Young Gug Seol³, Nae Eung Lee³ and Jinho Ahn¹; ¹Department of Materials Science and Engineering, Hanyang University, Seoul, South Korea; ²Information Display Research Institute, Hanyang University, Seoul, South Korea; ³School of Advanced Materials Science & Engineering, Sungkyunkwan University, Suwon, South Korea.

Recently, organic thin film transistors (OTFTs) attract a lot of attractions due to their potential advantages like flexibility and cost competitiveness. Applications based on OTFTs with pentacene semiconductors have already been demonstrated in the fields of memory devices, sensors, and flexible displays. All organic TFTs with polymer dielectrics, however, exhibit unstable threshold voltage, higher leakage current and higher operation voltage compared to the OTFTs with inorganic gate dielectrics. In this study, an inorganic/organic gate dielectric stack structure is introduced in order to overcome these drawbacks of OTFTs with polymer gate dielectrics. Atomic layer deposition (ALD) technique was used to deposit HfO2 inorganic film on spin-coated poly-4-vinylphenol(PVP) laminated dielectric. Pentacene was deposited as a semiconductor channel layer by evaporation technique. OTFT’s with HfO2/PVP stacked dielectrics exhibit improved channel mobility and on/off ratio. And also the operation voltage uniformity, hysteresis, and off-current level could be adjusted with the combination of organic/inorganic dielectric layers. The detailed experimental results and characterization methods will be discussed during the presentation.

O9.43 TRANSFERRED TO O7.5

O9.44
Self-Organized Soluble-Pentacene Crystals for High-Performance Organic Field-Effect Transistors Kilwon Cho, Wi Hyoung Lee, Do Hwan Kim, Yunseok Jang, Dong Yun Lee and Hwa Sung Lee; Chemical Engineering, Pohang University of Science and Technology, Pohang, South Korea.

In this study, we report the fabrication of high-quality one-dimensional (1-D) single-crystalline triisopropylsilylethynyl pentacene (TIPS_PEN) microribbons via a simple solution process with well-defined facets and unprecedented electrical characteristics, such as field-effect mobility as high as 1.4 cm2/Vs and well-resolved gate modulation. We found that the 1-D single-crystalline TIPS_PEN microribbons form spontaneously through the self-assembly of individual TIPS_PEN molecules as a result of solvophobic interactions in the solution phase, adopting preferential well-ordered inter-molecular π-π stacking along the ribbon axis. Further, we report the fabrication of one-dimensional microstructure arrays of TIPS_PEN via simple drop casting on a tilted substrate. Aligned rod shape crystals were formed on a substrate, and the device based on arrays of the fabricated rod shape crystals shows a high field effect mobility of 0.3 cm2/Vs, which results from the directed organization of π-conjugated molecules. Our studies provide a general and rational approach for 1-D assembly of π-conjugated organic materials by easy-to-process yet highly ordered supramolecular system, which is indicative of the good candidate for optoelectronic applications. Acknowledgement. This work was supported by a grant (F0004022-2006-22) from the Information Display R&D Center under the 21st Century Frontier R&D Program, ERC program (R11-2003-006-03005-0) of the MOST/KOSEF, and the BK21 Program of the Ministry of Education and Human Resources Development of Korea.


O9.45
Abstract Withdrawn


O9.46
Organic Field Effect Transistor Using Sol-gel Processed Ferroelectric Gates Sambit Pattnaik, Vijayraj Singh, Ashish Garg and Monica Katiyar; Department of Materials and Metallurgical Engineering, Indian Institute of Technology Kanpur, Kanpur, U.P., India.

Organic field effect transistors using ferroelectric gates are important for the development of low-cost non-volatile memory elements. Here, we report on the fabrication of an organic thin film transistor device comprising of an oxide perovskite structured ferroelectric material (PbTiO₃ and BiFeO₃) as gate dielectric and a organic material (pentacene) as semiconductor. First the oxides were deposited on heavily doped n-type Si (100) substrate using sol-gel technique. The ferroelectric thickness was varied between 10-100 nm. Phase evolution in these films was assessed using X-ray diffraction. C-V and I-V characteristics of the gate oxide were measured to assess its quality for the device operation. Subsequently, pentacene is deposited by thermal evaporation at the rate of 0.03-0.04 nm/sec. The substrate temperature for pentacene deposition was 65°C. The film thicknesses were measured using Tencor surface profilometer. Finally, gold source/drain electrodes were deposited using shadow mask with channel length of about 30 μm and gate width of 1 mm. The devices were characterized in air using Keithley SCS 4200 semiconductor parameter analyzer. We report effect of the ferroelectric thickness on transistor characteristics and memory effect in these devices.


O9.47
Abstract Withdrawn


O9.48
Engineering Properties of Organic Materials for Near Infra-red Applications Jian Li, Evan L. Williams and Ghassan E. Jabbour; School of Materials and Flexible Display Center, ASU, Tempe, Arizona.

Near infra-red diodes are being used in applications such as defense, security, telecommunications and bio-imaging. In this talk, we will present our progress in developing near infra-red organic light emitting devices (OLEDs). A typical OLED is comprised of several functional materials such as a hole-transporting material, an electron-transporting material and an emissive material, sandwiched between two electrodes. Our approach focuses on the use of heavy metal complexes as phosphorescent emitters which can harvest both electrically generated singlet and triplet excitons, leading to possible 100% internal quantum efficiency. This presentation will include discussion on materials development of novel, efficient cyclometalated Ir complexes including iridium(III) bis(1 pyrenyl isoquinolinato N,C’) acetylacetonate (NIR1) and their derivatives. To further increase the device efficiency, it is important to incorporate appropriate charge-transporting and host materials to achieve balanced charge injection and suitable confinement of excitons. In this regard, we will highlight the development of novel electron transporting materials and host materials that were used to achieve efficient near infra-red OLEDs with relatively high power density output.


O9.49
Abstract Withdrawn


O9.50
Engineering Vertical Composition Variations in Spin-Coated Thin Films of Low-Bandgap Polyfluorene Copolymers and PCBM for Solar Cell Applications. Cecilia Bjorstrom¹, Jakub Rysz², Andrzej Bernasik³, Andrzej Budkowski² and Ellen Moons¹; ¹Department of Physics, Karlstad University, Karlstad, Sweden; ²Institute of Physics, Jagiellonian University, Kraków, Poland; ³Faculty of Physics and Applied Computer Science, AGH- University of Science and Technology, Kraków, Poland.

Thin films of conjugated polymers blended with fullerene derivatives are frequently used as the active material in photovoltaic cells, where the blends are spin-coated from solution directly onto the bottom electrode. For such devices, so-called bulk-heterojunction solar cells, the blend film morphology has been shown to have a strong effect on the performance of the solar cell. The film formation during spin-coating is characterised by rapid solvent quenching and phase separation, and results in a non-equilibrium film morphology. There are several parameters that may affect the final film morphology. One important factor is the difference in surface energy between the blend components. This difference may give rise to surface-directed spinodal decomposition if one of the blend components is attracted to one or both of the external surfaces, i.e. the free surface with air or the interface with the substrate. This, in turn, will lead to the initial formation of vertically stratified phases that can either be frozen in or break up by interfacial instabilities and yield a lateral domain structure. Here we present results from morphology studies on spin-coated thin films of low-bandgap alternating polyfluorene copolymers (APFO’s) and fullerene derivative [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM). We show that the compositional depth profile of the films can be determined with a resolution of about 10 nm by means of dynamic secondary ion mass spectrometry (SIMS). The polymer component was identified by its sulphur and the CN signatures and for detection of PCBM we used deuterium labelling of the phenyl-group (d5-PCBM). We analysed blend films and bilayers. The blend films were spincoated from chloroform and chlorobenzene solutions, and we found that the morphology can be controlled from a vertical multilayer structure to a homogeneous distribution, by adjusting the volume ratio of the solvents. Rapid evaporation causes quenching into the two-phase region, which results in vertical variations in composition, expressed as multilayers of polymer-rich and PCBM-rich phases. In this spontaneously formed structure the surface of the film is strongly enriched with polymer, the low surface energy component. Despite this structure with the hole-transporting material near the low-workfunction metal electrode being electronically non-ideal, the solar cells produced from this active layer have shown the highest energy conversion efficiency among all low-bandgap polyfluorene:PCBM blend devices. To understand this better, we have fabricated bilayers with reversed structures as compared to the spontaneously formed multilayers. Their compositional depth profiles determined by SIMS are a clear proof for the bilayer structure. Depending on the preparation parameters, the bilayer structure can be formed with a sharp or more graded composition at the polymer/PCBM interface.

SESSION O10: Commercialization and Applications
Chair: J. Devin MacKenzie
Friday Morning, April 13, 2007
Room 2002 (Moscone West)
8:30 AM *O10.1
Dielectrics for All-additive Jet-printed Organic Electronics. Jurgen Daniel, Ana Claudia Arias, Steve Ready, Brent Krusor, Michael Chabinyc and Robert Street; PARC, Palo Alto, California.

In an all-additive process we use jet-printing and solution-coating methods to fabricate organic electronic circuits. This approach employs the lowest number of processing steps and consumes the least amount of materials. It is therefore promising for inexpensive, potentially disposable electronics. Active-matrix pixel circuits for paper-like displays are a promising initial application for our process since the feature size does not need to be extremely small and the performance requirements for the thin-film-transistors (TFT) pixel switches are not too challenging. However, there are many materials issues to solve with regards to the process integration and the materials performance in an electronic circuit. One of the key components is the gate-dielectric which significantly affects the TFT operation. We have investigated various polymer gate dielectrics for their dielectric properties, their TFT performance and their suitability as surface for subsequent jet-printing of conductors. We use jet-printed nanosilver to define the conductors such as gate and data lines as well as pixel pads. The printing of continuous silver lines depends strongly on the ink and on the properties of the print surface. Therefore, strongly hydrophobic surfaces, which are often desired for improving the molecular ordering of the polymer semiconductor, are not suitable for printing continuous data lines and pixel pads in bottom contact TFTs. With a specific ink and carefully chosen polymer gate dielectrics, printed silver lines around 50-60 microns were achieved. The gate dielectric is also critical for the TFT performance and we have investigated several polymers, such as the epoxy SU-8 and polyvinylphenol (PVP), which were deposited by spin-coating at a layer thickness around 200-500 nm. Capacitance and leakage properties of the materials were measured and the problem of pinholes in thin dielectric layers was investigated. As the organic semiconductor we used jet-printed polythiophene PQT-12. Typical saturation mobilities for our PQT-based TFTs were around 10-2 cm2/Vs. With further optimized conditions, mobilities around 10-1 cm2/Vs can be achieved in TFTs with PQT semiconductor. The best combination of materials was used to fabricate active matrix backplanes on low-temperature flexible substrates with a pixel size varying from 500 to 1000 microns. The pixel performance was measured on various pixel layouts in order to determine the ideal pixel design in terms of fast response time and low feedthrough voltage. We will also discuss approaches for multilayer pixel structures in which a ‘mushroom metal’ layer extends over the data and gate lines to shield their voltage potential. Such designs yield pixels with a high fill-factor and with improved performance for displays and image sensors. In one approach, micromolding of a polymer is used in combination with jet-printing to form an inter-layer dielectric with vias.


9:00 AM O10.2
UHF Rectification for Organic RFID Tags. Soeren Steudel^1,2, Jan Genoe¹ and Paul Heremans^1,2; ¹Polymer & Molecular Electronics, IMEC, Leuven, Belgium; ²Electrical Engineering, KU Leuven, Leuven, Belgium.

One of the main drivers of research on organic thin film transistors (OTFT) are low-cost integrated circuits, in particular for passive RFID tags. In order to be a competitive and viable product, the organic RFID tag needs to be able to work at the commercially used base-carrier frequencies either in the high-frequency band (13.56MHz) or preferable in the ultra high frequency band (433MHz, 869MHz, 915MHz), which is challenging because of the intrinsically low charge carrier mobility in organic semiconductors. The rectifying stage is by far, the most critical part regarding speed, because the rectifying element needs to be able to charge the load-capacitance of the rectifier in only a fraction of the carrier frequency period. The circuitry of the logic operates at a much lower frequency, commonly in the kilohertz range. Only recently the feasibility of an organic RFID tag working at 13.56MHz has been demonstrated. However, the wider reading distance resulting from the use of the UHF band would be desirable, even more so as the traditional Si-based RFID tags shift to this frequency band. We would like to present experimental results demonstrating for the first time rectification based on an organic diode in the UHF band. The resulting DC voltage is sufficient to drive our organic circuits. The utilized diode consists of evaporated pentacene sandwiched between Al/Au electrodes showing a space-charge-limited-current mobility of 0.13cm2/Vs while being able to sustain a current density exceeding 2000A/cm2. Those finding represent an important step in the realization of an organic radio frequency identification tag.


9:15 AM O10.3
Printable Temperature Llogging Device. Payman Tehrani¹, Nathaniel D. Robinson¹, Mats Robertsson², Anna Malmström², David Nilsson^1,2 and Magnus Berggren¹; ¹Dept. of science and technology, Linköping University, Norrköping, Sweden; ²Acreo AB, Norrköping, Sweden.

Here, we present a simple printable device that can monitor and store temperature data over time. Temperature history is recorded through a network of electrochemical sensor devices with a gradual propagation of over-oxidation induced in thin films of poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate) (PEDOT:PSS). The phase transition (in this case solid-to-liquid) in patterned electrolytes is utilized to modulate the electrochemical switching rate. This simple and robust device architecture together with printable organic materials promise for low-cost, roll-to-roll manufactured temperature loggers for monitoring the life cycle of packages during transportation. The quality of sensitive goods, such as food, is to a large extent predicted by the storage conditions (temperature and time) while transported along the logistic chain. A logging device, that record and store temperature over time, printed on individual packages can reduce losses in logistic chains and serve as a dynamic “best-before-date-indicator” to end customers.


9:30 AM O10.4
Flexible, Polymer-based Light Sensor Arrays on Active-matrix Backplanes Fabricated by Digital Inkjet Printing. Tse Nga Ng¹, William S Wong¹, Rene A Lujan¹, Raj B Apte¹, Michael L Chabinyc¹, Scott Limb² and Robert A Street¹; ¹Electronic Materials Lab, Palo Alto Research Center, Palo Alto, California; ²Hardware System Lab, Palo Alto Research Center, Palo Alto, California.

As macro-electronics applications scale towards larger areas, array fabrication will become increasingly complex with conventional methods of device processing, such that photolithography and vacuum deposition will reach a practical limit. Low-temperature integration of inorganic and polymeric materials onto flexible platforms would enable low-cost, large-area image sensors by reducing materials and fabrication costs. We have developed prototypes of flexible light sensor arrays fabricated using polymeric sensor materials on polyethylene naphthalate substrates. The photo-sensitive materials were comprised of layered organic semiconductors, in which a transparent, hole-transporting layer of tetraphenyldiamine is spin-casted on top of a low bandgap polymer that dictates the spectral response of the sensor. The sensor layer has shown quantum efficiency of ~1% at 488 nm and a dark current of 1.1 pA/mm^2. This layer is integrated onto a flexible a-Si:H active matrix backplane fabricated using digital inkjet printing to create a 180x180 pixel array with 75 dpi resolution. Sensor properties such as sensitivity, spectral response, and spatial resolution are determined and compared to those of conventional amorphous silicon photodiodes.


9:45 AM O10.5
High Performance Organic Field-effect Transistors (OFETs) Using High-κ Dielectrics Grown by Atomic Layer Deposition (ALD). Xiaohong Zhang¹, Benoit Domercq¹, Seunghyup Yoo¹, Xudong Wang², Zhong Lin Wang² and Bernard Kippelen¹; ¹School of Electrical and Computer Engineering and Center for Organic Photonics and Electronics (COPE), Georgia Institute of Technology, Atlanta, Georgia; ²School of Materials Sciences and Engineering, Georgia Institute of Technology, Atlanta, Georgia.

Organic field-effect transistors (OFETs) have recently gained a lot of interest as building blocks for a variety of low-cost and large-area electronic applications, such as radio-frequency identification tags (RFIDs), chemical sensors, and flat panel displays (FPDs). One of the key challenges with existing OFETs is to develop high quality, defect-free gate dielectrics compatible with various substrates for low temperature fabrication. Here we will present high performance pentacene OFETs using high-quality Al₂O₃ gate dielectric grown by atomic layer deposition (ALD) at a low temperature of 100 °C. The dielectric constant of Al₂O₃ (7.5 ± 0.2) grown by ALD is almost twice that of SiO₂ (3.9). Hole mobility values of 1.5 ± 0.2 cm²/Vs and 0.9 ± 0.1 cm²/Vs were obtained when using heavily n-doped silicon (n⁺-Si) and ITO-coated glass as gate electrodes, respectively. These transistors operate in enhancement mode with a zero turn-on voltage and exhibit a low threshold voltage (< -10 V), as well as a low sub-threshold swing (< 1 V/decade) and an on/off current ratio larger than 106. We will compare OFETs using ALD grown Al₂O₃ gate dielectric with OFETs using thermally-grown SiO₂ gate dielectric which is a predominant oxide in silicon integrated circuits (ICs).


10:30 AM *O10.6
Printed Organic Transistors for Large-area Sensors and Actuators. Takao Someya¹, Tsuyoshi Sekitani¹, Yoshiaki Noguchi¹, Shintaro Nakano¹, Shinya Takatani¹, Makoto Takamiya² and Takayasu Sakurai²; ¹Quantum-Phase Electronics Center, University of Tokyo, Tokyo, Japan; ²Center for Collaborative Research, University of Tokyo, University of Tokyo, Japan.

In this talk, we report recent progress of large-area, flexible sensors and actuators using printed organic field-effect transistors (FETs). The examples of large-area sensors are artificial electronic skins and sheet image scanners, while large-area actuators include sheet Braille displays. Furthermore, integrating large-area sensors with large-area actuators, we have recently developed wireless power transmission sheet. We first describe 33-cm-diagonal flexible pressure sensors using printed active matrices. Polyimide precursors and silver nano-particles are patterned on a polyimide film by using an inkjet printing system and to form gate dielectric layers and electrodes for organic FETs, respectively. Epoxy partitions are prepared by a screen printing system to define the device dimensions. The mobility of the transistors is 0.7 cm²/Vs. Spatial distributions of pressure are read out by an organic FET active matrix. We also report technical details on a large-area power transmission sheet manufactured by using printing technologies. In particular, we have manufactured printed plastic switches for high power electronics applications and combined them with printed organic transistors. The position of electronic objects on this sheet can be contactlessly sensed by electromagnetic coupling using an organic transistor active matrix. Then, power is selectively fed to the objects by an electromagnetic field using a two-dimensional array of copper coils that are driven by a printed plastic switching matrix. The effective power transmission area is 21 × 21 cm². Due to selective power transmission, we achieved a coupling efficiency of power transmission of 62.3%, and a power of 29.3 W was wirelessly received. The thickness and weight of the entire sheet are 1 mm and 50 g, respectively. This work was supported by Special Coordination Funds for Promoting and technology, JST-CREST, and TOKUTEI (15073204).


11:00 AM O10.7
High Resolution Organic TFTs for Active Matrix Organic Light Emitting Diode (AM-OLED) Hee Jung Kim¹, Sun-young Kim¹, Jeong-Hun Son¹, Jung-Nam An¹, JiHun Choi¹, Jae Won Chang¹, Sungeun Lee¹, Young Hwan Choi¹, Jason Locklin² and Zhenan Bao²; ¹Devices & Materials Lab., LG Electronics, Seoul, South Korea; ²chemical Engineering, Stanford University, Stanford, California.

Organic field effect transistors (OTFTs), using a thin film of small molecules as the semiconductor layer, have gained increasing interest for their low cost, lightweight, flexible, and large area electronics properties. As charge carrier mobility values in OTFTs continue to rise with optimization molecular design, morphological improvements and device architectures, operational stability under ambient conditions still remains a major issue. The poor stability of pentacene TFTs was caused by the instability of pentacene in the presence of oxygen and/or moisture. To improve the stability of OTFT, the thiophene-phenylene cooligomer, JLBO3T, with a relatively high ionization energy was used as an active material. JLBO3T was easily deposited on any substrate by thermal evaporation. Thin films deposited at different deposition conditions were characterized with AFM, FE-SEM, and XRD. We also fabricated TFT devices using this active material and evaluated by HP4145B semiconductor analyzer. The transistors exhibited high on currents of 3.2 μA, low off currents of 12 pA, on/off ratios of 10^5, threshold voltages of +11 V, and field effect mobilities of 0.02 cm^2/Vs. These results implied that JLBO3T was an appropriate semiconducting material to produce high quality TFTs with high stabilities. Therefore, we fabricated OTFT arrays for organic light emiting diode (OLED). Display size of fabricated OLED was 3.5” in diagonal and the pixel resolution was 160 (V)x 120 (H). ITO coated glass was used as a substrate and also used as a gate electrode. A gate insulator was formed by deposition of 200 nm of silicon dioxide (SiO2) by PE-CVD at 350 oC. The deposited SiO2 layer was treated by rapid thermal annealing (RTA) for 120 sec at 600 C. The SiO2 surface was cleaned by UV/O3 for 5 min, followed by surface treatment using octadecyl trichlorosilane (OTS) performed to improve performances of the device. JLBO3T films were deposited on the gate insulator by thermal evaporation at 80 C. And then, source/drain electrode using Au was also deposited by thermal evaporation. After finishing parylene passivation, the OLED layer was deposited by thermal evaporation continuously. Finally, the device was completed by the encapsulation. In this paper, we describe the fabrication, characterization, and performance of OTFTs applied with a thoiphene-phenylene cooligomer as an active material. In addition, we demonstrate a 3.5” QQVGA OLED driven by thiophene-phenylene cooligomer based TFTs.


11:15 AM O10.8
Fabrication of Long-Life Organic Light-Emitting Devices with Graded Composition Using an In-Line Evaporation Method Junji Kido^1,2, Yuji Fujita¹, Nobuhiro Ide² and Ken-ichi Nakayama^1,2; ¹Polymer Science and Engineering, Yamagata University, Yonezawa, Yamagata, Japan; ²Optoelectronic Industry and Technology Development Association, Bunkyo-ku, Tokyo, Japan.

One of the factors that appears to limit the lifetime of a general organic light emitting device with hetero-junction structure is charge buildup at the interface. In this study, the influence of the graded mixed-layer on the performance of organic light-emitting device was investigated. A typical device structure employed in this study was ITO/ polymer buffer layer (MCC-PC1020) / NPD:Alq (graded composition) / Liq / Al, which was fabricated by specially constructed in-line evaporation system. Various patterns of gradual mixing of NPD:Alq were evaluated. Dye-doped OLEDs were also fabricated to improve the efficiency and lifetime. When rubrene was used as a dopant dye, the lifetime of the graded device was improved to 37,000 h which was 1.6 times longer than that of a conventional hetero-junction structure (22,000 h). This improvement in lifetime is attributed, in part, to the suppression of the formation of fluorescence quenching cationic Alq species at the NPD/Alq interface.


11:30 AM O10.9
Advances in Plexcore™ Technology for OLED Displays and Lighting. Darin Laird, Brian Woodworth, Reza Stegamat and Mathew Mathia; Plextronics, Pittsburgh, Pennsylvania.

Plextronics develops active layer technology for printed electronics devices - OLED displays and lighting, polymer solar cells and plastic circuitry. Active layer technology is the printed semiconductors and conductors that drive device performance. Plextronics has unsurpassed control of polymer design and ink formulation, as well as an intimate understanding of device physics and its impact on device performance. This knowledge is applied to the creation of Plexcore™ technology. Plexcore technology is designed to maximize the efficiency, lifetime and stability of printed electronic devices. The tunable nature of Plexcore allows many "knobs to be turned" in both the polymer design and ink formulation stages, resulting in a stable, non-acidic ink that is optimized for a specific application or device. Plexcore inks can be printed into thin films using various printing techniques on flexible substrates. This talk will emphasize advances to our Plexcore™ HIL technology - a highly tunable hole injection layer technology which is designed to dramatically improve device efficiency and lifetime of flat panel displays and solid state white-lighting. The hole injection layer (HIL) in an OLED functions as a gatekeeper to balance the flow of electricity into the OLED which then generates the light. The HIL also smoothes the rough electrode that would otherwise cause rapid device failure. We will present data on lifetime and efficiency performance for Plexcore HIL in commercial emitters. In addition, we will discuss recent efforts to extend the technology for use with small molecule emitters. Initial results show improvements on industry standard HIL technologies.


11:45 AM O10.10
Low-operating Voltage Pentacene FETs with High Dielectric Constant Polymeric Gate Dielectrics and its Hysteresis Behavior. Se Hyun Kim, Sang Yoon Yang and Chan Eon Park; Polymer Research Institute, Pohang university of science and technology, Pohang, South Korea.

Organic field-effect transistors (OFETs) have received considerable interest in recent years, and intensive studies have made it possible to achieve device performances comparable to those of amorphous silicon transistors. However, OFETs often suffer from high operating voltage due to the low charge carrier mobility of organic semiconductors. Hence, for the applications that require high field-induced current at low voltage, OFETs are still not the suitable candidate. Since the field-induced current density is proportional to the capacitance of gate dielectric, the way to increase field induced current density is to use high dielectric constant materials as a gate dielectric or to reduce the thickness of gate dielectric. Therefore, we use amorphous polymer with high dielectric constant (k~12) as a gate dielectric. Since our polymer dielectric itself exhibit low dielectric strength and poor solvent resistance, we cross-linked the polymer gate dielectric by using a curing agent. This cross-linked dielectric exhibited good insulating properties (leakage current density~10-8Acm-2), smooth surface (rms roughness ~3A) and excellent solvent resistance. Using this dielectric, we prepared pentacene FETs which showed high mobility (~3cm2/Vs) at low operating voltage (-4V) and small subthreshold slope (120mV/decade). However, considerable hysteresis was observed in our devices during ID-VG characterization. Hysteresis behavior in our case may be due to slow polarization because artificially large currents appeared when devices were scanned with moderate speed. We believe that water molecules or ion impurities in the dielectric layer are responsible for slow polarization which can lead to the hysteresis measured during ID-VG characterization. To remove this effect, we reduced the thickness of dielectric layer and modified the curing condition. Hence, we can clarify the origin of the hysteresis and significantly reduce the hysteresis in pentacene FETs with high-k polymer gate dielectric.


SESSION O11: Novel Concepts and Devices
Chair: Nir Tessler
Friday Afternoon, April 13, 2007
Room 2002 (Moscone West)
1:30 PM *O11.1
Expanding The Frontiers Of Organic Electronics Using Sequence Independent Synthesis Tools. Yoav Eichen^1,2,3, Elena Gershman¹, Batya Blumer-Ganon¹, Michal Adler¹, Nir Tessler^2,3, Vlad Medvedev², Olga Solomeshch² and Alexey Razin²; ¹Schulich Faculty of Chemistry, Technion - Israel Institute of Technology, Technion City, Israel; ²Nanoelectronics Center, Technion - Israel Institute of Technology, Technion City, Israel; ³Peptronics Ltd., Ramat Gan, Israel.

Despite the long standing effort to develop effective and robust organic electronics, still progress in this field is impeded by the limited choice of materials. The inherent versatility of organic chemistry and the potential application of π-conjugated organic materials in structuring molecular electronic components focuses enormous efforts towards the preparation and characterization of new organic based electronic components. Since the field is largely material driven, a major obstacle in organic-based optoelectronics is the difficulty to reach the wide set of material properties required by current device technologies and in synthesizing complicated structures with molecular precision. Using conventional synthetic routes, one is limited by the fact that almost each synthetic step has different and specific conditions and usually produces a variety of byproducts that must be removed before any subsequent synthetic step is performed. As a result, only a small fraction of the practically limitless choice of organic materials is currently available to the organic semiconductor community. When versatility and reliability is in question, nature adopts a sequence independent synthesis, producing large numbers of materials that differ in structure and properties out of a very limited number of small building blocks (amino acids, nucleic acids). Due to the importance of these two reactions to molecular biology, two most powerful in-vitro syntheses were developed for the in-vitro solid-phase preparation of peptide and nucleic acid sequences. The talk will report on the harnessing of these in-vitro solid phase syntheses for the preparation of (almost) sequence independent materials for organic electronics based on bio-like and bio-based processes.


2:00 PM O11.2
Dry-stamping to Produce High-performance Polymer Thin-film Transistors. Akihiro Nomoto, Nobuhide Yoneya, Noriyuki Kawashima, Kazumasa Nomoto and Jiro Kasahara; Materials Laboratories, Sony Corporation, 4-16-1 Okata, Atsugi-shi, Kanagawa, Japan.

Deposition techniques such as drop casting, spin coating, and ink-jet printing are used to form polymer semiconductor (PSC) film for organic thin film transistor (OTFT). However, these techniques have two major problems, (1) most PSCs can be dissolved only in strong solvents such as toluene and chloroform, which can cause damage to the exposed gate insulator layer, (2) orientation and crystallinity of PSC films strongly depend on the surface energy of the exposed layer, which is restricted by the choice of a gate insulator with low leakage and good processability. In our dry-stamping technique, a PCS film is formed on a stamp after the PCS solution is applied, and then the film is stamped on the gate insulator layer. This method dose not expose the gate insulator layer to solvents and has a high degree of freedom for a choice of a plate to form the highly-oriented uniform PSC film. We have fabricated a poly(3-hexylthiophene) (P3HT) TFT using dry stamping with P3HT and an elastomeric stamp made with poly dimethylsiloxane (PDMS). The TFT device has a coplanar electrode structure with 200-500 nm thick "cross-linked" poly(vinyl-phenol) (PVP) as a gate insulator on a p⁺-doped Si wafer as a gate electrode. Source/drain electrodes were made of a multi-layer of 50 nm Au and 1 nm Cr. The TFT shows higher carrier mobility of 0.03 cm²/Vs than 1 x10^-3 cm²/Vs of TFTs with a P3HT film formed by conventional spin-coating. To investigate the origin of this improvement, we measured the UV absorption spectra of P3HT films formed by dry-stamping and spin-coating. The spectrum of the dry-stamped film showed a stronger zero-phonon absorption peak at around 610 nm than that of the spin coating film, indicating that the P3HT film transferred from a PDMS stamp has more highly-ordered lamella structure than the P3HT film spin-coated on a PVP film. The water-contact-angle measurement shows that PDMS has lower surface energy than PVP. This is consistent with the UV measurements as regards the extent of crystal growth. Therefore we consider that solvent-free dry stamping of a highly-oriented PCS film formed on a plate will be a key technique to make high-performance OTFTs. OTFT with dry-stamped P3HT on a PVP gate insulator hydrophobized with octadecyl-trichlorosilane (OTS) was also investigated. The OTFT shows higher mobility of 0.04 cm²/Vs and lower interface-trap density of 5.2x10¹² cm^-2, estimated from the subthreshold swing, compared with the sole PVP insulator. Similar results observed for spin-coated TFTs is usually considered to be caused by the enhancement of the crystallization. In the dry-stamping technique, surface hydrophobicity is unlikely to affect the P3HT crystallinity, because the crystallization is done on a PDMS stamp. Therefore this result suggests that hydrophobic surfaces may have a relatively lower carrier-trap density and so allow the carrier mobility to be enhanced.


2:15 PM O11.3
Tough, Semiconducting Polyethylene-Poly(3-hexylthiophene) Diblock Copolymers Christian Muller¹, Shalom Goffri², Jens W Andreasen³, Rene A J Janssen⁶, Henri D Chanzy^1,5, Dag W Breiby^3,4, Martin M Nielsen³, Christopher P Radano^6,7, Henning Sirringhaus², Paul Smith¹ and Natalie Stingelin-Stutzmann^1,8; ¹Department of Materials, ETH Zurich, Zurich, Switzerland; ²Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom; ³Danish Polymer Centre, Riso National Laboratory, Roskilde, Denmark; ⁴Centre for Molecular Movies, University of Copenhagen, Copenhagen, Denmark; ⁵CERMAV, CNRS, Grenoble, France; ⁶Laboratory of Macromolecular and Organic Chemistry, TU Eindhoven, Eindhoven, Netherlands; ⁷Degussa RohMax, Horsham, Pennsylvania; ⁸Department of Materials, Queen Mary, London, United Kingdom.

Semiconducting diblock copolymers of polyethylene and regioregular poly(3-hexylthiophene) are demonstrated to exhibit a rich phase behaviour, judicious use of which permitted us to fabricate field-effect transistors that show saturated charge carrier mobilities as high as 2.10e-2 cm2/Vs and ON-OFF ratios ~10e5 at contents of the insulating polyethylene moiety as high as 90 wt%. In addition, the diblock copolymers display outstanding flexibility and toughness with elongations at break exceeding 600 % and true tensile strengths around 70 MPa, opening the path towards robust and truly flexible electronic components. [C. P. Radano et al., Crystalline-Crystalline Block Copolymers of Regioregular Poly(3-hexylthiophene) and Polyethylene by Ring-Opening Metathesis Polymerization, J. Am. Chem. Soc.127, 12502 (2005).]


2:30 PM O11.4
Organic Transistors of Molecular Integrated Semiconductor-insulator Monolayer Films Using a Room Temperature Self-assembly Process. Kanan Puntambekar¹, Kinyip Phoa¹, Vivek Subramanian¹, Amanda Murphy², Clayton Mauldin², Jean M. J. Frechet², Dean M. DeLongchamp⁴, Daniel Fischer⁴ and Michael F. Toney³; ¹Electrical Engineering and Computer Science, Univ. of California, Berkeley, Berkeley, California; ²Department of Chemistry, Univ. of California, Berkeley, Berkeley, California; ³Radiation Laboratory, Stanford Linear Accelerator Center, Menlo Park, California; ⁴Polymer Division of the Materials Science and Engineering Department, National Institute of Standards and Technology, Gaithersburg, Maryland.

We explore the potential of molecular level functionalization as an approach toward integrated nanoscale molecular / organic devices, emphasizing a solution-based self assembly of active components. We have previously shown that solution-processed self-assembled thiophene-based organic semiconductors can be printed as ultra-thin films (≈ 1.5 nm thick) resulting in excellent electrostatics (swing, on-off, etc.) with virtually no degradation in on-state behavior.¹ Currently, we are scaling the device further by covalently linking the dielectric and semiconductor to create a combined molecule that spontaneously assembles into a heterostructure with dual functionality. The molecule is an asymmetric, end-functionalized quarterthiophene, where a C11 alkane chain is added as the dielectric element. By changing the bonding group moiety, these molecules can be self-assembled on different substrates to form smooth films, as observed by atomic force microscopy (AFM). Structural characterization of the self-assembled films on both silicon and gold substrates revealed varying degrees of crystallinity in the film. Grazing incidence X-ray diffraction measurements (GIXD) show distinct peaks in-plane confirming crystallinity in the self-assembled films. Further, both GIXD and near angle X-ray absorption spectroscopy (NEXAFS) studies of these monolayer films shows that the overall film alignment is predominantly with the molecular long axes perpendicular to the substrate, consistent with aligned parallel semiconductor and insulator lamellae in the monolayer (For NEXAFS measurements, dichroic ratio R ≈ 0.41 to 0.44). GIXD suggests a herring bone packing scheme similar to oligothiophene systems, indicating that the quarterthiophene semiconducting element of the molecule dictates the intermolecular packing. Variations in the degree of structural order as a function of the SAM deposition process as well as roughness and annealing temperature of the substrates were also investigated. For example, on gold gate substrates, we observe a distinct improvement in film crystallinity by changing the SAM deposition procedure as well as reducing the substrate roughness. Because both the semiconductor and dielectric are of molecular dimensions, (≈1 to 2 nm), novel device architectures are required to probe their electrical behavior. Preliminary measurements show transistor behavior in these devices. We are currently working on improving the performance and reproducibility of these devices based on the structural characterization data. 1. P. C. Chang, S. E. Molesa, A. R. Murphy, J. M. J. Fréchet, V. Subramanian, IEEE Device Research Conference, 1, 183-184, 2004.


3:15 PM O11.5
The SensorOLED Device: A new Integrated Organic Light Emitting Device for Direct Gas Detection. Stefan Sax¹, Evelin Fisslthaler¹, Sabrina Eder^1,4, Stefan Kappaun², Christian Slugovc², Josemon Jacob³, Klaus Müllen³ and Emil J.W. List^1,4,5; ¹Institute of Solid State Physics, Graz University of Technology, Graz, Austria; ²Institute for Chemistry and Technology of Organic Materials, Graz University of Technology, Graz, Austria; ³Max-Planck-Institute for Polymer Research, Mainz, Germany; ⁴Christian Doppler Laboratory Advanced Functional Materials, Institute of Solid State Physics, Graz University of Technology, Graz, Austria and Institute of Nanostructured Materials and Photonics, JOANNEUM RESEARCH, Weiz, Austria; ⁵NanoTecCenter Forschungsgesellschaft mbH, Weiz, Austria.

A decade after the effect was discovered organic materials have made it into commercial use. Conjugated organic oligomers and polymers are of considerable industrial importance as active materials in the emerging new technologies of organic light-emitting diodes (OLEDs), as well as of academic interest in the field of polymer lasers, photovoltaic devices (solar cells), and field-effect transistors. During the last years this class of materials was also frequently used for several sensor applications. Conjugated organic oligomers and polymers are easy tunable for different sensor applications and analytes. However, all of these sensor devices consist of a sensor film which is excited by an external light source (lasers and several inorganic and organic light sources have been used in the past), therefore belonging to the class of so called “passive” sensor devices. Compared to such a classical fluorescent sensor system, which comprises of a light source with a photoluminescence sensing layer and a detector system, an active organic sensor device has the enormous advantage of combining the light source with the sensing element within one layer, which reduces the fabrication effort and simplifies the whole sensor geometry- the sensor is incorporated in the organic layer, instead of requiring a bipartite structure. Especially when using phosphorescent sensing materials like metal complexes, which successfully have been employed in the past in several sensor applications, the use of electroluminescence effects instead of photoluminescence enhances the overall electroluminescence quantum yield of a SensLED. Those types of materials are also known for acting as high yield luminescence dyes in organic light emitting devices, by weakening the spin orbit coupling and using radiant triplet recombination for enhanced electroluminescence. In the case of oxygen detection, metal complexes like PtOEP or Ir-complexes have been used by taking advantage of quenching effects caused by the interaction of the heavy metal atom with the oxygen molecules. In particular we were able to build a reversible oxygen sensitive light emitting device by implementing the sensor functionality into the active layer of the light emitting device (SensLED). A SensLED consists of an ITO anode on a glass or flexible substrate, a PEDOT:PSS layer, the electroluminescent sensor layer (in our case comprising of PtOEP in poly(9-vinylcarbazole) matrix), and a top electrode that is structured for enhanced gas diffusion.


3:30 PM O11.6
Monolayer Transistor using a Highly Ordered Conjugated Polymer as Channel J. Campbell Scott¹, J. D. Jeyaprakash Samuel¹, Jennifer H. Hou², Chantel C. Tester³, Clara J. Cho¹, Charles T. Rettner¹, Sally A. Swanson¹ and Robert D. Miller¹; ¹IBM Almaden Research Center, San Jose, California; ²Cambridge Univ., Cambridge, United Kingdom; ³Mount Holyoke Coll., South Hadley, Massachusetts.

Many amphiphilic diacetylene monomers are known to form two-dimensional crystals at the air-water interface. By topotactic polymerization, using UV irradiation either before or after transfer to a solid substrate, a highly ordered polymer monolayer is obtained, consisting essentially of a single sheet of parallel conjugated chains. Domain sizes exceed 100 μm. We have fabricated field-effect transistor structures using such polymer monolayers as the channel. The polymer formed from pentacosa-10,12-diynoic ethanolamide exhibits modulation of source-drain current on application of a gating voltage. Comparison of the two-dimensional crystal morphology of this material with several closely related derivatives which show no gating, suggests that a high degree of alignment and order in the polymer chains is necessary for the observed transistor action.


3:45 PM O11.7
Tuning the Contact Resistance in Nanoscale Oligothiophene field-effect Transistors. Arne Hoppe¹, Joerg Seekamp¹, Torsten Balster¹, Guenther Goetz², Peter Baeuerle² and Veit Wagner¹; ¹School of Engineering and Science, International University Bremen, Bremen, Germany; ²Department Organic Chemistry II, Ulm University, Ulm, Germany.

Nanoscale organic field-effect transistors (OFETs) for high frequency applications are often limited by source/drain contact resistances. We report on a study of those contact resistances in nanoscale organic field-effect transistors by a systematic variation of the bandgap of the organic semiconductor. We use high mobility alkyl-substituted oligothiophenes (Dihexyl-n-thiophenes, DHnT) with n, the number of thiophene rings, ranging from four to seven. The bandgap variation also shifts the HOMO-level by 0.5 eV, which is expected to modify contact properties. To produce OFET devices the material was sublimed in ultra-high vacuum at substrate temperatures between 90 °C and 190 °C. Transistors with gold electrodes in bottom configuration on SiO2 and channel lengths ranging from 50 nm to 400 nm as well as reference structures with 40 µm channel length were investigated. Generally we find increasing mobility with the number of thiophene rings n and at the same time decreasing contact resistances. Best values determined for DH7T in air are µ = 0.12 cm2/Vs and Rc = 1 kWcm. We find for all materials a systematically decreasing mobility with decreasing channel length while the contact resistance remains constant with channel length. Thickness dependent in-situ electrical measurements on those nano OFETs are in agreement with the fact, that one monolayer is sufficient for electronic transport. We see monolayer-oscillations not only in the mobility, but also on the contact resistance. In contrast to the mobility, the contact-resistance shows minimal values with any closed Monolayer. Due to this fact, steps towards high frequency OFETs are demonstrated.


4:00 PM O11.8
An Organic Photodetector on a Scanning Probe Cantilever. Kwang Hyup An¹, Brendan O'Connor¹, Kevin P. Pipe¹ and Max Shtein²; ¹Mechanical Engineering, University of Michigan, Ann Arbor, Michigan; ²Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan.

A wide variety of organic-based optoelectronic devices have been demonstrated to date, including OLEDs, photovoltaic cells and photodetectors, transistors, biological sensors, and others. The ability to deposit molecularly smooth organic thin films on a variety of substrate materials and geometries without concern for lattice matching has allowed novel device form factors and fabrication approaches to be applied. Taking advantage of these properties of organic materials and deposition techniques, we demonstrate an organic photodetector integrated with a commercial AFM cantilever probe. The device consists of an electron donor-acceptor heterojunction sandwiched between two metal contacts. A photoactive region of sub-micron lateral dimensions is defined on the cantilever using selective focused ion beam milling of a pre-deposited insulator layer (parylene). Light enters the active region through a thin, semitransparent, vacuum-deposited silver electrode. Electrical power generation on the order of a nanowatt is observed under 470nm and 19 W/cm2 illumination of the probe, with a dark current of 24 nA at 0.4 V reverse bias and a signal-to-noise ratio of 15. High resolution spatial imaging can be achieved by scanning the probe across a feature while measuring light transmission through a broadly illuminated sample. Contactless high resolution 3-dimensional imaging of a light source may be possible via optical interference effects specific to ultra-thin organic thin-film heterostructure photovoltaic devices. Potential applications of this probe may be found in micropositoning, biological imaging and sensing, and non-destructive semiconductor and optoelectronic device testing.


4:15 PM O11.9
Single-layer Organic Non-volatile Memory Devices. Whitney Gaynor¹ and Peter Peumans²; ¹Materials Science and Engineering, Stanford University, Stanford, California; ²Electrical Engineering, Stanford University, Stanford, California.

Low-cost, thin-film devices that exhibit bistable behavior are of interest for non-volatile memory applications. We have developed very simple, air-stable, organic thin-film bistable devices by sequentially evaporating a thin (15nm) silver electrode, a 45nm layer of Ruthenium(III) acetylacetonate, and then a second thin (15nm) silver electrode. Such devices reproducibly show bistable behavior and have a conductivity that depends on the sweep direction. They can be programmed into a low resistance-state by pulsing with a high voltage (30V) and into a high resistance-state by pulsing at a low voltage (0.6V). At a readout voltage of 15V, the current density is 2.5mA/cm² in the low resistance-state and the on/off ratio is as high as 1.6 x 10⁵. The switching time between high and low current states was found to be <100ns, limited by the instrumentation. The retention time for the high current state exceeds 24 hours in air without packaging. These devices are unique in that they employ a single active layer and extremely thin metal electrodes, making the devices transparent. While the operating voltages are currently higher than that of other organic thin-film non-volatile memory devices that have been reported, their current densities and on/off ratios are comparable while their fabrication is potentially simpler. With optimization and packaging, it might be possible to lower the operating voltage and increase the on-state retention time. We will also present structural characterization results and the relation to device function.


4:30 PM O11.10
Role of Charge Interaction in the Behavior of Organic Thin Film Transistors. Christoph Erlen¹, Francesca Brunetti², Matthias Fiebig³, Bert Nickel³, Aldo Di Carlo² and Paolo Lugli¹; ¹Institute for Nanoelectronics, Technische Universität München, München, Germany; ²University of Rome "Tor Vergata", Rome, Italy; ³Department für Physik, Ludwig-Maximilians-Universität, München, Germany.

Pronounced fluctuations in the IV characteristics are a particularity of organic thin films transistors (OTFTs). Being a clear fingerprint of time dependent processes, they are most frequently encountered in form of transfer and output curve hysteresis. Up to now, there is no self-contained theory to describe and predict these effects, since research has mainly targeted the description of IV characteristics in terms of static models [1]. The associated inaccuracy in the modeling of device behavior adds to the difficulties encountered in the realization of organic circuits. In this contribution, we show that both mobile ions and traps effectively contribute to the hysteresis. The fabrication process and employed materials define to which extend they are present in a particular device. In order to demonstrate our model, we have built and simulated two different types of pentacene OTFTs, in either of which one of the effects is predominant. By varying measurement speed and temperature, characteristic signatures of ions and traps are identified. The ion-dominated OTFT is all-organic and realized using PV-OH as gate insulator. The fabrication technique focuses on the possibility to obtain micrometer resolution in the definition of PEDOT source/drain contacts. The process is based on a selective electrochemical growth of the conductive polymer on a prepatterned anode [2]. 20 nm of pentacene are deposited by thermal evaporation, and the PV-OH insulating layer has a thickness of 800 nm. The trap-dominated transistor is built on n-doped silicon substrate, which also serves as back gate. The insulating dielectric is a 200 nm thick layer of thermal SiO2. Gold source and drain contacts are defined using optical lithography. Oxygen plasma is applied to reduce surface contamination before a 48 nm layer of highly purified pentacene is evaporated. It has been demonstrated that finite element drift-diffusion simulations are suitable to enhance the understanding of device behavior allowing us to extract valuable TFT parameters such as traps and fixed charges at the semiconductor/insulator interface [3][4]. To analyze the hysteresis behavior, we have modified the commercial software tool SENTAURUS(TM) to account for the special nature of organic devices and implemented transient simulations of both devices. As a result, we can show that trapping and mobile ions indeed model the experimental IV characteristics. Furthermore, we are able to quantify trap density, energy, and lifetime in the silicon substrate OTFT as well as ion mobility and concentration in the all-organic device. [1] G. Horowitz, J. Mater. Res., vol. 19, p.1946, 2004. [2] E. Becker et al., Appl. Phys. Lett., vol. 83, p.4044, 2003. [3] A. Bolognesi et al., IEEE Trans. Electr. Dev., vol. 51, p.1997, 2004. [4] Erlen et al., J.Comp. Elec., in press, 2006.


4:45 PM O11.11
Low-Voltage Organic Field-Effect Transistors Gated via Polyanionic Electrolytes. Lars Herlogsson, Xavier Crispin, Elias Said, Nathaniel D. Robinson and Magnus Berggren; Department of Science and Technology, Linköping University, Norrköping, Sweden.

Organic field-effect transistors (OFETs) and other “plastic” electronic devices are currently scrutinized for use in printed, flexible, integrated electronics. Ideally, these systems are fast, operate at low voltage, and are robust enough to be manufactured using standard printing techniques. Tremendous efforts have been devoted to reach high capacitance (per area) between the gate and the channel to allow transistors to operate at low voltage. Since the dielectric constant of organic materials usually is quite low, very thin gate insulator layers are required in order to obtain a high capacitance. Molecular assembly and self-organization techniques have been utilized to manufacture gate dielectric layers only a few nanometres thick, resulting in large capacitance. These layers are usually quite complicated to produce and are therefore not very compatible with printing techniques. Another approach to reduce the operating voltage is to use an electrolyte as gate insulator. A small voltage applied to the gate electrode will redistribute the mobile ions in the electrolyte towards the electrodes, resulting in a high electric field at the interfaces. As a result, the drive voltage is essentially independent of the electrolyte thickness. However, the electrolyte-gated transistors reported so far have slow on-off switching, typically in the order of a second, which in most cases is due to electrochemical doping of the channel. We demonstrate a new generation of OFETs that are gated via polyanionic electrolytes. The novelty of this electrolyte-gated OFET resides in the mechanism to open the channel via the quick formation of highly capacitive (in order of 10 to 100 μF cm^-2) electric double layers at the semiconductor-electrolyte and electrolyte-gate interfaces, as the OFET is gated. By having only immobile anions in the electrolyte electrochemical doping of the semiconductor bulk is avoided. This results in fast-responding (< 0.3 ms), low-voltage (< 1 V) transistors. Employing a polyelectrolyte as gate insulator also eases design and manufacturing requirements since thicker polyelectrolyte layers are not detrimental for the performance of the OFETs. This extraordinary property of the insulator is due to the spontaneous formation of a nanometer-scale “capacitor” at the semiconductor-insulator interface as the gate is biased. The resulting flexibility together with recent progress in printing source-drain gate electrodes makes printing a promising manufacturing platform for such transistors in low-cost electronics. References L. Herlogsson et al., Adv. Mater. In Press. E. Said et al., Appl. Phys. Lett. 89, 143507 (2006). Acknowledgments The authors gratefully acknowledge The Swedish Foundation for Strategic Research (COE@COIN), VINNOVA, The Royal Swedish Academy of Sciences, and The Swedish Research Council for financial support of this project.