Abstracts - Symposium U: Charge Generation/Transport in Organic Semiconductor Materials

SYMPOSIUM U


Charge Generation/Transport in Organic Semiconductor Materials

November 27 - December 1, 2011

Chairs
Alejandro L. Briseno
Dept. of Polymer Science and Engineering
University of Massachusetts
Amherst, MA 01003
413-577-1213

 
        Vitaly Podzorov
Dept. of Physics
Rutgers University
P. O. Box 849
Piscataway, NJ 08854
732-445-5500 x-2528
 
Iian McCulloch
Dept. of Chemistry
Imperial College London
Exhibition Rd.
London, SW7 2AZ United Kingdom
44-20-7594-5669
 
        Jun Takeya
Institute of Science and Industrial Research
Osaka University
8-1 Mihogaoka
Osaka, 560-0047 Japan
81-66-879-8400
 
Antonio Facchetti
Polyera Corporation
8045 Lamon Ave.
Skokie, IL 60077
847-677-7570 x-101
 
         


Proceedings to be published in electronic-only format (see MRS Online Proceedings Library at www.mrs.org/opl) as Volume 1402E of the Materials Research Society Symposium Proceedings Series.



* Invited paper

 

TUTORIAL




U: Organic Semiconductor Crystals 101
Sunday, November 27
9:00 am-5:00 pm
Hynes Convention Center


The performance of organic semiconductors has increased dramatically over the last decade, partly due to the development of new materials, but more significantly due to the ability to form high-quality crystals and crystalline films of these materials. Organic crystals are promising materials for a multitude of applications, including fundamental transport measurements, structure-property studies, and as the active semiconductor elements for organic electronic devices. The tutorial will cover several important areas including crystallization techniques from both solution phase and vapor phase, crystal growth from small-molecule and polymer semiconductors, design and synthesis of new organic semiconductor materials, crystal engineering approaches to high-performance organic semiconductors, fabrication of various types of single-crystal transistors, optical properties of organic crystals, energy transport (exciton dynamics), photoconductivity, and Hall effect measurements. A broad survey of crystal packing motifs will be presented, demonstrating how subtle changes in crystal packing can yield dramatic changes in crystal shape (and thus, film morphology) and transport properties.

Instructors:
Alejandro L. Briseno
, University of Massachusetts-Amherst
John E. Anthony, University of Kentucky
Vitaly Podzorov, Rutgers University
Jun Takeya, Osaka University, Japan



SESSION U1: Thin Film Transistors
Chairs: Alex Briseno and Jun Takeya
Monday Morning, November 28, 2011
Room 311 (Hynes)

8:00 AM U1.1
Dithienobenzothienothiophenes (DTBTTs), Novel Highly-Fused Heterocycles for Organic Thin Film Transistors. Jeong-il Park, Bang-Lin Lee, Jong Won Chung, Joo Young Kim, Jiyoul Lee, Ji-young Jung, Hyuk Kim, Bon Won Ku, Yongwan Jin and Sangyoon Lee; Display Device Lab., SAIT, Yongil-si, Gyeonggi-do, Korea, Republic of.

Highly π-extended heteroacenes are attracting current attention for their air-stability over oligoacenes as organic semiconductors in the electronic devices. Recently, Yamamoto and Takimiya reported that Cn-DNTT showed the highest hole mobility of 10 cm2/Vs which was fabricated by hot-solution process. And Zschieschang demonstrated that DNTT TFTs with small channel width/length ratio were able to drive blue organic LEDs to a brightness well above that required for active-matrix displays. Here, we introduce a novel air-stable fused-ring structure, dithienobenzothienothiophenes (DTBTTs), which differ from DNTT by replacing the terminal benzenes of DNTT with two thiophenes instead. We synthesized it by a facile four-step procedure starting from thieno[3,2-b]thiophene and ring-closing with amberlyst 15, where the terminal thiophene’s orientation (syn or anti) can be controllable. Though the two isomers look similar, but their spectroscopic and molecular orbital configurations are quite different. We fabricated thin-film transistor using DTBTTs as active layers by a vapor deposition method and their hole mobilities are compatible to DNTT. The experimental details and comparison with DNTT will be presented.


8:15 AM U1.2
A Hydrogen-Bonded Quaterthiophene and Its Use in Organic-Field-Effect-Transistors.

The performance of organic semiconductors has increased dramatically over the last decade, partly due to the development of new materials, but more significantly due to the ability to form high-quality crystals and crystalline films of these materials. Organic crystals are promising materials for a multitude of applications, including fundamental transport measurements, structure-property studies, and as the active semiconductor elements for organic electronic devices. The tutorial will cover several important areas including crystallization techniques from both solution phase and vapor phase, crystal growth from small-molecule and polymer semiconductors, design and synthesis of new organic semiconductor materials, crystal engineering approaches to high-performance organic semiconductors, fabrication of various types of single-crystal transistors, optical properties of organic crystals, energy transport (exciton dynamics), photoconductivity, and Hall effect measurements. A broad survey of crystal packing motifs will be presented, demonstrating how subtle changes in crystal packing can yield dramatic changes in crystal shape (and thus, film morphology) and transport properties. , University of Massachusetts-Amherst, University of Kentucky, Rutgers University, Osaka University, Japan, Bang-Lin Lee, Jong Won Chung, Joo Young Kim, Jiyoul Lee, Ji-young Jung, Hyuk Kim, Bon Won Ku, Yongwan Jin and Sangyoon Lee; Display Device Lab., SAIT, Yongil-si, Gyeonggi-do, Korea, Republic of.Highly π-extended heteroacenes are attracting current attention for their air-stability over oligoacenes as organic semiconductors in the electronic devices. Recently, Yamamoto and Takimiya reported that Cn-DNTT showed the highest hole mobility of 10 cm2/Vs which was fabricated by hot-solution process. And Zschieschang demonstrated that DNTT TFTs with small channel width/length ratio were able to drive blue organic LEDs to a brightness well above that required for active-matrix displays. Here, we introduce a novel air-stable fused-ring structure, dithienobenzothienothiophenes (DTBTTs), which differ from DNTT by replacing the terminal benzenes of DNTT with two thiophenes instead. We synthesized it by a facile four-step procedure starting from thieno[3,2-b]thiophene and ring-closing with amberlyst 15, where the terminal thiophene’s orientation (syn or anti) can be controllable. Though the two isomers look similar, but their spectroscopic and molecular orbital configurations are quite different. We fabricated thin-film transistor using DTBTTs as active layers by a vapor deposition method and their hole mobilities are compatible to DNTT. The experimental details and comparison with DNTT will be presented. Jan Gebers, Stephane Suarez, Michel Schaer, Philippe Bugnon and Holger Frauenrath; Institute of Materials, Ecole Polytechnique Federale de Lausanne (EPFL), Lausanne, Switzerland.

Supramolecular self-assembly represents a convenient pathway to create optoelectronic materials from monodisperse π-conjugated oligomers. Our work focuses on the preparation of novel oligothiophenes bearing substituents capable of hydrogen-bonding to enhance crystalline order, optimize the π-π stacking interaction, and, thus, improve their electronic properties. Here, we present an improved synthetic pathway towards amine-substituted oligothiophenes and the synthesis of a quaterthiophene diacetamide. This hydrogen-bonded quaterthiophene was vaccum-deposited onto silicon-dioxide substrates, and the influence of the deposition parameters (subtrate temperature and evaporation speed) on the morphology of the obtained thin films and the transistor performance was studied. At substrate temperatures of 20°C the molecules were found to adopt a random orientation on the dielectric, and grain sizes were on the order of 100 nm. By contrast, films deposited at 140°C and at low evaporation rates revealed epitaxially grown layers in which the molecules were oriented perpendicular to the substrate, and grain sizes of several microns were observed. Furthermore, these differences in morphology drastically influenced the performance of OFET devices fabricated from these thin films. The observed field effect mobility for the films deposited at elevated temperatures was remarkably high and almost four orders of magnitude higher than for the films deposited at ambient temperature.


8:30 AM U1.3
P- and N-Type Solution-Crystallized Organic Transistors and High-Performance Printable Inverters.Junshi Soeda1, Takafumi Uemura2, Yu Mizuno2, Akiko Nakao3, Yuri Hirose2, Masakazu Yamagishi2, Mayumi Uno2, Kengo Nakayama2, Yasuhiro Nakazawa1, Kazuo Takimiya4, Antonio Facchetti5 and Jun Takeya1,2; 1Graduate School of Science, Osaka University, Toyonaka, Osaka, Japan; 2The Institute of Scientific and Industrial Research, Osaka University, Ibaraki-shi, Suita, Japan; 3High Energy Accelerator Research Organization, Tsukuba, Japan; 4Graduate School of Engineering, Hiroshima University, Higashi Hiroshima, Hiroshima, Japan; 5Poryera, Skokie, Illinois.

Solution-processed organic field effect transistors (OFETs) are considered for attractive next-generation switching devices due to their advantage in large-area, low-temperature and low-cost production routes on plastic substrates. However, their major disadvantage has been relatively low performance as compared to vacuum-deposited thin-film devices. In this presentation, we introduce solution crystallization method which forms organic semiconductor films with excellent crystallinity, so that essentially improved device characteristics are achieved. The films are formed regulating direction of the crystal growth on oriented substrates. In comparison with typical charge carrier mobility in spin-coated or drop-casted poly-crystalline films which usually suffer from harmful effects from their grain boundaries, our present devices of crystallized C8-BTBT thin films demonstrate hole mobility up to 5 cm2/Vs, though it appears that in-gap charge-trapping states just above rather low HOMO level of the air-stable C8-BTBT molecules causes significant threshold gate voltage [1]. In order to improve this issue, we have developed a simple method of dipping the crystalline thin films into a solution of F4-TCNQ, which is known as a strong accepter, to reduce the threshold voltage of such devices stable in air but not operative at low gate voltages. Indeed, the threshold voltage is drastically reduced to almost 0 V by the treatment. Interestingly, UV-vis absorption spectra indicate that almost monolayer F4-TCNQ is attached on the surface of the C8-BTBT layers. The off-current does not rise at all as the result of the doping, indicating that the HOMO level of C8-BTBT is slightly deeper than the LUMO of F4-TCNQ so that the holes are doped only at the trap states in the band gap, not to the HOMO band itself [2]. Secondly, highly oriented PDIF-CN2 crystals are fabricated from solution to form high-mobility n-type organic semiconductor films, which is necessary for CMOS inverters. The maximum electron mobility reaches 1.3 cm2/Vs [3]. Moreover, the value did not diminish for at least two weeks during the whole duration of the stability test. In order to realize such high mobility, we have formed the crystalline film from solution confined in a very narrow wedge-shaped gap on the substrate, so that even a low-energy/hydrophobic dielectric surface of minimized electron traps can be used without significant de-wetting. Finally, the complementary devices of above p- and n-type solution-crystalized OFETs show excellent inverter characteristics with the gain exceeding 100, as the result of the extremely ordered molecular stacking in the crystals. [1] T. Uemura, J. Soeda, J. Takeya et al., Appl. Phys. Exp. 2, 111501 (2009). [2] J. Soeda, K. Takimiya, J. Takeya et al., Adv. Mater., published online. [3] J. Soeda, A. Facchetti, J. Takeya et al., Adv. Mater., in press.


8:45 AM U1.4
Synthesis and Characterization of Fused Thienoacene Organic Semiconductors and Their Application to Organic Field Effect Transistors.Hayden T. Black1, Shubin Liu2 and Valerie V. Sheares1; 1Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina; 2Information Services Technology, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina.

The synthesis and characterization of two new fused thienoacene organic semiconductors is presented. The compounds exhibit planar two dimensional structures which extend the area of π-conjugation, leading to crystal structures with exceptional π-π overlap. Density functional theory calculations have provided the HOMO and LUMO orbital geometries for the two compounds. In addition, dimeric orbitals have been calculated using the crystal structures, allowing for analysis of electronic communication between molecular pairs in the solid state. This work highlights the importance of synergy between crystalline packing and molecular orbital geometry for obtaining optimal orbital interactions in the solid state of organic semiconductors. Preliminary application of these materials to field effect transistors is also presented.


9:00 AM U1.5
Solution-Processable High Performance-OFETs of Alkyl-Dinaphthothienothiophene (DNTT) by Using Aryl-Substituted Aminoalkylsilyl Self-Assembled Monolayers.Toshihiro Okamoto1, Yuri Hirose1, Masakazu Yamagishi1, Junshi Soeda1, Mayumi Uno1, Takafumi Uemura1, Kazuo Takimiya2 and Jun Takeya1; 1ISIR, Osaka Univ., Ibaraki, Osaka, Japan; 2Graduate School of Engineering, Hiroshima Univ., Higashi-Hiroshima, Hiroshima, Japan.

Solution-processable organic thin-film transistors (OTFTs) having high performance are attractive for next-generation technology such as printable electronics. As rubrene single-crystal transistors, crystallized organic semiconductors are generally favored for high mobility carrier transport and have highly-ordered molecule packing compared to polycrystalline or amorphous films. Recently, we have developed a simple method of forming highly crystalline films of 2,9-didecyldinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (C10-DNTT) with which intentionally positioned matrix arrays were fabricated from hot solution at the temperature of approximately 100 °C. The obtained crystalline-film of C10-DNTT shows very high hole mobility of more than 10 cm2/Vs and the threshold voltage (Vth) of +10 V on DTS-treated SiO2 substrate. To obtain the reproducibility of forming high quality-crystalline films and control the electrical characteristic of OTFTs, the important issue is to produce the suitable self-assembled monolayers (SAMs) molecules as the organic semiconductors/dielectric interface. In this work, we have designed and developed several aryl-substituted amino- alkyl SAMs, which were synthesized in a similar method to reported literature. From the result of the contact angle measurements of all SAMs, it is found that aryl-substituted aminoalkyl SAMs exhibit the higher surface energy than the decyltriethoxylsilane (DTS)-treated SAM with water and aromatic solvents applied for solution-process. Notably, aryl-substituted aminoalkyl SAM has the high SAM-stability on SiO2, detected by time-dependent of water contact angle measurements compared to DTS-SAM, expecting the high reproducibility of forming high crystalline-film from solution process. In fact, the FET devices of C10- and C12-DNTT molecules were fabricated by gap-casting method, which have been reported recently, to give well-ordered crystallized film. The performance of these devices showed the hole mobility of 6-8 cm2/Vs and the Vth value of -5 V which was negative shift to 15 V compared to DTS-treated substrate. As a result, this allowed us to retain the mobility and successfully shift the Vth value, indicating the production of normally-off devices. The devices using other-type of amino SAM molecules will also be presented.


9:15 AM U1.6
A New Twist in Semicrystalline Pi-Conjugated Organic Semiconductors and Polaron Delocalization.Li-Hong Zhao1, Rui-Qi Png1, Jing-Mei Zhuo2, Han Guo1, Jie-Cong Tang2, Loke-Yuen Wong1, Richard H. Friend1,3, Lay-Lay Chua1,2,3 and Peter Ho1; 1Physics, National Univ Singapore, Singapore, Singapore; 2Chemistry, National University of Singapore, Singapore, Singapore; 3Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom.

The issue of the nature of the polaron state in pi-conjugated organic semiconductors is central to the science and technology of these materials. Previously it was widely believed that the polarons are substantially localized by disorder particularly in polymeric materials, although there has been evidence for interchain delocalization in regioregular poly(3-alkylthiophenes). Here I will describe evidence from infrared and optical charge-modulation spectroscopy (CMS) that polarons in tetradecyl-substituted poly[2,5-bis(3-alkylthiophene-2-yl)thieno[3,2-b]thiophene-2,5-diyl] (PBTTT), a model polymer OSC with even better-ordered pi-stacked lamellae, can in fact be nearly completely delocalized just slightly above room temperature. The delocalization transition broadly coincides with the ring-twist (Tr) transition of the pi-conjugated backbone. This phenomenon, which is counter-intuitive in view of the expected thermal-induced localization, is the consequence of frustration of the intrachain lattice relaxation of the polaron which then promotes its interchain delocalization. Nevertheless the delocalized polaron is still confined by thermal disorder and exhibits hopping transport behavior. The results show that polymer OSCs can support delocalized polarons and can approach close to the boundary of macroscopic band transport. Suppressing electron-lattice coupling is key to promoting the required polaron delocalization that will ultimately enable even higher carrier mobilities.


9:30 AM *U1.7
Bias-Stress Induced Charge Trapping at Polymer Gate-Dielectric in Organic Field-Effect Transistors.Kilwon Cho and Hyun Ho Choi; Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang, Korea, Republic of.

Here we present charge trapping mechanisms at polymer gate-dielectric under the bias stress applied to organic field-effect transistors (OFETs). By comparing threshold voltage shifts of the devices with air-gap dielectric and polymeric gate-dielectric, we could separate trapping dynamics within semiconductor and polymer gate-dielectric. We found that the chain-ends of gate-dielectric polymers work as metastable trap-sites for mobile charges and their density significantly affects the electrical stability. OFETs with polymer gate-dielectric having higher chain-end density exhibited large threshold voltage shift under the bias stress while no significant variation in the field-effect mobility was observed. These results can possibly be caused by the penetration of water molecules into the polymer chain-ends having large free volume, thus forming metastable trap-sites under the bias stress. Moreover, considering the depth profile of chain-end distribution, we could simulate the depth profile of trap-sites within gate-dielectric induced by the bias stress.


10:30 AM U1.8
Electronic Response to Structural Changes in 5-, 6-, and 7- Ringed Soluble Heteroacenes.Katelyn P. Goetz1, Zhong Li2, Jeremy W. Ward1, Cortney Bougher3, Jonathan Rivnay4, Jeremy Smith5, Brad R. Conrad3, Sean R. Parkin2, Thomas D. Anthopoulos5, Albert Salleo4, John E. Anthony2 and Oana D. Jurchescu1; 1Physics, Wake Forest University, Winston Salem, North Carolina; 2Chemistry, University of Kentucky, Lexington, Kentucky; 3Physics and Astronomy, Appalachian State University, Boone, North Carolina; 4Materials Science and Engineering, Stanford University, Stanford, California; 5Physics and Centre for Plastic Electronics, Imperial College London, London, United Kingdom.

Field-effect transistors with organic active layers have the potential to greatly impact the consumer market thanks to their promise of low production costs, as well as their compatibility with flexible substrates, such as plastic, textiles, or paper. This drives the need for an increase in the understanding of charge transport mechanisms in organic semiconductors. Studies of such materials have highlighted the role that molecular packing plays in the value of the hole mobility; specifically, varying the functional group attached to the aromatic backbone of the material directly affects packing morphology. This, in turn, varies charge transport capability. In order to explore the effect of backbone length on electronic properties, we have analyzed three novel, soluble heteroacenes chemically engineered to have the same functional group - tri-sec-butylsilyl ethynyl (TSBS) - but varying numbers of rings in the aromatic backbones. They are difluoro- TSBS anthradithiophene (ADT), difluoro- TSBS- tetracenedithiophene (TDT), and difluoro- TSBS- pentacenedithiophene (PDT). The latter compound, PDT, is the largest stable and soluble small molecule organic semiconductor investigated to date. We found that the ADT derivative displays a one-dimensional π-π stacking that results in low mobilities (10^-3 cm^2/Vs). The TDT derivative exhibits increased two-dimensional π-π stacking, but slight molecular rotations and a subsequent lack of cofacial orientation in crystals limits mobilities to 10^-2 cm^2/Vs. All transistors fabricated with the PDT derivative have shown excellent performance, with a characteristic mobility of 1 cm^2/Vs, and some devices as high as 1.8 cm^2/Vs. The brick-like two-dimensional π-π stacking of PDT indicates a strong correlation between this type of molecular orientation and good charge transport. PDT also proved amenable to use in blended semiconductors, which allowed for spin-coat deposition of very uniform films, with a small spread in mobility.


10:45 AM U1.9
Exploration of the Polarization Effects of Analyte Interactions with Receptor Molecules on the Trapping and Transport Mechanism in Organic Field Effect Transistor (OFET) Based Sensors.Davianne A. Duarte, Bradley J. Holliday and Ananth Dodabalapur; Electrical Engineering, University of Texas Austin, Austin, Texas.

Small receptor molecules offer a novel method for increasing the specificity and partition coefficient of organic field effect transistor (OFET) based sensors without adjusting the polymers through functional group and side chain additions. In this study, we have utilized concepts of polarization and molecular dipole moments to produce a model that describes the transport and trapping mechanisms of an OFET based sensor with receptor molecules. The sensors exhibit signal enhancement in the device characteristics upon analyte exposure without adding complexity to the OFET fabrication; the receptors are put into a solution and easily deposited through drop casting or spin-coating on the transistors. The Circle A receptor used in this study alters the device characteristics through hydrogen bonding with the analyte molecules. The receptor molecule has electronegative nitrogen atoms, which draw the electrons from the covalently bonded carbon and hydrogen atoms. This leaves the hydrogen atom with a slightly positive charge and the ability to form a strong interaction with the carbonyl group on analytes such as ketones, aldehydes, and esters. The hydrogen bonding induces a shift in the dipole moment of the receptor, causing a shift in the interaction energy through polarization effects. This produces an increase the potential barrier for carriers at those sites characterized by a current decrease and threshold voltage shift.


11:00 AM U1.10
High Carrier Mobility of 3.8cm2/Vs in Polydiacetylene Thin-Films Polymerized by Electron Beam Irradiation. Takuji Kato1, Yasukiyo Ueda2 and Chihaya Adachi1; 1Center for Organic Photonics and Electronics Research, Kyushu University, Fukuoka, Japan; 2Kobe University, Kobe, Japan.

Polydiacetylenes (PDA) are well-known organic semiconductors having main chain conduction path that can be fabricated by irradiating UV-light and/or heating of diacetylene monomers. In a theoretical study, the possibility of extremely high carrier mobility over 10,000 cm2/Vs has been anticipated (ref. 1). Indeed, one-dimensional carrier mobilities of 10 cm2/Vs have been reported by using the flash-photolysis-time-resolved microwave conductivity measurement, which allows the estimate of carrier mobilities in single crystalline domains and intra-molecular chains, for several PDA single crystals (refs. 2-5). Therefore, there is no doubt that polymerized diacetylenes should have intrinsically high conduction property. However, the highest carrier mobility in field-effect transistors (FETs) having a polydiacetylene active layer has been limited to less than 1 cm2/Vs (ref. 6). In this study, we introduced the technique of electron beam (EB) irradiation to control the morphology of polymerized diacetylene films to enhance the FET mobilities. The highest carrier mobility of polydiacetylene (PDA) thin-films in field-effect transistors has been limited to less than 0.8 cm2/Vs, although the main chain conduction should show higher carrier mobility potentially. We revealed that the cause of the low carrier mobility is due to the presence of local upheaval regions generated by the volume change through the polymerization process of diacetylene monomers. In order to suppress the occurrence of the upheaval regions, we found that electron beam (EB) irradiation is effective, resulted in the highest carrier mobility of μmax = 3.8 cm2/Vs. References 1) K. J. Donovan and E. G. Wilson, Philos. Mag., 9, (1981) 44. 2) J. M. Warman, M. P. de Haas, G. Dicker, F. C. Grozema, J. Piris, and M. G. Debijs, Chem. Matter., 16, 4600 (2004). 3) K. J. Donovan and E. G. Wilson, J. Phys. C: Solid State Phys., 12, 4857 (1979). 4) B. R. Wegewijs, G. Dicker, J. Priris, A. A.Garcia, M. P. de Haas and J. M. Warman, Chem. Phys. Lett., 332, 79 (2000). 5) G. P. van der Laan, M. P. de Haas, D. M. de Leeuw, and J. Tsibouklis, Synth. Met., 69, 35 (1995). 6) J. Nishide, T. Oyamada, S. Akiyama, S. Sasabe and C. Adachi, Adv. Matter., 18, 3120 (2006) .


11:15 AM U1.11
Long-Term Stability and Durability of n-Channel Polymer Thin Film Transistors.Felix Sunjoo Kim1 and Samson A. Jenekhe1,2; 1Chemical Engineering, University of Washington, Seattle, Washington; 2Chemistry, University of Washington, Seattle, Washington.

We report the air-stability and cycling durability of n-channel organic thin film transistors based on the n-type polymer semiconductor, poly(benzobisimidazobenzophenanthroline) (BBL). In investigation of long-term air-stability in terms of field-effect mobility and current on/off ratio, BBL transistors fabricated by solution-processing have been found to exhibit stable n-channel characteristics in air, and have outperformed p-channel devices based on poly(3-hexylthiophene) (P3HT). Treatment of the dielectric surface is shown to affect the performance of BBL transistors, resulting in increase in mobility by factor of 10-100 compared to devices on bare silicon dioxide. The devices with a modified dielectric layer showed stable electrical characteristics for 1000 cycles of duty scans, whereas severe hysteresis and threshold voltage shift were observed from devices without the dielectric layer modification.


11:30 AM U1.12
Short-Channel Top-Contact Organic Thin Film Transistors for the Study of Transport Properties.Carol Newby1, Jin Kyun Lee2 and Christopher K. Ober1; 1Materials Science, Cornell University, Ithaca, New York; 2Department of Polymer Science and Engineering, Inha University, Ithaca, Korea, Republic of.

The fabrication of top-contact organic thin film transistors is more difficult that bottom contact architectures because the aggressive solvents used to spin coat and develop the resist severely damage the underlying organic semiconductor material. However, the presence of the bottom contacts can be enough to disrupt the molecular packing of organic semiconductors, making it difficult to measure material properties of the organic semiconducting channel. Top contact transistors have been fabricated previously using shadowmasking [1] and contact printing methods [2] but neither of these routes can achieve dimensions small enough to study microstructure. In this work we use recently developed Orthogonal Processing, which uses non-damaging fluorous solvents[3], to successfully fabricate high-resolution metal electrodes on top of commonly used organic semiconductors such as P3HT and pentacene. Material combinations are carefully selected in order to minimize contact resistance effects, and the processing is optimized in order to ensure a good electrical contact between the electrode and semiconductor. The dimensions of the top contacts of the devices fabricated are on the same length scale as the grains of the organic semiconductor materials used. This therefore makes it is possible to directly investigate transport properties such as the intrinsic carrier mobility of these organic semiconductor materials rather than infering it by extrapolation of the change in mobility with the change in grain size. 1. M. Stein, J. Maple, J.B. Benzinger, S.R. Forrest, App. Phys. Lett. 81, 268 (2002) 2. J. Zaumseil, K. Baldwin, J. Rogers, J. Appl. Phys., 93, 6117 (2003) 3. A. A. Zakhidov, J.-K. Lee, H. H. Fong, J. A. DeFranco, M. Chatzichristidi, P. G. Taylor, C. K. Ober, G. G. Malliaras, Adv. Mater., 20, 348 (2008)


11:45 AM U1.13
Charge Carrier Transport and Density of Trap States in Balanced High Mobility Ambipolar Organic Thin-Film Transistors.Tae-jun Ha1, Prashant Sonar2 and Ananth Dodabalapur1; 1Microelectronics Research Center, University of Texas at Austin, Austin, Texas; 2Institute of Materials Research and Engineering Agency for Science, Technology and Research (A*STAR), Singapore, Singapore.

There have been very few organic semiconductors reported to date with balanced electron and hole mobilities that both are more than 0.1 cm2/V-s. One such ambipolar organic semiconductor is diketopyrrolopyrrole-benzothiadiazole copolymer (PDPP-TBT). We have fabricated ambipolar PDPP-TBT thin-film transistors (TFTs) which possess the highest balanced electron and hole field-effect mobilities of up to 0.6 cm2/V-s. Significantly, there have been no prior studies on charge carrier transport and the density of trap states (trap DOS) for both electrons and holes in high mobility ambipolar organic semiconductors. We are reporting on the results of such a study on PDPP-TBT. Temperature and gate-bias dependent field-effect mobility measurements are employed to extract the activation energies and trap DOS to understand the unique high mobility balanced ambipolar charge transport properties. Activation energy decreases with increasing VGS-VON in both holes and electrons and the value of μo is about 5 cm2/V-s. The combination of a fairly high mobility and activation energy of ~ 100 meV strongly suggests that the main charge tranport mechanism in this orgnaic semiconductor is multiple trap and thermal release. The symmetry between the electron and hole transport characteristics, parameters and activation energies is remarkable. We have also calculated the trap DOS in ambipolar PDPP-TBT TFTs based on activation energy as a function of gate voltages using two analytical methods following the approaches by Lang et al. and Kalb et al. We believe that this work represents the first charge transport measurement of a truly ambipolar organic/polymer based field-effect transistor.


 

SESSION U2: Organic Semiconductors for Charge Transport I
Chair: Iain McCulloch
Monday Afternoon, November 28, 2011
Room 311 (Hynes)


1:30 PM *U2.1
Photoactive Polymers: Dreams and Reality.Mathieu Turbiez, BASF Schweiz AG, Basel, Switzerland.

BASF has a strong commitment in the field of Organic Electronics. Significant research and development efforts target a range of innovative material solutions for Printed Electronics, OLED and OPV applications. In the past years BASF has successfully designed and synthesised materials with unique properties (e.g. efficiency, mobility, stability, etc.) and developed new technologies driven by our understanding of customer relevant applications. BASF also heavily invested in developing complex material production processes from g- to multi-kg scale to guaranty our customers the best quality in a reproducible manner. This latter challenge should not be underestimated and must be taken seriously if the market potential behind these technologies is to be effectively realised.


2:00 PM *U2.2
Donor-Acceptor Conjugated pi-Systems.Mark Watson, Mark S. Seger, Thilanga Liyanage and Guang Zhang; Department of Chemistry, University of Kentucky, Lexington, Kentucky.

This presentation will describe our recent synthetic approaches to regulate the (opto)electronic properties and self assembly of small molecules and polymeric materials. Approaches include systematic increases in the steric bulk of donor monomer side chains in the near vicinity of polymer backbones, incorporation of mildly to strongly electron-withdrawing groups, and stringing together electron-withdrawing groups in series.


2:30 PM U2.3
Film Structure and FET Performance of Pentacene from Soluble Photoprecursor.Ken-ichi Nakayama1,2,4, Yoshisato Oikawa1, Chika Ohashi1, Junji Kido1,2 and Hiroko Yamada3,4; 1Department of Organic Device Engineering, Yamagata University, Yamagata, Japan; 2Research Center for Organic Electronics, Yamagata University, Yamagata, Japan; 3Graduate School of Material Science, Nara Institute of Science and Technology, Nara, Japan; 4CREST, Tokyo, Japan.

Pentacene is the most standard and high performance organic semiconductors, but its film is difficult to be prepared by solution process. Some soluble pentacene derivatives have been reported like TIPS-pentacene, and some soluble precursors of pentacene that can be thermally converted have been reported. 6,13-pentacene diketone is a promising material of soluble pentacene precursor that can be converted by photo irradiation. The ketone moieties are eliminated by photochemical reaction and eliminated two CO molecules do not remain in the system. Therefore, this photoprecursor can solve the problem in precursor system, and give a new possibility to control the film structure by "light" having many degrees of freedom. In this study, we investigated film preparation process and relationship between the film structure and device performance of field-effect transistor. In particular, we focused on the effect of solvents and their composition. The solution of 6,13-pentacene diketone was spin-coated on a highly doped silicon substrate (gate) with an oxide layer (300 nm) in the glove box system. We used chloroform as a main solvent and various types of solvents as additive. The monochromatic light of 470 nm was irradiated for 10 min ~ 60 min also in the glove box system and the precursor was converted to a pentacene channel layer. The gold source and drain electrodes were prepared by vacuum deposition through a metal shadow mask. The OFET performance was measured using semiconductor parameter analyzer. The film structure and morphology were investigated by atomic force microscopy (AFM) and X-ray diffractometry (XRD). The converted film showed similar UV-VIS spectrum to a pentacene deposited film. Good OFET performance was observed when mixed solvents of chloroform and some solvents with high boiling point. When o-dichlorobenzene was used as additional solvent, the field-effect mobility around 0.37 cm2/Vs was achieved. This value is basically comparable to a standard performance of vacuum-evaporated pentacene OFET. It should be noted that the converted film showed almost no peak in XRD, and small isotropic grains in AFM. These results suggest that the photo-converted pentacene film from pentacene 6,13-diketone follows a different principle to achieve high field-effect mobility compared to conventional vacuum-deposited pentacene film.


2:45 PM U2.4
Correlation of Channel Mobility in OTFTs with Domain Cluster Size Directly Measured Using PSoXS.Brian A. Collins1, Justin E. Cochran2, Eliot Gann1, Hongping Yan1, Cheng Wang3, Michael L. Chabinyc2 and Harald Ade1; 1Physics, NC State University, Raleigh, North Carolina; 2Materials, University of California Santa Barbara, Santa Barbara, California; 3Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California.

Mobilities in organic thin film transistors (OTFTs) have to date largely been attributed to, first, the fundamental material properties and, second, to domain size and molecular order. Correlations in orientational order between domains have also been proposed as a major factor in OTFT device performance, but no direct evidence has so far indicated such a connection. Here we demonstrate this connection in OTFTs with the polymer poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophene (pBTTT) as a channel material using a new technique that utilizes scattering of polarized soft x-rays (PSoXS). In this study, different device preparation conditions resulted in varying domain cluster sizes within which orientations of the ordered individual domains were aligned. The PSoXS technique utilizes the contrast between anisotropic bonds within the aromatic rings prevalent in soft matter giving it excellent sensitivity to orientational correlations of molecules within a material. From the different preparation conditions of the devices, a varying domain cluster size (average number of domains in a cluster) was observed in the scattering patterns. When compared with saturation mobility measurements in devices identically prepared, this variation in cluster size resulted in a 99.2% Pearson correlation, suggesting a causal relationship. Notably, individual domain size and contrast with domain boundaries is also observed with the PSoXS technique and showed no correlation with device performance. This suggests that orientational alignment between domains can greatly enhance charge transport across domain boundaries in these materials and is an important factor in OTFT device performance.


3:30 PM *U2.5
Exploring the Larger Acenes.John E. Anthony, Chemistry, Univ of Kentucky, Lexington, Kentucky.

Over the past few years, we have been trying to push the collection of pentacenes useful for electronic applications past the ubiquitous pentacene. To that end, we have performed detailed studies of the decomposition mechanisms for functionalized acenes, and developed a set of guidelines for the stabilization of acenes larger than pentacene. Using both steric and electronic modifications (including heteroatom substitution), and relying heavily on our crystal-engineering approach to enhance pi-stacking in the solid state, we have developed 6- and 7-ringed acenes that have yielded impressive performance in field-effect transistor applications, and were sufficiently stable for detailed studies of their thin-film properties, including their behavior in photovoltaic devices. Following recent reports of so-called "persistent" nonacenes with inexplicable photophysical properties, we applied our stabilization techniques to that larger hydrocarbon, and were rewarded with three nonacene derivatives sufficiently stable for crystallographic and other studies. Unlike the prior reported nonacene, the photophysical properties of our acenes follow established photophysical trends, and in support of recent theoretical predictions, these acenes appear to have significant diradical character in the ground state. Such a phenomenon has interesting implications, as the acenes represent oligomers of the narrowest variety of graphene nanoribbons.


4:00 PM *U2.6
Recent Progress in Thienoacene-Based Organic Semiconductors.Kazuo Takimiya, Hiroshima Univ, Higashi-Hiroshima, Japan.

Thiophene-containing fused-aromatic compounds, often called as thienoacenes or heteroarenes, have been regarded as potential organic semiconductors for organic field-effect transistors (OFETs) [1]. Among numbers of thienoacenes structures, diacene-fused thieno[3,2-b]thiophenes including [1]benzothieno[3,2-b][1]benzothiophenes (BTBT) [2] and dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT) [3] represent a promising material class, and OFETs based on their derivatives can afford very high hole mobilities up to 8 cm2/Vs [4]. Recent synthetic efforts have enabled us to add a new member with a further p-extended homologue, dianthra[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DATT) consisting of eight fused-aromatic rings, which also acts as the active semiconducting layer in OFETs [4]. Another interesting material class among thienoacenes is linear-shaped acenedithiophenes represented by benzo[1,2-b:4,5-b']dithiopehene (BDT) and anthra[2,3-b:6,7-b']dithiopehene (ADT) [5]. Naphtodithiohpenes (NDTs) had been missing members in this class, owing to the lack of efficient synthetic methods. We have recently established selective synthetic routes to four isomeric NDTs [6] and explored their utilities in the development of organic semiconductors [7]. In the present contribution, the syntheses, molecular and electronic structure of materials, packing structures, and properties of these new thienoacene-based materials will be discussed. References [1] For example, Anthony, J. E. Chem. Rev. 2006, 106, 5028. [2] Ebata, H. et al., J. Am. Chem. Soc. 2007, 129, 15732. [3] Yamamoto, T. et al., J. Am. Chem. Soc. 2007, 129, 2224. [4] Kang, M. J. et al., Adv. Mater. 2011, 23, 1222. [5] Niimi, K. et al., J. Am. Chem. Soc. 2011, 133, 2011, 133, 8732. [6] Shinamura, S. et al., J. Am. Chem. Soc. 2011, 133, 5024. [7] Osaka, I. et al., J. Am. Chem. Soc. 2011, 133, 6852.


4:30 PM U2.7
Theoretical Modeling and Design of Organic Semiconductors with High Carrier Mobility.Xiao Ma, Chang-Gua Zhen and John Kieffer; University of Michigan, Ann Arbor, Ann Arbor, Michigan.

Charge transport in organic materials can be quite different from that in inorganic materials. The weak van der Waals interaction between organic molecules invalidates the band model used widely in inorganic materials. We have applied a multiscale hopping model based on Marcus’ electron transfer theory to study the carrier mobility in a pentacene single crystal structure. The pentacene single crystal adopts a herringbone stacking, which strongly limits the π-orbital overlap between neighboring molecules, resulting in poor charge carrier transfer and long-range mobility. Two possible ways have been considered to improve the charge transport performance of pentacene-related organic materials: (i) functionalize the pentacene with polyhedral oligomeric silsesquioxanes (POSS) cages to induce a parallel configuration; (ii) control structural defects in the pentacene single crystal. These two factors have been investigated using a combination of density functional theory calculations and Monte Carlo simulations, in order to predict the most effective materials design approach for achieving higher charge carrier mobility in organic semiconductors. The temperature dependence is also analyzed in our model.


4:45 PM U2.8
Polymorph Selection in Pentacene Thin-Films: Molecular Dynamics Simulation Study.Makoto Yoneya1, Masahiro Kawasaki2 and Masahiko Ando2; 1Nanosystem Res. Inst., National Inst. of Advanced Industrial Sci. & Tech. (AIST), Tsukuba, Japan; 2Hitachi Res. Lab., Hitachi, Ltd., Kokubunji, Japan.

Molecular dynamics simulations were performed for mono- and multi-layer pentacene thin-films using a simple model surface to study polymorphs of the films. It is known that pentacene has at least four different crystallographic morphologies (polymorphs) that are sensitive to temperature, thickness, surface energy and other parameters [1]. Recently, Natsume et al. [2] reported that the crystalline structure of solution-processed pentacene thin-films has a similar structure to the thin-films reported by Campbell et al. [3]. This ``Campbell-polymorph" was found to correspond to the high-temperature (HT) polymorph described by Siegrist et al. [4]. In contrast, Mattheus et al. [1] reported the low-temperature (LT) polymorph for solution-processed single crystals. These two HT and LT pentacene polymorphs were compared by simulating their stability as thin-films on a substrate and under free surface conditions. It was found that the LT polymorph was destabilized by the presence of substrate and free surface effects, which transformed the HT-polymorph over the course of our simulations [5]. Suppression of the molecular fluctuations along the long molecular axis by these surface effects was suggested as the source of this destabilization. On the other hand, the vacuum deposition process generally yields a ``thin-film’’ polymorph in films thinner than 100nm [6]. Even thinner monolayer deposited on a silicon oxide substrate, it is found to be a completely molecular tilt-free polymorph, i.e., ``monolayer phase’’ [7]. To study these polymorphs in the deposition process, we simulate the layer by layer growth starting from the first monolayer and examine the change of the film structure. Simulated monolayer was found to be the tilt-free (``monolayer phase’’) and with the successive second layer, spontaneous molecular tilting was occurred. The tilt angle of this bilayer (around 8 degrees) was similar to that of the reported ``thin-film’’ phase. Although, the tilt angle was slightly increased with successive 3rd and 4th layer, it kept smaller value than that of the HT and LT polymorphs (around 30 degrees). These ``thin-film” and ``monolayer’’ phases are often called substrates induced phases [5]. However, we found that the tilt-free monolayer and the spontaneous tilting with the second layer deposition were occurred even without the substrate interactions. Our simulation results imply that the ``monolayer” and ``thin-film’’ phases are both not substrates induced, but are induced with in-layer and across-layer interactions of the pentacene itself. [1] C. Mattheus et al., Synth. Mat., 138, 475 (2003). [2] Y. Natsume et al., Synth. Mat., 159, 338 (2009). [3] R. Campbell et al., Acta Cryst., 15, 289 (1962). [4] T. Siegrist et al., Adv. Mater., 19, 2079 (2007). [5] M. Yoneya et al., J. Mat. Chem., 20, 10397 (2010). [6] I. P. M. Bouchoms et al., Synth. Met., 104, 175 (1999). [7] S. C. B. Mannsfeld et al., Adv. Mat., 21, 2294 (2009).


 

SESSION U3: Optical and Electronic Properties
Chairs: Iain McCulloch and Jun Takeya
Tuesday Morning, November 29, 2011
Room 311 (Hynes)


8:00 AM U3.1
Charge Noise in Organic Photovoltaic Materials and in Polymeric Semiconductors as Determined by Electric Force Microscopy.Nikolas Hoepker1, Justin L. Luria2, Swapna Lekkala2, Roger F. Loring2 and John A. Marohn2; 1Department of Physics, Cornell University, Ithaca, New York; 2Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York.

Despite a large body of research, degradation pathways in many organic photovoltaic materials and polymeric semiconductors remain unknown. However, it is known that these devices suffer from degradation due to charge trapping. Here we measure charge trapping as a fluctuation in the local surface potential as observed in an electric force microscope experiment. In bulk heterojunction solar cells we found that in the presence of light, cantilever frequency noise increases by almost two orders of magnitude. We attribute the increase in noise to charge trapping and de-trapping. We have developed a theory that reasonably reproduces the magnitude, spectral shape, and distance, frequency, and tip-voltage dependence of the observed fluctuations. Our measurements elucidate details in the trapping mechanism and allow us to determine the density of trapped charges. With accurate modeling, such measurements give valuable information on charge trapping and inform the synthetic process. In polymeric semiconductors there is an ongoing controversy on the charge density and electric field dependence of mobility. While the correlated-disorder model correctly predicts the electric field dependence of mobility, models that predict a density dependence of mobility rely on uncorrelated site-to-site energies. A resolution of these controversies calls for new tools to study carrier motion in organic semiconductors. By measuring the frequency fluctuations experienced by a cantilever near a surface, we hope to microscopically probe carrier motion in organic materials. Measurements of frequency noise in polymeric transistors are under way.


8:15 AM U3.2
Energy Levels in Organic Electronics.Scott E. Watkins, Fiona Scholes, Jacek Jasieniak, Tino Ehlig and Birendra Singh; Materials Science and Engineering, CSIRO, Melbourne, Victoria, Australia.

In this paper we will discuss the characterisation of energy levels of organic semiconductors through the use of Photo Electron Spectroscopy in Air. This technique enables the rapid analysis of films prepared under real fabrication conditions. Through the use of bilayer device architectures and a mix of solution and vacuum deposition conditions we will demonstrate changes to energy levels and correlate these directly with device performance.


8:30 AM *U3.3
Electronic Traps in Organic Thin Films: Interface Electronic Structure, Charge Transport and Chemical Doping.Antoine Kahn, Electrical Engineering, Princeton University, Princeton, New Jersey.

Numerous experiments and theoretical investigations have shown over the past few years that a significant density of electronic states (or traps) tail into the gap of organic semiconductors.1-4 These states derive from structural or chemical defects, and/or static and dynamic disorder in the films. Trap densities are found to be already substantial in highly crystalline bulk organic crystals and larger in polycrystalline and amorphous films.3 These states are largely responsible for the uniformly observed pinning of the Fermi level close to highest occupied or lowest unoccupied molecular orbitals (HOMO, LUMO). We discuss the issue of “band bending” observed in organic films in contact with electrodes with work function close to either the film ionization energy or electron affinity. Band bending is predominantly due to the need for the Fermi level to move out of the density of trap states. Trap states are also discussed in terms of the formation of interface dipoles in organic/organic heterojunctions, in particular in donor/acceptor pairs. We then turn to chemical doping, with HT materials like α-NPD or pentacene p-doped with the strongly oxidizing molecule Mo(tfd)3,5 or the ET polymer P(NDI2OD-T2) n-doped with our new air-stable [RhCp2]2.6 We demonstrate how chemical doping results in filling these gap states, moving the Fermi level closer to the HOMO or LUMO level and increasing charge carrier mobility by reducing activation energy in the hoping process.5 1. Hulea et al., Phys. Rev. Lett. 93, 166601 (2004) 2. Tal et al., Phys. Rev. Lett. 95, 256405 (2005) 3. Kalb et al., Phys. Rev. B 81, 155315 (2010) 4. Street et al., Phys. Rev. B 83, 165207 (2011) 5. Qi et al., J. Am. Chem. Soc. 131, 12530 (2009) 6. Barlow et al. (in preparation)


9:00 AM *U3.4
Organic Light-Emitting Transistors: Status and Perspectives of an Emerging Technology.Michele Muccini, CNR-ISMN, Consiglio Nazionale delle Ricerche, Istituto per lo Studio dei Materiali Nanostructturati Via P. Gobetti, Bologna, Italy; E.T.C. s.r.l., via P. Gobetti, Bologna, Italy.

Organic light-emitting transistors (OLETs)1 are emerging as an alternative driving scheme to generate electroluminescence from organic materials, that offers fundamental advantages over the well established and more mature organic light-emitting diodes (OLEDs). Indeed, the three electrodes planar field-effect device architecture, as opposed to the vertical structure of OLEDs, enables a better control of charge injection, current flows, charge recombination and light emission, holding the promise of higher efficiency, higher brightness and more efficient light out-coupling. Here, recent advances and future prospects of light-emitting field-effect transistors will be discussed, with particular emphasis on organic semiconductors and the role played by the material properties, device features and the active layer structure in determining the device performances. In particular, we discuss the concept of using a p-channel/emitter/n-channel trilayer semiconducting heterostructure in OLETs,2 providing a new approach to markedly improve OLET performance. In this architecture, exciton-charge annihilation and electrode photon losses are prevented. Our devices are >100 times more efficient than the equivalent OLED, >2X more efficient than the optimized OLED with the same emitting layer and >10 times more efficient than any other reported OLETs. [1] M. Muccini, A bright future for organic field-effect transistors. Nature Mater. 5, (2006) 605-613. [2] R. Capelli, S. Toffanin, G. Generali, H. Husta, A. Facchetti and M. Muccini, Organic light-emitting transistors with an efficiency that out-performs the equivalent light-emitting diodes. Nature Mater. 9 (2010) 496-503.


9:30 AM U3.5
High-Resolution near-Field Optical Investigation of the Thin Film Morphology of Oligomeric PQT-12.Sergei Kuehn1, Patrick Pingel2, Markus Breusing1, Thomas Fischer3, Joachim Stumpe3, Dieter Neher2 and Thomas Elsaesser1; 1Max-Born-Institute Berlin, Berlin, Germany; 2Institute of Physics and Astronomy, Postdam, Germany; 3Fraunhofer Institute for Applied Polymer Research, Postdam, Germany.

Organic thin films processed from solution exhibit a hierarchical dispersion of morphologies spanning length scales from nano- to millimeters. A comprehensive characterization is therefore challenging and seldom undertaken notwithstanding the fact that the morphology is recognized as a crucial determinant for such key performance figures as the charge carrier mobility. Typically, solution processed polymers form high mobility crystalline domains embedded in a host of disordered material of low mobility. High molecular weight (MW) polymers can bridge disordered regions and reach a higher overall mobility than low MW oligomers. Grain boundaries and packing defects are the major impediment to current flow in these thin films. At the level of the single crysal the mobility shoots up in consequence. A peculiar observation was made on oligomeric PQT-12 thin films prepared by spin coating: In defiance of the general trend the mobility fully recovers at low MW corresponding to the dimmer. This implies a morphological transition between poly- and single-crystallinity. We have characterized the structure and morphology of this transitional state in a non-perturbative manner by optical polarization spectroscopy with a near-field scanning probe microscope (NSOM) to elucidate and substantiate information obtained from various other techniques. X-ray diffraction shows a highly crystalline order with the molecular backbone and the pi-stacking direction parallel to the substrate. Polarization microscopy reveals an interconnected network of orientational domains with near perfect anisotropy several ten microns in size. A plate-like morphology is supported by TEM. Selected area electron diffraction, however, shows an orientational distribution at the micron scale stemming from either an ensemble of smaller crystallites or from a continuously bending crystal axis. Similarly, confocal absorption spectroscopy reveals heterogeneity of the crystalline quality down to the half micron resolution limit. The AFM topography reveals micron-sized rafts decorated by differently oriented nanocrystallites, which completes the size hierarchy of features. The size scale determining the crystalline domain size where revealed utilizing an NSOM. An overall high crystalline quality of the film is confirmed by spatially resolved absorption spectroscopy with 100 nm resolution. Disorder related imperfections are homogeneously distributed in the material and lead to a flexible crystal order permitting a continuous adjustment of the crystal axis. This fact is confirmed by the local dichroism and molecular orientation measured by NSOM. Similar to a liquid crystal, high angle grain boundaries are hence reduced facilitating a superior field mobility. We thus provide a link between performance and structure and present a broadly applicable non-destructive method to characterize as-prepared thin films with very high spatial resolution.


9:45 AM U3.6
Optical Spectroscopy of Electron Trapping in Polymer Semiconductor Devices.Riccardo Di Pietro, Michael C. Gwinner, Yana Vaynzof, Kathryn Greenberg, and Henning Sirringhaus;Physics, University of Cambridge, Cambridge, Cambridgeshire, United Kingdom.

We have developed a high sensitivity optical spectroscopy technique that allows monitoring subtle changes of the optical absorption of an organic device at a sensitivity level of < 5 part in 10000 as a function of time. This technique allows in-situ spectroscopic investigation of some of the key electronic processes in fully functional device structures. The technique has been successfully applied to study the mechanism of charge transfer doping of conjugated polymer films by MoO3, quantify doping efficiency and investigate the doping efficiency under different atmospheric conditions. In this work we use the technique to investigate the mechanism for bias stress degradation in n-type organic FETs of poly{[N,N9-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,59-(2,29-bithiophene) (P(NDI2OD-T2). By detecting subtle changes in the optical transmission spectrum and in the Charge Induced Absorption while bias stressing the device it is possible to spectroscopically investigate mechanisms for electron trapping and transport degradation in such devices, and correlate them to modification induced in the spectroscopic signature of mobile and trapped electrons. The fundamental role of atmospheric contaminants in the stressing mechanisms is determined by studying stress effects in different environments (vacuum, H2O/N2, O2/N2, ambient air), and the results are analysed in view of possible electrochemical reactions happening at the interface with the dielectric, and how this affects the electrical characteristics of the device.


10:30 AM U3.7
Enhanced Ambipolar Charge Injection and near-IR Emission in Semiconducting Polymer/Carbon Nanotube Field-Effect Transistors. Florian Jakubka1, Michael C. Gwinner2, Florentina Niebelschuetz1, Henning Sirringhaus2 and Jana Zaumseil1; 1Materials Science and Engineering, University of Erlangen, Erlangen, Germany; 2Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom.

Semiconducting polymers (e.g. polyfluorenes) containing single-walled carbon nanotubes (SWNT) are interesting hybrid systems for organic electronic devices such as solar cells, light-emitting diodes and field-effect transistors (FETs). Such hybrid systems are easily produced by selective dispersion of SWNT in polymer solutions by ultrasonication. The produced polymer-wrapped nanotubes are very well de-bundled and show unusually high photoluminescence efficiencies in the near infrared. They also form type I heterojunctions with most polyfluorenes, which makes them ideal near-infrared emissive dopants in polymer light-emitting devices. Here we demonstrate that nanotubes at concentration levels well below the percolation limit have a significant impact on charge injection and transport in polymer field-effect transistors. Even at very low nanotube concentrations the charge injection of holes and electrons into poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) and poly(9,9-dioctylfluorene) (PFO) is significantly enhanced leading to reduced threshold voltages and lower contact resistances than in FETs with pristine polymer films. At the same time the charge carrier mobility decreases slightly. Overall the maximum ambipolar currents and thus visible light emission due to electron-hole recombination are considerably enhanced. This effect is investigated in terms of nanotube type and concentration and their spatial distribution within the active polymer layer. The improved device characteristics of ambipolar polymer FETs with dispersed carbon nanotubes allow us to study charge and excitation transfer from the polymer matrix to the nanotubes and the resulting near-infrared electroluminescence. Combining good charge transport within the polymer matrix and efficient near-infrared emission from SWNTs could lead to novel optoelectronic devices operating efficiently in the near-infrared telecommunication wavelength window.


10:45 AM U3.8
Design of Conductivity Doped Transport Systems for Organic Light Emitting Devices.Asanga B. Padmaperuma, Lelia Cosimbescu, Phillip K. Koech, Liang Wang, James S. Swensen and Evgueni Polikarpov; PNNL, Richland, Washington.

There has been a growing interest in organic electronic devices such as organic light emitting diodes, organic solar cells, and organic transistors due to the unique electrical and optical properties of the organic materials containing them. In the push to develop organic light emitting diodes (OLEDs) for flat panel displays and solid state lighting applications, obtaining low drive voltage is key to minimizing power consumption. The transport layers can be doped to increase the carrier density and thus reduce the drive voltage. A major breakthrough for small molecule OLEDs was the successful application of “conductivity” doping as means of reducing the voltage. The doping of organic electron transport layers with inorganic donors or doping the hole transport layer with inorganic acceptors lead to OLEDs with lower voltages. However, these inorganic dopants generate small, mobile ions which diffuse or drift under the high electric fields present in operating conductivity doped OLEDs, limiting device lifetimes. Better stability was envisioned by using larger organic-based molecular dopants with suitable electron affinities for p-doping and ionization energies for n-doping. We developed new classes of molecular dopants and charge transport materials based on rational design for OLEDs using computational methods. The respective energies of the HOMO and the LUMO states as well as the triplet energy of the new molecules were tuned by modifications of the chemical structure. The design parameters, and properties of these new materials will be presented.


11:00 AM U3.9
Design of Semiconducting Materials and Polymer Dielectrics for Printed Transistor Applications.Silvia Janietz, Kerstin Schulze, Tatjana Egorov-Brening and Marcel Schmidt; FhG-IAP, Potsdam, Germany.

Much research was done in the manufacture of organic field effect transistors (OFET) which is the most famous component for the realization of integrated circuits. OFETs based on solution-processible polymers as well as small organic molecules have obtained impressive improvements in performance in the last years. But the aim is to realize integrated circuits based only on organic components to reach the potential for low cost processing. A critical point in the OFET is the complete system organic semiconductor/ organic dielectric. On the one side the interface between the organic dielectric and the organic semiconductor could be influenced through modification of the chemical structure of the applied semiconductor polymer. Therefore a novel poly(3-[hexyl-co-3,6-dioxaheptyl]-thiophene) was synthesized. The dioxaheptyl-groups in the side chain of the polymer backbone result in good wetting behavior of the PMMA-solution on top of the P3HT-doh layer which we do not observe when we use P3HT-layers. This means a difference at the interface between the semiconductor and the dielectric in dependence of the side chains of the thiophene unit occurs. The measured performance on flexible OFETs regarding mobility seems comparable as well. The copolymer was formulated for inkjet printing processes and we determine a comparable behavior characteristic of the OTFT to devices that based on spin-coated semiconductor layers. The OTFTs show good air-stability when using PMMA as dielectric in a top-gate configuration. On the other side new adapted organic dielectrics are developed to adjust the interface to the organic semiconductor. For this reason new thermally crosslinkable dielectrics are synthesized which have the opportunity to integrate different kind of monomer units to adapt the organic semiconductor. At first inert polymers were prepared with two different crosslinkable units. These polymers have the possibility to be crosslinkable in a thin layer through a following thermal or UV-initiated step. The polymers were synthesized through a radical solution polymerisation. The copolymers consist of phenol- and phenoxymethyloxiran-units in different ratios in the polymer backbone. This synthetic strategy offers the possibility to realize very thin dielectric layers to reduce the operation voltage of OFETs to some volts. The electrical stability of thin layers using this dielectric as well as the functionality in OFETs were proven.


11:15 AM U3.10
Investigations on Charge Transport in Organic Semiconducting Single Crystals via Polarized Infrared Spectroscopy.Alessandro Fraleoni Morgera1, Beatrice Fraboni2, Marta Tessarolo2, Andrea Perucchi1, Leonetta Baldassarre1 and Stefano Lupi3; 1Sincrotrone Trieste SCpA, Basovizza (TS), Italy; 2Dept. of Physics, Univ. of Bologna, Bologna, Italy; 3Dept. of Physics, Univ. of Roma "La Sapienza", Roma, Italy.

The electronic behaviour of organic semiconductors is a topic of hot debate. Organic semiconducting single crystals (OSSCs) present favorable properties (like absence of grain boundaries, defined molecular arrangements, lack of defects) for helping to investigate transport in organic semiconductors, and are hence the subject of intense studies [1-3]. Recent reports on 4-hydroxycyanobenzene (4HCB) single crystals evidenced reproducible, three-dimensional anisotropic electronic properties, such as carrier mobilities, [4,5] density of state distribution, deep trap concentration and energy levels [6]. Accurate polarized IR studies evidenced interesting anisotropic optical properties, like 2D anisotropic hydrogen bonding degree and a 3D anisotropic rotation of the electric dipole vector upon electrical polarization of the crystal. Moreover, it was found that a marked 2D spectral anisotropy allows to identify reproducibly and beyond any doubt the crystal axes with no need for X-ray or electrical measurements. When the crystal is the active layer of an organic field effect transistor (OFET) and is probed by polarized IR under actual charge transport conditions, a quite interesting 2D anisotropic IR screening effect of the charge transport layer is detected [7]. These features will be presented together with novel, polarized IR investigations carried out on 4HCB crystals in an OFET configuration. Under these conditions it is seen that the charge transport layer is either a few nanometers thin, with a very high (metal-like) carriers density, or several tens of nanometers thick, with a semiconductor-like carriers density. Moreover, around 1000-7000 cm-1, polaronic signatures were observed along the planar axis a, but no corresponding peak shifts were detected in the MID-IR range. This is unexpected, since polaronic entities have quinoid structures, rather than benzenoid ones, hence they should present detectable spectral differences. Therefore, further investigations of the polarized IR spectral behaviour of 4HCB crystals under actual current flow (in an OFET configuration), at temperatures down to 77 K, will be presented, shedding further light on the charge transport phenomena occurring in 4HCB crystals. References [1] O. Ostroverkhova et al., Appl. Phys. Lett., 88, 162101 (2006) [2] A. Troisi, Adv. Mater., 19, 2000 (2007) [3] D. Braga, G. Horowitz, Adv. Mater., 21, 1473 (2009) [4] B. Fraboni, R. DiPietro, A. Castaldini, A. Cavallini, A. Fraleoni-Morgera, L. Setti, I. Mencarelli, C. Femoni, Org. Electron., 9, 974 (2008) [5] B. Fraboni, C. Femoni, I. Mencarelli, L. Setti, R. Di Pietro, A. Cavallini, A. Fraleoni-Morgera, Adv. Mater., 21, 1835 (2009) [6] B. Fraboni, A. Fraleoni-Morgera, A. Cavallini, Org. Electron., 11, 10 (2010) [7] A. Fraleoni-Morgera, B. Fraboni, M. Tessarolo, L. Baldassarre, A. Perucchi, S. Lupi, paper submitted


11:30 AM U3.11
Current Deep Level Transient Spectroscopy with a Bipolar Rectangular Weighting Function to Detect Traps in Organic Light Emitting Diodes.Yutaka Tokuda1, Tatsunari Shibata1, Hidenari Naitou1, Tetsuya Katou2 and Masayuki Katayama2; 1Department of Electrical and Electronics Engineering, Aichi Institute of Technology, Toyota, Japan; 2Research Laboratories, DENSO CORPORATION, Nisshin, Japan.

Current deep level transient spectroscopy (DLTS) with a bipolar rectangular weighting function has been developed to characterize traps in organic light emitting diodes (OLEDs). Current transients resulting from emission of carriers from traps are multiplied by a bipolar rectangular weighting function (BRWF) and integrated during the period of a BRWF to produce DLTS signals in the unit of coulomb (BRWF-CDLTS). The present BRWF-CDLTS method has been verified by characterizing traps with discrete energy levels in He-implanted n-type silicon. It is demonstrated that the BRWF-CDLTS has some advantages over current DLTS in the unit of ampere [1] and is different from charge-based DLTS, employed to characterize traps in OLEDs [2]. The BRWF-CDLTS has been applied to characterize traps in the polymer-based OLEDs. The DLTS spectrum in the temperature range from 80 to 300 K reveals one peak around 210 K with the DLTS time constant of 9.1 ms. The apparent thermal emission activation energy is determined to be 0.21 eV from the Arrhenius analysis of the thermal emission time constants. The trap concentration is estimated to be 1.1x1016 cm-3 from the DLTS peak height. However, the DLTS spectra broaden over the temperature range from 100 to 300 K, which suggests the presence of several traps with different emission activation energies or defects bands. The detailed analysis of the broad DLTS spectra observed in the polymer-based OLED will be presented. The authors acknowledge Sumitomo Chemical Company Ltd. for supplying us with the polymer material. [1] Y. Tokuda and A. Usami, Japan. J. Appl. Phys. 22, 371 (1983). [2]C. Renaud and T-P Mguyen, J. Appl. Phys. 104, 113705 (2008).


11:45 AM U3.12
Photoresponsive Organic Thin-Film Transistors Using Photochromic Molecules.Emanuele Orgiu1, Nuria Crivillers1, Martin Herder2, Lutz Gruber2, Michael Paetzel2, Johannes Frisch3, Egon Pavlica4, Gvido Bratina4, Norbert Koch3, Stefan Hecht2 and Paolo Samori1; 1Institut de Science et d'Ingénierie Supramoléculaires (I.S.I.S.), Université de Strasbourg, Strasbourg, France; 2Department of Chemistry, Humboldt-Universität zu Berlin, Berlin, Germany; 3Institut für Physik, Humboldt-Universität zu Berlin, Berlin, Germany; 4Laboratory for Organic Matter Physics, University of Nova Gorica, Nova Gorica, Slovenia.

Photochromic systems are capable of undergoing efficient and reversible photochemical reactions, i.e. to switch between two (meta)stable isomers featuring markedly different properties. Such photochemically controlled bi-stable building blocks can then be employed to translate an incoming light stimulus into a macroscopic property change of the materials.[1] Among photochromic systems, diarylethenes (DREs)[2] are the most popular scaffolds in molecular electronics[3] because their open and closed isomers are associated with markedly different electronic properties, including the Highest Occupied Molecular Orbital (HOMO) - Lowest Unoccupied Molecular Orbital (LUMO) levels and gap and hence absorption, emission as well as redox characteristics. Such unique features were recently employed to change interfacial properties either by using a DRE-based multilayers as photochemically tunable hole-injection layer in an organic light-emitting diode (OLED)[4] or by employing a DRE interlayer between the organic semiconductor and the dielectric to fabricate a prototype of memory.[5] Promising electrical current switching properties achieved by combining DREs and organic semiconductors in blends were theoretically predicted.[6] Here we report for the first time the blending of a photochromic system with an organic semiconducting polymer, and used such a bi-component film as electroactive layer for organic thin-film transistors (OTFTs), to confer dual functionality to the device. Photoelectron Spectroscopy (PES) was used to characterized the energetics of the electroactive blend layer. A tunable-wavelength laser, working at low intensities, was employed to characterize the response times of the FET devices to the light stimuli. Our strategy is fully compatible with ink-jet printing and large-area exploitation towards a new generation of organic phototransistors. References [1] M.-M. Russew, S. Hecht, Adv. Mater. 22, 3348 (2010). [2] M. Irie, Chem. Rev. 100, 1683 (2000). [3] A. J. Kronemeijer et al., Adv Mater 20, 1467 (2008). [4] P. Zacharias, M. C. Gather, A. Kohnen, N. Rehmann, K. Meerholz, Angew Chem 48, 4038 (2009). [5] M. Yoshida et al., Jpn J Appl Phys 49, (2010). [6] F. L. E. Jakobsson et al., Journal of Physical Chemistry C 113, 18396 (2009).


 

SESSION U4: Device Physics
Chairs: Vitaly Podzorov and Jun Takeya
Tuesday Afternoon, November 29, 2011
Room 311 (Hynes)


1:30 PM *U4.1
Charge Transport Physics of High Mobility Molecular and Polymer Semiconductors.Henning Sirringhaus, University of Cambridge, Cambridge, United Kingdom.

Conjugated organic semiconductors offer new opportunities for the controlled manufacturing of active electronic circuits by a combination of solution processing and direct printing. In recent years both small molecule and conjugated polymer semiconductors with charge carrier mobilities above 1 cm2/Vs have been discovered. In this talk we will discuss recent insights into the charge transport physics of these materials with a particular focus on the microscopic processes that limit the field-effect mobility in these systems and discuss differences in the transport physics of such high mobility conjugated polymers and molecular semiconductors.


2:00 PM U4.2
Device Physics of Organic Ferroelectric Field-Effect Transistors.Jakob Jan Brondijk1, Kamal Asadi1,3, Paul Blom1,2 and Dago de Leeuw1,3; 1Molecular Electronics, Zernike Institute for Advanced Materials, University of Groningen, Groningen, Netherlands; 2TNO Holst Centre, Eindhoven, Netherlands; 3Philips Research Laboratories, Eindhoven, Netherlands.

The ability to store information is crucial to many of the envisioned applications of organic electronics. For example, RFID tagsneed to send, receive and store information. Therefore low-cost, non-volatile and rewritable memory is required. An organic ferroelectric field-effect transistor (FeFET) is an attractive candidate as a memory element. The ferroelectric gate insulator in a FeFET can be polarized and is bistable. Organic FeFETs have been demonstrated using the ferroelectric polymer poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)).[1,2] Depending on the direction of the polarization of the ferroelectric insulator, the conductance of the transistor channel can be programmed to a low or high conductance state. After removal of the gate bias the polarization remains, making a FeFET a non-volatile memory element. The conductance state can be read non-destructively at a low bias. Despite the technical advancement, the fundamental understanding of organic FeFETs is still in its infancy. Good understanding of the device physics is essential for performance improvement and a critical step toward commercial realization of FeFET memories. In this contribution we present for the first time a unified model which describes the polarization dependent charge transport in organic FeFETs. Unlike inorganic semiconductors, the mobility in organic semiconductors is not constant but depends on temperature and charge carrier density. We based our organic FeFET model on a variable range hopping model which was previously used successfully to model non-ferroelectric organic transistors. The unified FeFET model is developed by integrating a ferroelectric polarization model into the transport description. Ferroelectric capacitors and unipolar p-type FeFETs were fabricated and characterized. First, the capacitors were adequately modeled by the polarization model. The saturated polarization loops as well as the partially polarized loops were described. Next, transfer characteristics of the FeFETs were perfectly fitted by the model using parameters that were derived experimentally. We show that in a unipolar p-type FeFET only one polarization state is stable. The other polarization cannot be achieved due to lack of compensation charges (electrons for a p-type semiconductor). The model reproduces the current as a function of gate bias accurately. To further support this finding, we model ambipolar FeFETs. In an ambipolar FeFET both electron and hole transport are present. As a result, the transport in an ambipolar FeFET can be accurately modeled when both polarization directions are stable. [1] R. C. G. Naber et al., Advanced Materials, 22, 933 (2010) [2] S. J. Kang et al., Advanced Functional Materials, 19, 1609-1616 (2009)


2:15 PM U4.3
Interface State Density in Pentacene Thin-Film Transistors Analyzed by Field-Effect Thermally-Stimulated-Current Method: Variation by Air Exposure. Takahiro Fujii1, Shigekazu Kuniyoshi1, Masatoshi Sakai1, Kazuhiro Kudo1, Ryosuke Matsubara1,2 and Masakazu Nakamura1,2; 1Graduate School of Engineering, Chiba University, Chiba, Japan; 2Graduate School of Materials Science, Nara Institute of Science and Technology, Ikoma, Japan.

Gate insulator/organic interfaces in organic thin-film transistors (OTFTs) have carrier trap states, which greatly influence on the field-effect mobility and gate threshold voltage of OTFTs. However, quantitative and reliable study on the density and the energetic distribution of the interface trap states is still insufficient. It is partly because extrinsic traps tend to be unexpectedly formed by water and other ambient gases. Furthermore, discrimination between bulk and interface traps is generally difficult. In this work, we have therefore fabricated a special measurement system for field-effect thermally stimulated current (FE-TSC) measurements in an ultra-high vacuum chamber to selectively analyze the density of interface trap states as a function of electron energy. Bottom-contact type OTFTs were fabricated on bare and HMDS-treated SiO2/Si substrates. High-purity pentacene films were deposited on the substrate and FE-TSC was measured without breaking the vacuum or after the air exposure. For the FE-TSC measurements, trapping and detrapping of the carriers to the pentacene/insulator-interface states were controlled by the charging, or accumulating, gate voltage (VcG) and the discharging gate voltage (VdG), respectively. By sweeping the sample temperature during the discharging cycle, the detrapped holes were collected as a drain current. The depth of the interface trap was estimated from the initial activation energy of the TSC signal and the trap density was estimated by the difference of the integral intensity of the TSC signal against step-wise decrease of VcG. As the results of FE-TSC measurements in vacuum, the origin of the FE-TSC signal was concluded to be dominated by the interface trap states. The calculated density of trap states has a narrow distribution at around 0.08-0.1 eV above the HOMO edge almost independently of the chemical composition of the gate insulator surface. This isolated distribution of trap states is therefore concluded as an intrinsic one in pentacene polycrystalline film. Variation of the trap distribution by air exposure was quite different between the samples with bare and HMDS-treated SiO2/Si substrates. HMDS-treated samples showed only the quantitative change of the narrow trap distribution. On the other hand, completely new trap states became dominant in case of bare SiO2 samples. Detail of the energy spectra of the interface state density, its variation by air exposure, and influence on the transistor characteristics will be presented.


2:30 PM U4.4
Electric Field and Disorder Dependence of Meyer-Neldel Energy in Organic Field Effect Transistors.Mujeeb Ullah1, Almantas Pivrikas2, I. I. Fishchuk3, Andrey Kadashchuk4,5, Clemens Simbrunner1, Niyazi S. Sariciftci2 and Helmut Sitter1; 1Institute of Semiconductor and Solid State Physics, Johannes Kepler University Linz, Austria, Linz, Austria; 2Linz Institute of Organic Solar Cells (LIOS), Johannes Kepler University Linz, Linz, Austria; 3Institute for Nuclear Research, Natl. Academy of Sciences of Ukraine, Kyiv, Ukraine; 4IMEC, Leuven, Belgium; 5Institute of Physics, Natl. Academy of Sciences of Ukraine, Kyiv, Ukraine.

We systematically studied Meyer-Neldel rule behavior for charge carrier mobility measured in C60-based organic field-effect transistors (OFETs) [1,2,3] at different applied source drain voltages in semiconducting fullerene films with different disorder. A decrease in the characteristic Meyer-Neldel energy (EMN) from 36 meV to 32 meV was observed with changing electric field in the OFET channel. Concomitantly a decrease from 34 meV to 21 meV was observed too by increasing the grain size and the crystallinity of the active C60 layer in the device. These empiric findings are in agreement with the hopping-transport model for the temperature dependent OFET mobility in organic semiconductors with a Gaussian density of states (DOS) [3,4,5]. Experimental results along with theoretical descriptions are presented. References [1] Mujeeb Ullah, T.B. Singh, H. Sitter, N.S. Sariciftci, Applied Physics A, Materials Science & Processing 97 (2009), 521. [2] Mujeeb Ullah, A. Pivrikas, I. I. Fishchuk, A. Kadashchuk, C. Simbrunner, P. Stadler, N. S. Sariciftci and H. Sitter, APL 2011(accepted). [3] Mujeeb Ullah, I. I. Fishchuk, A. Kadashchuk, P. Stadler, A. Pivrikas, C. Simbrunner, V. N. Poroshin, N. S. Sariciftci, and H. Sitter, Appl. Phy. Lett. 96,213306 (2010). [4] I. I. Fishchuk, A. K. Kadashchuk, J. Genoe, Mujeeb Ullah, H. Sitter, Th. B. Singh, N. S. Sariciftci, and H. Bässler, Phys. Rev. B 81, 045202 (2010). [5] I. I. Fishchuk, A. Kadashchuk, Mujeeb Ullah, H. Sitter, A. Pivrikas, J. Genoe and H. Bässler, PRB 2011(submitted)


2:45 PM U4.5
Organic Electronic Ratchets as Efficient Brownian Motors.Martijn Kemerink1, Erik Roeling1, Wijnand Germs1, Barry Smalbrugge2, Erik Jan Geluk2, Tjibbe de Vries2 and René Janssen1; 1Applied Physics, Eindhoven University of Technology, Eindhoven, Netherlands; 2COBRA Research Institute, Eindhoven University of Technology, Eindhoven, Netherlands.

Recently, we developed an organic electronic ‘ratchet’ device that may be considered the DC equivalent of the AC transformer. [1] The ratchet is based on a modified organic field effect transistor (OFET) and employs a periodic but spatially asymmetric potential to rectify a non-directed AC drive. Ratchet systems were introduced by Smoluchowski and later Feynman [2] and seem to violate the second law of thermodynamics. However, they operate in a non-equilibrium regime where the second law no longer applies. In this work, we address both fundamental and practical aspects of organic electronic ratchets. We show that they should be considered as true Brownian motors, as they rectify random thermal motion. They do this at remarkable efficiencies; the charge displacement and power efficiencies can be as high as 50% and 7%, respectively. Our ratchet is based on an OFET in which asymmetrically spaced interdigitated finger electrodes are embedded in the gate dielectric. An oscillating asymmetric potential is applied to the accumulated charges in the OFET channel via these finger electrodes. The DC output current at the source and drain electrodes is measured as a function of drive frequency, drive amplitude and gate bias for devices that vary in asymmetry and number of repetitions of the (asymmetric) potential repeat unit. The results are interpreted using a full 2D drift-diffusion model. The ratchets operate up to RF frequencies at room temperature and generate currents and voltages that are orders of magnitude larger than previously reported ratchets. In addition, the device characteristics have maintained the intriguing, highly non-linear behavior that is typical of ratchets. In particular, we show that the occurrence of current reversals with driving frequency results from a complex interplay between drift, diffusion, the asymmetry of the repeat unit and the termination by the electrical contacts. By using the scaling relations that are derived from the experiments, the geometry and drive can be optimized to yield power efficiencies (defined as useful output power over total power supplied to the system via the electrodes) up to 7%. In terms of practical applications, we foresee the use of ratchet based receivers in wireless power transmission to low-power devices as (radiofrequency identification) tags, implants and sensors. We demonstrate the use of our ratchets by powering a simple logic circuit with them. [1] E.M. Roeling et al, Nat. Mater. 10, p51 (2011). [2] M. v. Smoluchowski, Physikalische Zeitschrift 13, p12 (1912); R.P. Feynman et al., ‘The Feynman lectures on Physics’, Addison-Wesley, 1989.


3:30 PM U4.6
Universal Scaling in Highly Doped Conducting Polymer Films.Auke Jisk Kronemeijer1,2, Eek H. Huisman1,3, Ilias Katsouras1, Paul A. van Hal4, Tom C. T. Geuns4, Paul W. M. Blom1,5, Sense J. van der Molen6 and Dago M. de Leeuw1,4; 1Molecular Electronics, Zernike Institute for Advanced Materials, University of Groningen, Groningen, Netherlands; 2Optoelectronics Group, Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom; 3Center for Electron Transport in Molecular Nanostructures, Columbia University, New York, New York; 4Philips Research Laboratories, Eindhoven, Netherlands; 5TNO Holst Centre, Eindhoven, Netherlands; 6Kamerlingh Onnes Laboratory, University of Leiden, Leiden, Netherlands.

A fundamental and still unresolved question concerns the nature of electrical transport in conjugated polymers. Polymer chains can be regarded as one-dimensional systems while disorder in polymer thin films yields localization of carriers, leading to hopping conduction resembling a three-dimensional system. Recently, charge transport in the conjugated polymer PBTTT has been shown to obey scaling onto a universal scaling curve described by the Luttinger Liquid theory of one-dimensional metals [1]. To address the influence of dimensionality we studied charge transport as a function of bias and temperature in highly-doped PEDOT:PSS. While PEDOT:PSS cannot be regarded as a one-dimensional Luttinger system due to the morphology of the thin films, it is shown that the electrical transport nevertheless exhibits universal power-law scaling [2]. The complete J(V,T) characteristics are described by a single equation using two fit parameters. Possible origins of the universal behavior, such as dissipative tunneling processes, are discussed. [1] J. D. Yuen, R. Menon, N. E. Coates, E. B. Namdas, S. Cho, S. T. Hannahs, D. Moses, A. J. Heeger. Nature Materials 8, 572 (2009). [2] A. J. Kronemeijer, E. H. Huisman, I. Katsouras, P. A. van Hal, T. C. T. Geuns, P. W. M. Blom, S. J. van der Molen, D. M. de Leeuw. Phys. Rev. Lett. 105, 156604 (2010).


3:45 PM U4.7
Low Voltage, Lithographically-Processed, Pentacene-Based Thin Film Transistors.Melissa A. Smith2,1 and Akintunde I. Akinwande3,1; 1Microsystems Technology Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts; 2Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts; 3Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts.

In this work, we present the fabrication of low voltage (VDS and VGS at ~5V) integrated circuits based on dual threshold voltage pentacene thin film transistors (TFTs) using a near-room-temperature (≤ 95°C) completely lithographic process that is compatible with both flexible and rigid substrates. As photolithography uses various solvents, it is typically incompatible with organic semiconductors which are highly sensitive to moisture. As a result, shadow-masking and similar patterning techniques are often used to demonstrate the efficacy of a technology. However, these techniques may lack the precision required for building large-scale integrated circuits. As reported in literature, many devices fabricated via these processes often suffer from poor reproducibility from misalignment and low performance due to large overlap capacitances. One way to increase drive currents is to accumulate more charge in the semiconductor channel. To minimize the gate voltage required to accumulate charge, a material with a large dielectric constant (κ) as the gate insulator or a thinner gate insulator can be used. Pentacene-based TFTs typically use organic gate insulators as these surfaces enable optimal charge transport at the dielectric/semiconductor interface. However, these materials generally have a low-κ, which necessitates larger operating voltages (> 20V) in most cases. Further, thinning these insulators can lead to significant gate leakage and poor reliability. It is well documented that high-κ insulator materials are effective in reducing operating voltages in TFTs while minimizing current leakage through the gate. We have developed a process with a high-κ gate insulator that is compatible with pentacene. Specifically, BZN (Bi1.5Zn1Nb1.5O7) is a paraelectric pyrochlore system that features a high dielectric constant (~40), which can be deposited at room temperature through RF sputtering. For semiconductor patterning, a conformal organic material, parylene-C (deposited via chemical vapor deposition at room temperature) is used to protect the pentacene from moisture during photoresist spin-coating and development. The parlyene-C and pentacene are then etched in an oxygen plasma. It is shown that the threshold voltage of pentacene-based TFTs fabricated with BZN can be shifted from positive (~2V) to negative (~-0.5V) by modifying the dielectric/semiconductor interface with a thin layer (< 8nm) of parylene-C. These characteristics make it a viable insulator for improving pentacene-based TFT performance and enabling advanced circuit design based on dual threshold voltage TFTs. TFT electrical performance is evaluated through standard current-voltage and quasi-static capacitance-voltage measurements. AFM, XRD, and SEM are used to analyze and characterize materials and processes.


4:00 PM U4.8
Long-Range Electron Transport Ability of Ru-Complex Multilayer on ITO Surface.Takao Ishida1, Kei-ichi Terada1, Hisao Nakamura1, Katsuhiko Kanaizuka2, Masa-aki Haga3 and Yoshihiro Asai1; 1NRI-AIST, Tsukuba, Ibaraki, Japan; 2Deapertment of Chemistry, Yamagata Univ, Yamagata, Yamagata, Japan; 3Department of Applied Chemistry, Chou Univ, Bonkyo-ku, Tokyo, Japan.

Formerly, electron transfer through molecular layer or organic semiconductor has been extensively studied, as one of the most fundamental and important properties of molecular layer. Long-range electron-transport ability of redox-active molecular multilayer greater than 20 nm has been found recently. Here we discuss a microscopic mechanism of the small β problem experimentally focusing a special role of metal nuclear centers. We use a free-standing Ru complex with eight phosphonic acid groups which is designed to stand perpendicular to the surface by intramolecular steric repulsion. Our Ru-complex has long π-conjugated chain as well as two Ru metals asredox centers. The distance between the two Ru atoms is expected to be ca 1.5 nm, thus, we expect that our Ru-complex multilayer film also enable to exhibit a long-range electron-transport ability because molecular unit layer structure resembles to that of long-range electron-transport redox-active Co or Fe complexes as previously reported. Before transport measurements through the Ru-complex multilayer, we confirmed the Ru-complex layer growth with XPS. At pH3, Ru-complex multilayer was spontaneously formed via hydrogen bonds. The β value was estimated by solid state sandwiched cells having PEDOT:PSS/Ru-complex layer/ITO junction. We confirmed the small β through the Ru complex multilayers of 0.009- 0.011 Å-1, indicating that our Ru complex layers on ITO surface exhibit long-range electron-transport abilities. Based on the results of theory-experiment collaboration, we propose a modified tunneling mechanism which we named “stepping stone mechanism”.


4:15 PM U4.9
Charge Trapping by Self-Assembled Monolayers Demonstrated as the Origin of the Threshold Voltage Shift in Organic Field-Effect Transistors.Fatemeh Gholamrezaie1,4, Anne-Marije Andringa1, Christian Roelofs2, Alfred Neuhold3, Martijn Kemerink2, Paul Blom5 and Dago de Leeuw4; 1Organic semiconductor, University of Groningen, Groningen, Netherlands; 2University of Eindhoven, Eindhoven, Netherlands; 3Graz University of Technology, Graz, Austria; 4Philips, Eindhoven, Netherlands; 5holst center, Eindhoven, Netherlands.

Crucial for any application of organic field-effect transistors is the threshold voltage (Vth). This voltage should be set at a given value and, furthermore, be identical for all devices in a circuit. Any deviation yields a reduced gain of logic gates, a decreased noise margin of integrated circuits. For standard Si transistors, the threshold voltage can be accurately set by the amount of doping applied by ion implantation. In organic transistors, however, local doping of individual transistors is not an option. To get around this constraint and to externally set Vth, several options have been published. A suggested method to set the threshold voltage is by application of a self-assembled monolayer (SAM) on the gate dielectric. [1] The value of the threshold voltage has been tuned by applying a SAMs on the gate dielectric and the shift depends on the type of SAM. The shift has tentatively been explained by the macroscopic dipole moment of the SAM molecules. However, device modeling has shown that the shifts are too big [2] and the microscopic origin is still under debate. A recent publication [3] stated that an experimental demonstration to accurately explain the voltage shifts is still lacking. We fabricated field-effect transistors with various SAMs on the gate dielectric. The threshold voltage did vary over tens of volts. To investigate the origin we delaminated the semiconductor after electrical characterization and probed the surface potentials of the revealed SAMs with SKPM (Scanning kelvin probe microscopy). The surface potentials of the stripped devices perfectly correspond with the value of the threshold voltage. This agreement clearly shows that the shift is not due to a dipolar contribution but due to charge trapping by the SAM.[4] The origin of the charge trapping and the temporal behavior will be discussed. [1] Kobayashi, S.et al. Nature Materials, 2004 ,3, 317. [2] Possanner, S. K.; Zojer, K.; Pacher, P.; Zojer, Schurrer F. Adv. Funct. Mater. 2009, 19, 958. [3] Chung, Y.; Verploegen, E.; Vailionis, A.; Sun, Y.; Nishi, Y.; Murmann, B.; Bao Z. Nano Lett. 2011,11,1161. [4] Gholamrezaie, F.et al. Submited to Nano Letters, 2011.


4:30 PM U4.10
Clustering of MoO3 Doped into Organic Thin Films Studied by TEM, AFM and IR Spectroscopy.Daniela Donhauser1,2, Michael Kroeger1,2, Katrin Schultheiss1,2, Martin Pfannmoeller3, Rasmus R. Schroeder3,2, Tobias Glaser4,2, Sven Tengeler4,2, Milan Alt4,2, Annemarie Pucci4,2, Bernd Lunkenheimer5,2, Andreas Koehn5 and Wolfgang Kowalsky1,2; 1Institut für Hochfrequenztechnik, Technische Universität Braunschweig, Braunschweig, Germany; 2InnovationLab GmbH, Heidelberg, Germany; 3CellNetworks, Universität Heidelberg, Heidelberg, Germany; 4Kirchhoff-Institut für Physik, Universität Heidelberg, Heidelberg, Germany; 5Institut für Physikalische Chemie, Johannes Gutenberg-Universität Mainz, Mainz, Germany.

Electrochemical doping is essential to overcome limitations in organic devices imposed by low intrinsic conductivity and high injection barriers at the contacts. It has been shown that MoO3 has very strong accepting properties and serves well as a p-dopant in a variety of organic materials. Nonetheless doping efficiencies (charge carrier per MoO3 unit) tend to be rather low and have been cited in the range of 1% or even below [1]. We present a comprehensive study using AFM, IR spectroscopy and TEM to investigate the physical origin of the low doping efficiency. AFM-measurements of the neat CBP and MoO3 films reveal an amorphous film structure. Upon doping of CBP with MoO3 lateral feature sizes as wells as the roughness of the observed thin films decrease. Our results are in accordance with previous results upon MoO3 doped NPB [2]. FTIR-spectra of the intrinsic and doped films were compared to DFT calculations and the strongest absorption bands were assigned to the corresponding vibrational modes. It can be expected, that larger fractions of MoO3 agglomerate within the organic matrix during the evaporation process. The appearance of new peaks is likely to originate from ionized CBP molecules resulting from electron transfer towards the MoO3 dopant. To gain information about the lateral distribution of the MoO3 clusters electron spectroscopic imaging is used. The energy loss peak of the Mo3p1/2 transition allows mapping of the local distribution of MoO3 within the organic matrix. At present we investigate the nano-clustering of the MoO3 dopant as also seen in literature by bright field TEM [3]. We conclude from IR and TEM results, that MoO3 has a strong tendency to form nano-clusters when codeposited with organic materials such as CBP. Under these conditions it becomes clear, that the effective density of doped carriers must be lower than the number one would expect for the case of ideally dispersed MoO3 dopants. [1] Kröger et al., Org. Electronics 10, 932 (2009) [2] Wang et al., Org. Electronics 9, 985 (2008) [3] Lee et al., Org. Electronics 12, 950 (2011)


 

SESSION U5: Organic Single-Crystalline Structures and Properties I
Chairs: Alex Briseno and Vitaly Podzorov
Wednesday Morning, November 30, 2011
Room 311 (Hynes)


8:00 AM U5.1
Photoelectron Yield Spectroscopy of Anthracene Single Crystal: How Does Surface Roughness Affect on the Electronic Structure? Hiroumi Kinjo1, Naoki Ogawa1, Yasuo Nakayama2 and Hisao Ishii1,2; 1Graduate School of Advanced Integration Science, Chiba University, Chiba-shi, Japan; 2Center for Frontier Science, Chiba University, Chiba-shi, Japan.

The electric properties of organic electronics are dominated not only by intrinsic electronic structure such as HOMO and LUMO, but also by extrinsic factors like gap-state and defect state in practical devices. It has been widely believed that structural disorder induces broadening of intrinsic energy level and the formation of gap-state. But the direct observation of such a gap-state is still limited, and the origin and formation mechanism are not well understood. In order to investigate such effects, the direct comparison in electronic structure between well-ordered and disordered systems should be desired. In this study, we have investigated the electronic structures of very flat and rough surfaces of anthracene single crystals (SC) to examine the roughness-induced effect to the electronic structure. Because anthracene has high vapor pressure, sublimation-induced morphology change is expected, and this system is suited for the above purpose. The measurement of electronic structure was performed with current-detection type photoelectron yield spectroscopy (PYS) . This technique is tough against sample charging problem, and can be applied for insulating thick sample like organic single crystals. Atomic force microscope measurements revealed that (i) the surface is very smooth with clear terrace-step structure, (ii) the sublimation of anthracene molecure in air slowly induces further surface smoothness, (iii) the sublimation in vacuum drastically induces very rough surface, (iv) UV irradiation in PYS measurement in vacuum extremely accelerates the molecular desorption. Thus we can control the surface morphology by changing the atmosphere. PYS spectra of anthracene SCs in air showed clear threshold structure, and Ionization energy estimated from this onset was 5.8eV. For a rough surface, PYS spectra taken in vacuum showed a tail structure on the lower energy side. This result suggests that surface-disorder can induce gap-state ranging to 400meV above the HOMO edge. The derivative of yield curve of PYS, which roughly reflects the density of states at valence top reigon, showed clear shoulder and peaks. These structures well correspond to the calculated band structure. In the presentation, the formation mechanism of gap-state will be discussed with the result of UV photoemission spectroscopy. This research is granted by the Japan Society for the Promotion of Science (JSPS) through the “Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST Program),” initiated by the Council for Science and Technology Policy (CSTP).


8:15 AM U5.2
Short-Channel and High-Mobility p- and n-Type Organic Single-Crystal Transistors with Air-Gap Structures.Mayumi Uno1,2, Takafumi Uemura2, Kazumoto Miwa2, Antonio Facchetti3 and Jun Takeya2; 1TRI Osaka, Izumi, Osaka, Japan; 2ISIR, Osaka Univ., Ibaraki, Osaka, Japan; 3Polyera Corporation, Skokie, Illinois, Illinois.

With recent development of high-mobility organic semiconductors, it is getting more realistic to argue possible applications to high-speed logic-circuit components, which would provide a lot of attractive devices such as flexible driver circuits in full-flexible displays, REID tags, and light-weight and rollable processing arrays for wearable computers. Here we employ organic single-crystal field-effect transistors (SC-OFETs) with air-gap structures to investigate intrinsic high-frequency carrier dynamics in organic semiconductors, minimizing influences of grain boundaries and trap levels at the semiconductor / insulator interfaces. Rubrene SC-OFETs were reported to exhibit very high carrier mobility exceeding 10 cm2/Vs, though they were not suitable for high-speed applications because of the long channels above 100 μm. Since the maximum operating frequency of an OFET is proportional to the transconductance gm and inversely proportional to the square of the channel length L, it is ideal to realize high gm in very short-channel configurations, which is still a challenge because of large contact resistance between organic semiconductors and electrodes. In this presentation, we report both p-type and n-type air-gap SC-OFETs with the channel lengths as short as several μm. The air-gap structures were fabricated using SiO2/Si by partially etching Si substrates and oxidizing their surfaces. After depositing source, drain and gate electrodes with gold from strictly normal direction to the substrates, thin platelets of rubrene (for p-type) and PDIF-CN2 (for n-type) single crystals were placed on the structures. FET measurements were carried out in ambient condition for rubrene OFETs, and in anaerobic conditions for PDIF-CN2 OFETs. Interestingly, the effect of contact resistance turned out to be minor because the transconductance increases almost inversely proportional to L^2 down to the lengths of 6 μm. The observation is contrasting to that in conventional devices with solid gate dielectrics, in which the increase in the transconductance was much less rapid. As the result, hole transport with the mobility as high as 10 cm2/Vs is demonstrated in the rubrene devices and electron transport with the mobility of 2.3 cm2/Vs is achieved in PDIF-CN2 devices even for the channel length of 6 μm. The maximum switching frequencies of these transistors can be calculated to be as high as 44 MHz and 10 MHz, respectively, when drain voltage of 10 V is applied if one can neglect parasitic capacitance. We speculate that the excellent maximum speed is attributed to highly oriented molecular arrangement in the very vicinity of the interfaces of electrodes and organic semiconductors, which is specific to the present devices with laminated single crystals on the air-gap structures. The present experiments demonstrate that even higher speeds can be realized with shorter devices for future applications to organic complementary circuits.


8:30 AM U5.3
Magnetic Field Effect on Photoconductivity of Single-Crystalline Pentacene and Perfluoropentacene Field-Effect Transistors.Toan S. Pham, Yoshitaka Kawasugi and Hirokazu Tada; Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka, Japan.

An intrinsic magnetic-field-dependent current observed in organic semiconductors, so-called, organic magnetoresistance (OMAR) is of great scientific interest since it has several interesting properties that make it unique compared to other magnetoresistive effects. The OMAR effect can have relatively large magnetoresistance (MR) value (up to 10%) at room temperature and small magnetic fields of ~10 mT without using ferromagnetic contacts. Many models are suggested to explain this effect, but debate still exists about its nature [1,2]. The common device structure used for studying OMAR is two terminal devices. Comparing with two-terminal devices, three-terminal devices have advantage that the mobility and density of charge carriers can be determined independently. Only a few works have done about MR on organic field-effect transistors (FET) [3,4]. In this work, we used high mobilities single-crystalline pentacene (C22H14) and perfluoropentacene (C22F14) to study OMAR effect. Single crystals, with typical diameters around 1 mm, were grown by conventional physical vapor transport under a flow of pure nitrogen gas [5]. We selected pieces of single crystals with the thickness less than 500 nm to make the bottom-contact FET by laminating on n+-Si/SiO2 substrate with patterned gold electrodes. The photo-induced negative MRs of 0.5% at magnetic field of 75mT can be observed and it strongly depends on the gate voltage. References [1] Hu B., Yan L. & Shao M. Advanced Materials, 21(14-15), 1500-1516 (2009). [2] Wagemans W. & Koopmans B. Physica Status Solidi (B) 13, 1-13(2010). [3] Nishioka M., Lee Y.-B., Goldman A M., Xia Y. & Frisbie C. D. Applied Physics Letters, 91(9), 092117(2007). [4] Reichert T. & Saragi T. P. I. Applied Physics Letters, 98(6), 063307(2011). [5] Laudise R. Journal of Crystal Growth 187, 449-454(1998).


8:45 AM U5.4
Effect of Contact Asymmetry and Diffusion on the Space Charge Limited Current in Organic Single Crystals.Javier Dacuna1 and Alberto Salleo2; 1Electrical Engineering, Stanford University, Stanford, California; 2Materials Science and Engineering, Stanford University, Stanford, California.

One of the major obstacles that hinders the development of novel organic semiconductors is the lack of a fundamental understanding of what limits their charge transport properties. Localized states, or traps, within the transport band gap have been shown to strongly affect their performance. A reliable characterization of the energetic trap distribution is of the utmost importance to fully understand the relation between charge transport and disorder, chemical or morphological defects and impurities, which are common causes of these localized states. Indirect measurements of the trap distribution can be obtained by analyzing its effect on the current-voltage characteristics of electronic devices by means of a charge transport model. The accuracy and reliability of the obtained parameters will depend on the ability of the model to completely characterize the measured device. In this work we develop and apply a mobility edge model that takes into account drift and diffusion currents to characterize the space charge limited current in a hole-only rubrene single crystal device. In addition, the model allows the utilization of asymmetric contacts, thus describing the effect of the built-in potential in the device. An energy offset of 570 meV between the injecting and extracting contact, in agreement with photoelectron spectroscopy measurements and ascribed to differences in the deposition techniques (lamination vs. evaporation), was essential to correctly interpret the shape of the current-voltage characteristics. The model is able to extract quantitative values of mobility and density of states. In particular it predicts a band mobility of 0.12 cm^2/Vs; such low value is not surprising as the SCLC is measured in the “slow” transport direction of the crystal. The energetic distribution of traps was modeled as a Gaussian centered at 156 meV above the HOMO level with a total density of states equal to 1.7x10^16 cm^-3 and a standard deviation of 48 meV. The analysis of the sensitivity of the model to the different regions of the density of states allows us to establish limits on the accuracy of the obtained parameters, directly related to the incorporation of the diffusion current term, and shows that it is not possible to resolve the shape of the trap distribution for energies higher than 0.3 eV above the VB edge.


9:00 AM *U5.5
Ionic Liquid Gating and Charge Transport at High Carrier Densities in Organic Semiconductor Single Crystals and Films.Daniel Frisbie, Chemical Eng & Materials Sci., University of Minnesota, Minneapolis, Minnesota.

This talk will describe the results of variable temperature transport measurements on single crystals and films of organic semiconductors at very high states of charge. Surface charge densities above 0.3 holes per unit cell in rubrene crystals and 0.3 holes per monomer in polythiophene films can be obtained readily by using ionic liquid gates in a transistor configuration. Both crystals and films exhibit a pronounced peak in conductivity versus charge density (or gate voltage) that is approximately independent of the type of ionic liquid. Explanations for this unusual behavior will be discussed in the context of both variable temperature and magnetoresistance results. In addition, careful analysis of the ionic liquid gate capacitance and real surface charge density will be described. The talk will end with a discussion of future prospects for employing ionic liquid gating to understand fundamental transport phenomena in organic semiconductors.


9:30 AM *U5.6
The Structural Origin of Traps in Organic Semiconductors: Disorder, Grain-Boundaries and Transport Regimes.Albert Salleo, Department of Materials Science and Engineering, Stanford University, Stanford, California.

From the fundamental standpoint, organic semiconductors are fascinating as they are neither crystalline nor amorphous and their microstructure plays a central role in governing charge transport. I will show that understanding disorder is the key to determining charge transport mechanism. Using advanced synchrotron-based X-ray characterization techniques we are able to define and measure structural order at different length-scales. We are particularly interested in cumulative disorder (paracrystallinity), where the lattice parameter takes on a Gaussian distribution about its mean value. The disorder parameter g allows us to rank materials quantitatively on a continuous scale, from a perfectly crystalline material (g<1%) to an amorphous one (g>10%). Surprisingly, even the polymers that are considered to have the highest crystallinity (PBTTT) have a g in the π-stacking direction close to that of an amorphous material (~7%). Using first principle calculations and tight binding methods, I will show that paracrystallinity in the π stacking direction provides a fundamental mechanism for the existence of an exponential distribution of localized tail states in the gap. The larger the degree of disorder the higher the trap density and the deeper their energy. Using disorder as a ranking parameter, I will discuss the differences in transport between small molecule and polymer films as well as their respective inherent limitations and bottlenecks. This work may help devising design rules for new materials with desirable transport properties for polarons and excitons.


10:30 AM *U5.7
From Photoexitation to Photocurrent in Rubrene: The Role of Singlet and Triplet Excitons.Ivan Biaggio, Physics, Lehigh University, Bethlehem, Pennsylvania.

It is a characteristic of rubrene that only a minority of the singlet excitons created by the absorption of visible light will directly emit luminescence or dissociate into charge carriers. Studies of photoluminescence dynamics indicate that an isolated singlet exciton in rubrene has a significantly larger probability to undergo fission into two triplet excitons. This efficient creation of triplet excitons has several dramatic consequences, among them efficient long-range exciton diffusion and peculiar photoluminescence and photocurrent dynamics after impulsive photoexcitation. This work discusses exciton dynamics and its implications for the impulsive generation of photocurrent in rubrene, which is clearly dominated by a delayed component with a build-up time of up to 100 microseconds at illumination intensities in the visible spectral range and at moderate excitation densities. In this regime a small current can appear within 10 ns after excitation, corresponding to fast photocarrier release from photoexcited singlet, but this fast contribution to the current is about 10 times smaller than the delayed one. The experimental conditions under which rubrene’s dominant delayed photocurrent is observed correspond to a low excitation density, which is important when comparing to other experiments in the literature that reported a fast release of charge carriers. The observed dynamics of delayed photoluminescence and delayed photocurrent indicate that the latter must originate from triplet exciton dissociation. In fact, a simple analysis based on triplet-triplet interaction correctly predicts how higher excitation densities lead to both a shortening of the build-up time of the delayed photocurrent and a saturation of its amplitude. These effects make rubrene a particularly interesting example of an efficient path leading from photon absorption to charge carrier release through singlet exciton fission, triplet diffusion, and triplet dissociation.


11:00 AM U5.8
Effect of Phase Inhomogeneity on Photoluminescence in Rubrene.Yuanzhen Chen1, Bumsu Lee1, Danni Fu1 and Vitaly Podzorov1,2; 1Physics and Astronomy, Rutgers University, Piscataway, New Jersey; 2Institute for Advanced Materials and Devices for Nanotechnology, Rutgers University, Piscataway, New Jersey.

Phase inhomogeneity in organic semiconductors is a common phenomenon that originates from natural phase coexistence present during material growth. For example, in physical vapor transport growth of molecular crystals, vapor, liquid (amorphous melt) and solid (crystalline) phases coexist in a relatively small volume of the growth apparatus. As a result, an amorphous phase may become trapped in a growing crystal. Differences in the physical properties of different phases might be responsible for substantial variations in the material characteristics, such as photoluminescence, frequently observed in nominally identical samples of organic semiconductors. Such variability hinders understanding the intrinsic properties of each individual phase. In this study, we experimentally investigate such phenomenon in a benchmark organic semiconductor, rubrene, known for its high charge carrier mobility, long-range exciton diffusion, and other interesting photo-physical properties. In spite of the large amount of work carried out on rubrene, many fundamental questions remain open and are under intense debate. One particularly interesting and important issue is the role of oxygen in determining the electrical and optical characteristics of rubrene. Previous studies have suggested that oxygen can dope rubrene by introducing band gap states. Certain features, for example, a photoluminescence band at 650 nm, have been assigned to oxidation. Our results, however, suggest that this band is not related to chemical impurities, such as oxygen, but rather it is due to recombination of molecular excitons intrinsic to amorphous phase of rubrene present in small amounts in rubrene single crystals. In most of the single-crystal samples with photoluminescence dominated by the 650-nm band, superior charge and exciton transport properties are preserved, indicating that the amorphous phase constitutes a very small fraction of the crystals. Strong quenching of the 650-nm band by surface-restricted functionalization of the crystals with an exciton quencher suggests that triplet excitons, migrating throughout the crystal, optically “pump” the amorphous inclusions by transferring energy to these defects, hence giving rise to an efficient 650-nm photoluminescence. This model of triplet excitons “feeding” the defects, or defects acting like optical “antennae” harvesting mobile triplets, naturally explains how a very small concentration of defects can give rise to very prominent features in photoluminescence spectra of organic materials. This mechanism also provides another confirmation that exciton diffusion length in highly crystalline rubrene is very large: indeed, without a long-range exciton diffusion, the effect of optical “pumping” of a few distributed defects would not be possible.


11:15 AM U5.9
Direct Imaging of Large Exciton Diffusion in the Organic Molecular Crystal Rubrene.Pavel Irkhin and Ivan Biaggio; Physics, Lehigh University, Bethlehem, Pennsylvania.

We developed a direct imaging technique allowing us to visualize the spatial distribution of excitons in molecular single crystals. By imaging the photoluminescence emitted by diffusing excitons under localized photoexcitation conditions on well-defined crystal surfaces, we investigate the exciton diffusion along different crystallographic directions. In rubrene single crystals, we observed a strong anisotropy in the mobility of the excitons, which is large for the same crystallographic direction where higher charge carrier mobilities were observed. In particular, photoexcited singlet excitons in rubrene are known to undergo fission to create long-lived triplet excitons with a lifetime of the order of 100 microseconds, and the triplet excitons themselves are known to be able to fuse to re-create singlet excitons that can then radiatively recombine to emit the luminescence light. From our diffusion data, we have been able to extract a large triplet diffusion length of 4.0 ± 0.4 micrometers along the crystallographic direction of closest molecular packing, with no relevant diffusion in the others. The imaging technique we have demonstrated should fundamentally allow the investigation of exciton diffusion under different experimental conditions and in different materials, and will be used to further investigate exciton transport and dynamics in rubrene. This is interesting because exciton dynamics in organic molecular crystals is a key to uncovering the role of intermolecular interactions in exciton transport, and is of prime interest towards the optimization of photocarrier generation in organic photovoltaics, where exciton diffusion is a key process limiting organic photovoltaic efficiency.


11:30 AM U5.10
Fast Transient Photocurrent of Organic Semiconductors: From Modelling to Devices. Heinrich Diesinger1, Majid Panahandeh Fard2, Zilong Wang2, Emmanuel Sevin1,2, Dominique Baillargeat1 and Cesare Soci1,2; 1CINTRA CNRS/NTU/THALES, UMI 3288, Singapore, Singapore; 2Physics and Applied Physics, Nanyang Technological University, Singapore, Singapore.

The sub-nanosecond carrier dynamics of organic semiconductors is crucial for devices working at steady-state, in particular polymer photovoltaics used in bulk heterojunctions, since competing processes occurring immediately after charge carrier photogeneration have a tremendous effect on their efficiency. Fast transient photocurrent measurements in the picosecond time domain can be used to study polaron photogeneration, thermalization, trapping, transport and recombination. The photocurrent response obtained in these measurements depends on both, the response of the active material and the response of the switch consisting of the electrodes and the transmission line access. A hybrid electromagnetic and circuit model has been developed to decompose the characteristics of the switch into the intrinsic response of the material and the response due to the transmission line, allowing the extraction of the intrinsic properties of the active region from the experimentally observed transients. Subsequently, the geometry of the photoconductive switch can be optimized to increase the overall bandwidth of the measuring system: examples of photoconductive switches with bandwidth of more than 70 GHz and suitable for fabrication on organic semiconductors (incompatible with standard lithography) will be given. Transient photocurrent measurements in different organic semiconductor systems including photoconductive polymers and molecular crystals will be discussed and used to validate the model.


11:45 AM U5.11
A Solution-Based Method for Self-Organized Organic Single Crystal Arrays with Controlled Crystal Orientation. Akichika Kumatani1, Takeo Minari1, Chuan Liu1, Yun Li1,2, Peter Darmawan1,2, Kazuo Takimiya3 and Kazuhito Tsukagoshi1,2; 1MANA, NIMS, Tsukuba, Japan; 2CREST, JST, Kawaguchi, Japan; 3Institute for Advanced Materials Research, Hiroshima University, Higashi-Hiroshima, Japan.

The ongoing proliferation of electronic devices and growing concerns about increasing environmental burdens are leading to demands for solution-based fabrication methods. The use of soluble organic semiconductors enables fabrication of plastic electronics devices. Here, we report the direct formation of solution-processed organic single crystal arrays with controlled orientation at the desired area. We first patterned the substrate surface into regions that were wettable and unwettable using a fluoropolymer, Cytop. An anisole solution of dioctylbenzothienobenzothiophene (C8-BTBT) and poly(methylmethacrylate) (PMMA) was applied to the substrate. This resulted in trace amounts of the semiconductor solution entering the wettable trenches. In order to realize oriented growth of organic crystals, we employed polymer assisted solvent vapor annealing (PASVA). The PASVA facilitated crystallization of C8-BTBT on PMMA surface in each trench. This enables organic molecules to reorganize into single crystals. The restriction of the growth area size allows crystals to self-organize the direction of crystal orientation during PASVA. An analysis of the optical micrographs showed that 83% of the 270 trenches were occupied by crystals, 98% of which were aligned in the same crystal direction [100], confirmed by polarized optical microscopy with the diagonal and extinction positions. The crystal length was typically longer than 50 μm, with the largest being longer than 600 μm. For the case of two perpendicular trenches on a tilted substrate, the crystals grew equally well within both trenches, indicating no dependence on substrate inclination. Similar results were also found for a tilted substrate having eight trenches along different directions. These results clearly indicate that crystals were self-organized along the trench direction, and substrate inclination does not have any effect. For more information on the crystal alignment mechanism, we observed the crystal growth process during PASVA using real-time video. The crystal gradually oriented itself along the trench direction as it grew longer. One minute after the PASVA process started, a crystal was almost aligned perpendicular to the trench direction. As time advanced, the ends of the crystal reached the edges of the trench, and then started to change the growth direction along the trench direction. Thus, the PASVA process causes strong alignment of the crystal along the trench direction. Based on this technique, we also have fabricated organic single crystal field effect transistors. The self-organized C8-BTBT single crystals towards crystal direction [100] imply that the charge transport via this crystal direction is minimal of π-π stacking, which expects to give high device performance. The device has shown the typical field effect mobility of 3.4 cm2V-1s-1. Therefore, our technique indicates to be applicable for practical devices with organic single crystals.


 

SESSION U6: Surface Morphology and Interface Characterization
Chairs: Alex Briseno and Jun Takeya
Wednesday Afternoon, November 30, 2011
Room 311 (Hynes)


1:30 PM *U6.1
Control of Surface and Interfacial Properties in Organic Electronic Devices by Self-Organization.Keisuke Tajima1,2, Akira Tada1, Yanfang Geng1,3, Qingshuo Wei2 and Kazuhito Hashimoto1,2; 1Department of Applied Chemistry, The University of Tokyo, Tokyo, Japan; 2HASHIMOTO Light energy conversion project, JST-ERATO, Tokyo, Japan; 3Department of Materials, Beijing Institute of Technology, Beijing, China.

The control of surface and interfacial properties of organic semiconductors is particularly important for thin-film organic electronic devices, since the charge transport and generation processes often take place only at the interfaces. We have proposed that the surface segregation phenomenon of fluoroalkylated compounds could be applied to control them in the organic electronic devices such as field effect transistors and photovoltaics. We recently synthesized a regioregular polythiophene with alternating alkyl and semifluroalkyl chains (P3AFT) and used for this purpose. The polymer synthesis was successfully done by Ni-catalyzed coupling reaction of the corresponding bisthiophene monomers. We observed that P3AFT also spontaneously segregate to the surface of the polymer layer due to the low surface energy of the fluorocarbon and forms a very thin ordered monolayer. The polymer showed high crystallinity probably due to the ordered packing of the regioregular semifluoroalkyl and alkyl side chains. The results on the field-effect carrier mobility and photovoltaics will be discussed in term of the formation of the surface and interfacial dipole moments.


2:00 PM *U6.2
Characterization of the Phase Separation Process in Polymer-Fullerene Bulk Heterojunctions.Michael L. Chabinyc, Materials Department, Univ California, Santa Barbara, California.

Bulk heterojunction (BHJ) photovoltaics have achieved efficiencies greater than 8% . In these systems, the nanoscale phase separation of an electron donating material and an electron accepting material has a profound impact on the efficiency of charge generation and extraction. We present recent work on characterization of the evolution of the phase-separated morphology in polymer:fullerene BHJs. Using a model system of poly(3-hexylthiophene), P3HT, and PCBM, we have examined temperature dependent x-ray scattering, diffusion kinetics of PCBM using dynamic secondary ion mass spectrometry and in-situ current-voltage measurements to develop insight into morphological changes during thermal annealing. We find significant mixing of PCBM in P3HT and discuss the implications for device operation. The extension of these studies to BHJs with low band gap polymers, such as PSBTBT, and novel fullerenes will be presented.


2:30 PM U6.3
Phase Separation in Ternary Charge Transfer Complexes Studied by IR Spectroscopy, XRD and TEM.Michael Kroeger1,2, Diana Nanova3,2, Katrin Schultheiss1,2, Martin Pfannmoeller4,2, Rasmus R. Schroeder4,2, Tobias Glaser3,2, Milan Alt3,2, Sebastian Beck3,2, Annemarie Pucci3,2, Jens Pflaum5 and Wolfgang Kowalsky1,2; 1Institut für Hochfrequenztechnik, TU Braunschweig, Braunschweig, Germany; 2Analytics Competence Center, InnovationLab GmbH, Heidelberg, Germany; 3Kirchhoff-Institut für Physik, Universität Heidelberg, Heidelberg, Germany; 4CellNetworks - Cluster of Excellence, Universität Heidelberg, Heidelberg, Germany; 5Experimentalphysik VI, Universität Würzburg, Würzburg, Germany.

We study the properties of thin films of organic charge transfer (CT) complexes, which are deposited via thermal evaporation. CT complexes in the presented context consist of segregated stacks of donor and acceptor molecules at a 1:1 mixing ratio. The band gap and structural properties of CT complexes correlate with the degree of charge transfer between donor and acceptor molecules. Due to a significant absorption in the IR spectral range, CT complexes may be suitable for future photovoltaic applications. Materials used for this study include dibenzo-tetrathiafulvalene-tetracyanquinodimethane (DB-TTF/TCNQ) and dibenzo-tetrathiafulvalene-tetrafluorotetracyanquinodimethane (DB-TTF/F4-TCNQ). We determined the degree of charge transfer in the complexes with IR-spectroscopy using the linear relationship between the shift in the excitation energy of the C-N- stretching mode of the acceptor TCNQ and the charge transfer. For DB-TTF/TCNQ, the C-N stretching mode appears at 2209 cm-1. Here, we infer -consistent with DFT calculations- a partial charge transfer between donor and acceptor. In the case of DB-TTF/F4-TCNQ position and shape of the C-N stretching mode feature correlate with a full charge transfer between donor and acceptor, which reflects the stronger accepting properties of the F4-TCNQ molecules. By AFM, we observe that the morphology of CT thin films deposited via thermal evaporation is strongly dependent on the underlying substrate. On KBr we observe highly ordered growth patterns, whereas on Si less ordered structures can be seen. This observation is in agreement with XRD measurements, indicating a higher degree of crystallinity for thin films grown on KBr. In addition, we studied the thin film structure of DB-TTF/TCNQ and DB-TTF/F4-TCNQ grown on KBr using TEM and electron diffraction. For DB-TTF/F4-TCNQ, we detect very distinct diffraction patterns, which indicate a pronounced long-range order. DB-TTF/TCNQ thin films possess a more polycrystalline texture. In both cases, the diffraction patterns can be explained by a molecular packing, where the molecules are standing upright on the substrate (Lunt et al., Phys. Rev. B 83, 064114). Further, we studied, how mixing of two CT complexes at different composition ratios by substituting the acceptor molecules, might allow adjustment of the optical and structural properties. The electron diffraction patterns we collected on samples of a ternary CT system DB-TTF/F4-TCNQ1-x/TCNQx do not show any indication of a mixed crystalline phase or of a novel crystalline order, but indicate a phase separated growth of DB-TTF/TCNQ and DB-TTF/F4-TCNQ crystals. However, upon mixing, the crystallinity of the individual phases degrades with a significant dependence on mixing ratio. In addition, there is no indication for a shift in the IR absorption spectrum nor for the appearance of new peaks. In summary this means, that, in the ternary system, a phase-separated growth mode dominates the film morphology.


2:45 PM U6.4
Study of the Microstructure and Morphology of High-Performance Polymer-Small Molecule Blend Organic Field-Effect Transistors.Kang Wei Chou1, Jeremy Smith2, Kui Zhao1, Dongkyu Cha3, Rachid Sougrat3, Marsha Loth4, John Anthony4, Martin Heeney5, Iain McCulloch5, Thomas Anthopoulos2 and Aram Amassian1; 1Materials Science and Engineering, KAUST, Thuwal, Saudi Arabia; 2Department of Phyics, Imperial College London, South Kensington, United Kingdom; 3Imaging and Characterization Core Laboratory, KAUST, Thuwal, Saudi Arabia; 4Department of Chemistry, University of Kentucky, Lexington, Kentucky; 5Department of Chemistry, Imperial College London, South Kensington, United Kingdom.

The fluorinated acene 2,8-difluoro-5,11-bis(triethylsilylethynyl) anthradithiophene (diF-TES-ADT) blended with amorphous poly(triarylamine)s (PTAA) has shown great promise for organic field-effect transistor (OFET) electronics due to its surprisingly high charge carrier mobility, ease of solution processing and device performance uniformity. The microstructure and morphology of the diF-TES-ADT:PTAA blend film are believed to be the key factors in achieving these high mobility values. The small-molecule provides efficient charge transport pathways, while the amorphous polymer PTAA acts as a matrix and aids uniformity and ease of processing. A critical concentration of the small-molecule is required for the blend system to obtain high performance OFETs. For diF-TES-ADT concentrations greater than 20 wt.%, the small-molecule-rich phase starts to form, but percolation - which is critical for in-plane carrier transport - occurs when a significant fraction of the molecular crystallites form conduction pathways. A mobility threshold is observed at about 39 wt.% diF-TES-ADT. Above this value uniform, polycrystalline films can be fabricated yielding OFETs with hole mobilities approaching 2 cm2/Vs. However, high concentrations of diF-TES-ADT lead to the formation of non-uniform films and significantly reduced hole mobility. We report on the spectro-microscopic investigation of the diF-TES-ADT:PTAA blend film using scanning transmission x-ray microscopy (STXM) and grazing incidence wide-angle x-ray scattering (GIWAXS). The STXM is equipped with an elliptically polarizing undulator, allowing us to image grain boundaries and the degree of in-plane misorientation between different molecular crystallites. The percolation pathways, domain misorientations, and grain boundary formation were studied for different concentrations of the small-molecule with both techniques. In addition, the stratified, phase separated structure was imaged by energy-filtered cross-sectional transmission electron microscopy (TEM).


 

SESSION U7: Organic Single-Crystalline Structures and Transistors
Chairs: Iain McCulloch and Vitaly Podzorov
Wednesday Afternoon, November 30, 2011
Room 311 (Hynes)


3:30 PM U7.1
Hall Effect and Charge Transport in Organic Thin-Film Transistors.Takafumi Uemura1, Masakazu Yamagishi1, Yuichi Takatsuki2, Yugo Okada2, Yasuhiro Nakazawa2 and Jun Takeya1,2; 1The Institute of Scientific and Industrial Research (ISIR), Osaka University, Osaka, Japan; 2Department of Chemistry, Osaka University, Osaka, Japan.

Development of functional materials and understanding of the microscopic mechanisms mutually benefit through their close interaction. To accelerate development of organic semiconductor films for industrial application to flexible electronics devices, it is essential to understand mechanisms of charge transport in conjunction with molecular-scale charge transfer. Here, we examine universality of the idea that practically attractive high-mobility charge transport in organic transistors is caused by band-like carrier dynamics using several different molecular systems as the active semiconductor layers. We employ Hall-effect measurement which differentiates the diffusive band transport from site-to-site hopping. We prepared both solution-crystallized single-crystal and vacuum-evaporated polycrystalline samples with 2,9-didecyl-dinaphtho[2,3-b:2’,3’-f]thieno[3,2-b]thiophene (C10-DNTT), 2,7-dialkyl[1]benzothieno[3,2-b]benzothiophenes (C8-BTBT), pentacene, C60, all of which have mobility exceeding 1 cm2/Vs. The result of measurement shows Hall coefficient identical to inverse charge density for the devices of C10-DNTT and C8-BTBT, indicating well extended nature of the carriers accumulated at the surface of these semiconductors. On the contrary, the devices of pentacene and C60 have exhibited obvious discrepancy between the Hall coefficient and inverse charge density at room temperature, meaning the free-electron like picture is violated. Measuring temperature dependence of the devices, we argue about fundamental difference in the electronic states between the organic molecular systems of the above two categories. In pentacene single-crystal and thin-film transistors, at lower temperatures, the deviation of the Hall coefficient and inverse charge density gradually diminishes similarly for all the samples to approach the free-electron behavior. We discuss possible mechanism that large molecular fluctuation at room temperature partially destroys electronic coherence in pentacene and C60 devices. The results indicate that straight-rod pentacene and spherical C60 molecules are readily affected by the thermal fluctuation as compared to DNTT or BTBT derivatives. The observation can give a prescription to design molecules which realize high-mobility band transport.


3:45 PM U7.2
Surface Potential Mapping of TIPS-Pentacene OFETs under Device Operation.Rebecca Saive1, Lars Mueller1,3, Michael Kroeger1,2 and Wolfgang Kowalsky1,2; 1InnovationLab GmbH, Heidelberg, Germany; 2Institut für Hochfrequenztechnik, Technische Universität Braunschweig, Braunschweig, Germany; 3Kirchhoff-Institut für Physik, University of Heidelberg, Heidelberg, Germany.

The mapping of potential characteristics in organic devices under operation is a powerful technique for a basic understanding of device functionality. Yet, there has been reported on surface potential measurements with scanning Kelvin probe microscopy (SKPM) e.g. on pentacene (Appl. Phys. Lett. 83, 5539 (2003)) and Acene based organic field effect transistors (OFETs) (Adv. Mater. 2008, 20, 4513-4516). Organic field effect transistors are considered as a promising technology for various applications such as RF-ID tags. We studied OFETs based on TIPS-Pentacene with different grain sizes achieved by using a drop cast deposition from Toluene and THF respectively, as described in Appl. Phys. Lett. 93, 103302 (2008). We could see a strong dependency of transistor performance on grain size and on surface treatment with self assembling monolayers (SAM). With SKPM we measured topography and surface potential simultaneously such that we were able to correlate both properties. Contrary to other groups we studied films with different thicknesses ranging from a few hundred nanometers to a few microns. For thin films (thickness around 300 nm) and for films with small grain sizes (around 200 nm), we measured voltage drops in the channel region as it has already been seen for other materials than TIPS-Pentacene. (e.g. Adv. Mater. 2007, 19, 2267). In thick films with grain sizes larger than the channel size, we could observe that surface voltage drops no longer in the channel region but on grain boundaries lying a few microns away from the channel. Furthermore, we performed focused ion beam (FIB) assisted tomography on our samples, which gives the absolute layer-thicknesses and reveals 3D-resolved images of grain boundaries in the regions of interest.


4:00 PM U7.3
A Study of Rubrene Surface Evolution.Robert J. Thompson1,2, Benjamin Yadin2, Zara J. Grout2, Steven Hudziak1, Christian L. Kloc3, Oleg Mitrofanov1,2 and Neil J. Curson1,2; 1Electrical & Electronic Engineering, UCL, London,, London, United Kingdom; 2London Centre for Nanotechnology, UCL, London, London, United Kingdom; 34Nanyang Technological University, Singapore, Singapore.

Crystalline rubrene, a material of great interest and importance in the area of field effect transistors (FETs) due to its high mobility, was recently shown to have surface processes that are, as yet, undefined and unknown. As these processes occurring on the surface of the material are of importance in understanding it’s functionality as a FET knowledge of the surface is required for the materials development within the field of device physics. To address this issue we have used atomic force microscopy (AFM) to show the surface evolution of the material in its single crystal form from an environmentally unexposed state. Investigating cleaved rubrene surfaces provided two major benefits over the ‘as-grown’ surfaces: The newly formed surface is un-oxidized allowing controllable studies of chemical doping processes; and large molecularly flat terraces can be reproducibly created allowing reliable device fabrication. We show that after cleaving, the rubrene surface undergoes molecular reorganization consisting of two distinct processes: the formation and three-dimensional growth of nanoscale beads; and the formation and two-dimensional growth of narrow one molecule high fingers in excess of 1 micrometer long. These processes are shown to have an environmental dependence and occur in atmospheric conditions implying the incorporation of an atmospheric component, likely oxygen or water, into the material. Using conductive AFM we show how the electrical properties at the surface vary at the location of features. These conductive properties where seen to change with time, differing from that of as-grown rubrene. Nanoscale beads show an insulating behavior and a band bending effect on the surface nearby. Extensive investigations into the observed features showed that: Beads were initially seen between 30 minutes and several hours after the new surface was exposed to oxygen. When initially detected they had an average height of ~3nm and a bead-to-bead spacing of 250nm - 600nm ranging from sample to sample; Fingers were seen to appear several hours after cleaving and their growth originated from the base of step edges. The fingers had an anisotropic growth, preferring to grow along the b-axis with a length to width ratio of up to 7:1. These results shed light on the natural evolution of the surface of rubrene within atmospheric conditions, and suggest that these surface processes could have a significant impact on the performance of rubrene FETs and other electronic components considering that exposed surfaces are used in rubrene-based devices. Furthermore, our detailed characterization of the molecular redistribution on the surface provides the data on which models of molecular surface dynamics can be tested, potentially leading to the development of a long-sought fabrication method for ordered rubrene thin films.


4:15 PM U7.4
Electrical Investigation of the Interface Band Structure in Rubrene Single-Crystal/Nickel Junction.Yuta Kitamura1, Eiji Shikoh1, Satria Z. Bisri2,3, Taishi Takenobu3 and Masashi Shiraishi1; 1Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka, Japan; 2Department of Physics, Graduate School of Science, Tohoku University, Sendai, Miyagi, Japan; 3Department of Applied Physics, Waseda University, Shinjuku-ku, Tokyo, Japan.

Organic spintronics has attracted significant attention as a means of utilizing novel functionality of the spin degree of freedom to molecular electronics. Because organic molecules comprise light elements such as carbon and hydrogen, the spin orbit interaction in organic molecules is smaller than that in inorganic materials. Therefore, spin carriers in organic molecules can be less scattered, resulting in longer spin relaxation time and length. However, very large Schottky barriers are often formed between ferromagnet and organic materials, which prevents efficient and conclusive spin injection. Therefore, the investigation of an interface electronic structure between organic molecules and ferromagnetic metal is indispensable for further studies. In this study, a Au/rubrene single-crystal/Ni structure, which equipped a lateral and a top electrode configuration, was fabricated and the interface band structure between Ni and rubrene was investigated by using an electrical method from a viewpoint of spintronics applications of organic single-crystals [1]. Current-voltage (I-V) characteristics were measured in the range of 270-350 K in 10 K intervals, where a typical diode characteristic was observed. The I-V characteristics obtained were fitted by using the equation based on the thermionic emission theory by Bethe [2]. The ideal factor n was estimated to be 1.49 ± 0.54, indicating that our devices were good diodes. The temperature dependence of the inverse saturation current allows us to investigate the interface electronic band structure between Ni and rubrene single-crystal, and it was clarified that he Schottky barrier and the vacuum-level shift at the interface were 0.56 eV and 0.66 eV, respectively. It was also found that deep level traps in the rubrene single-crystal device were located 0.18 eV above the highest occupied molecular orbital level. Reference: [1] Y. Kitamura et al., submitted to Appl. Phys. Lett., [2] T. Kaji, T. Takenobu et al., Adv Func. Mat., 21 (2009) 3689-3693.


4:30 PM U7.5
High Performance Single-Crystal OFETs with Suspended Polymer Membrane Insulators.Hee Taek Yi1, Yuanzhen Chen1 and Vitaly Podzorov1,2; 1Physics and Astronomy, Rutgers University, Piscataway, New Jersey; 2Institute for Advanced Materials and Devices for Nanotechnology, Piscataway, New Jersey.

Organic semiconductors are promising materials for development of novel technologies, such as flexible displays, bio-sensors, electronic paper and solar cells. In order to better understand fundamental properties and limitations of organic semiconductors, it is necessary to study the charge transport and optical properties of materials with minimized disorder. Ideal candidates for such research are organic molecular single crystals. Herein, we introduce a novel method of fabrication of high-performance p-type and n-type single-crystal Organic Field-Effect Transistors (OFETs), called a “FoodSaver” approach. In brief, this method uses commercially available thin insulating polymer membranes (2.5 μm-thick Mylar®, or 11 μm-thick Glad® or Saran® food wraps) conformably collapsed onto a flat surface of an organic single crystal by a gentle vacuum to form a gate insulator. This method has numerous advantages: (1) it is very quick - fabrication of a typical transistor takes a few minutes; (2) it eliminates the usage of undesirable, expensive and time consuming processing, such as a high-vacuum evaporation, high-temperature furnaces, plasmas, etc; (3) it works for different types of surface, including n-type organic crystals and surfaces coated with Self-Assembled Monolayer (SAM); (4) it is completely reversible and non-destructive - many different polymer membranes can be applied sequentially to the same organic crystal without degradation of it’s surface, which is important for investigating intrinsic properties of OFETs, while avoiding sample-to-sample variations; (5) the method is very inexpensive.


4:45 PM U7.6
Novel Soft X-Ray Structural Characterisation of a Highly-Crystalline Thienothiophene Polymer.Torben Schuettfort1, Mi Jung Lee1, Benjamin Watts2, Henning Sirringhaus1 and Christopher R. McNeill3; 1Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom; 2Swiss Light Source, Paul Scherrer Institut, Villigen, Switzerland; 3Department of Materials Engineering, Monash University, Clayton, Victoria, Australia.

The high performance polymer poly(2,5-bis(3-alkylthiophene-2-yl)thieno[3,2-b]thiophene) (PBTTT) offers large potential as a p-type organic semiconductor for solution processed circuits. Previous morphological studies revealed large degrees of self ordering and an in-plane π-π packing motif beneficial for charge transport. Near edge X-ray absorption fine structure (NEXAFS) spectroscopy in particular allowed the molecular alignment of the backbone and side-chains to be determined. Several different morphologies which exhibit different device performance and surface morphologies have been reported, which can form terraced layers with in-plane π-π stacking and molecular step height. In order to investigate the structure-performance relationship and self-ordering mechanism we analyse Carbon edge NEXAFS spectra, scanning transmission X-ray microscopy (STXM) and atomic force microscopy (AFM) images of PBTTT thin films to compare with transistor performance. Surface-sensitive NEXAFS measurements identify the orientation of molecules at the charge transporting interface with chemical selectivity on the conjugated backbone or the side-chains. This aids understanding of the backbone alignment of zone-cast films, where alignment is induced at the top surface of the film. The STXM measurements show the length scales of quasi-domains with parallel backbone alignment in the bulk and at the surface, which show disclinations typical for liquid crystals. Furthermore, our novel data analysis methods allow the local degree of order and tilt angle of the molecules in the bulk of the film to be calculated for different morphologies. This comprehensive morphological study will aid in the understanding of charge transfer in highly crystalline organic semiconductors.


 

SESSION U8: Poster Session: Organic Devices/Materials I
Wednesday Evening, November 30, 2011
8:00 PM
Exhibition Hall D (Hynes)


U8.1
Acceptor Doping around Contact Electrodes in Bottom-Gate/Bottom-Contact p-Channel Organic Field-Effect Transistors.Yusuke Wakatsuki1, Kei Noda1, Yasuo Wada2, Toru Toyabe2 and Kazumi Matsushige1; 1Electronic Science and Engineering, Kyoto University, Kyoto, Japan; 2Bio-Nano Electronics Research Center, Toyo University, Saitama, Japan.

In recent years, the improvement of device performance in top-contact p-channel organic field-effect transistors (OFETs) has been demonstrated by contact-area-limited doping, where an electron-acceptor-doped layer was deposited at the interface between a semiconductor layer and a contact electrode. It has been commonly considered that p-type doping of organic layers decreases contact resistance, while acceptor/electrode charge transfer can also imitate p-type doping behaviors. Therefore, the origin of carrier doping effects in OFETs is still controversial. To understand the doping effects in OFETs, we examined p-type doping for bottom-gate/bottom-contact (BGBC) p-channel OFETs from both theoretical and experimental viewpoints. For BGBC p-channel OFETs in this study, acceptor-doped layers (p+ layers) are located a few tens of nanometers over source-drain contact electrodes in order to circumvent the acceptor/electrode charge transfer. Our device simulation using a Toyo University Organic Thin Film Transistor Advanced Simulator (TOTAS) [1] showed that p+ layers in this device structure can contribute to the increase of the drain current without any threshold voltage shift, although the influence of contact resistance has not been considered in the device simulation. It can be expected that p+ layers can compensate the shortage of hole at the source-channel interface during transistor operation. Next, these device simulation results were experimentally confirmed by fabricating p-channel BGBC OFETs on heavily-doped Si substrates with thermally grown SiO2. Pentacene and gold were deposited by vacuum evaporation as an active layer and contact electrodes, respectively. P+ layers were prepared with co-evaporation of pentacene and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ). These p+ layers are located 10 nm over the source-drain electrodes and do not contact these electrodes. Actually, the enhancement of drain current was also observed experimentally in pentacene BGBC OFETs with p+ layers. This study indicates that the control of carrier concentration with molecular doping is of great importance for improvement OFET device performance. [1] Y. Ishikawa, Y. Wada, T. Toyabe, J. Appl. Phys. 107 (2010) 053709


U8.2
Growth of Highly Crystalline Pentacene/C60 Multilayers.Kwangseok Ahn1, Jeong H. Cho2 and Dong R. Lee1; 1Physics, Soongsil University, Seoul, Korea, Republic of; 2Organic Materials and Fiber Engineering, Soongsil University, Seoul, Korea, Republic of.

Organic-organic heterostructures have been widely used for organic photovoltaics and ambipolar organic transistors and have potential for providing a new functionality. However, it is difficult to obtain crystalline organic-organic heterostructures with useful crystal size. Here we show that highly crystalline pentacene/C60 multilayers have been obtained by controlling the molecular wettability using the growth parameters such as the substrate temperature and deposition rate. Though an atomically flat pentacene monolayer has been shown to increase remarkably the crystallinity of C60 films, [1] here we have studied more detailed growth mechanism of C60 on pentacene and extended to the successive growth of pentacene on the C60 crystals formed on pentacene layers. We revealed that the surface of pentacene with two monolayers showed distinctly different wettability for C60 molecules from that with only the first monolayer, implying different surface energies between the surfaces of the first and second pentacene monolayers. This control of the wettability allows highly crystalline pentacene/C60 multilayers. We also discuss ambipolar transport characteristics observed in the organic field-effect transistors fabricated with these organic heteromultilayered films. [1] K. Itaka et al. Adv. Mater. 18, 1713 (2006).


U8.3
Time-of-Flight Technique Limits of Applicability for Thin-Films of Π-Conjugated Polymers.Marco R. Cavallari1, Vinicius R. Zanchin1, Cleber A. Amorim2, Gerson Santos1, Fernando J. Fonseca1, Adnei M. Andrade3 and Sergio Mergulhao2; 1Departamento de Engenharia de Sistemas Eletrônicos (PSI), Escola Politécnica - Universidade de São Paulo, São Paulo, São Paulo, Brazil; 2Departamento de Física, Universidade Federal de São Carlos, São Carlos, São Paulo, Brazil; 3Instituto de Eletrotécnica e Energia - Universidade de São Paulo, São Paulo, São Paulo, Brazil.

Charge transport in polymeric films can be investigated through stationary (e.g. current versus voltage) and transient (e.g. Time-of-Flight - ToF, Dark-Injection Space-Charge-Limited Current - DI-SCLC, Charge Extraction by Linearly Increasing Voltage - CELIV) current techniques [1,2]. Even though ToF is the most common technique for mobility characterization [3], it suffers from two limitations in nanometric films: (i) charge carrier transit time must be lower than dielectric relaxation time (ttr < τσ) and (ii) higher than RC constant (ttr > τRC). For such situations, a ToF variety proposed by Juška et al. [4] for high-mobility semiconductors is mandatory. In this context, this work suggests the application and comparison of CELIV, current and integral modes ToF to investigate transport in organic materials. Poly[2-methoxy-5-(3',7'-dimethylloctyloxy)-1-4-phenylene vinylene] (MDMO-PPV), poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1-4-phenylene vinylene] (MEH-PPV) and poly(3-hexylthiophene) (P3HT) were herein analyzed, due to their application in large-area electronics, low-cost printable electronics and flexible circuits [5]. An organic light-emitting diode (OLED) structure was used on indium tin oxide anode over glass (Delta, unpolished, 30 - 60 Ω/sq.). MDMO-PPV (Merck, Mw = 1 150 kg/mol, Mn = 170 kg/mol), MEH-PPV (Σ-Aldrich, Mn = 150 - 250 kg/mol) and P3HT (ADS, regioregularity > 95 %, Mw = 10 - 35 kDa, Mw/Mn = 2.0) in 7 - 25 mg/ml chloroform solutions were agitated for 24 hours and spun at 500 - 3 000 rpm for 60 s. Samples were then heated at 55 °C during 60 min. Resultant thickness of the films was 0.2 - 0.4 µm for MDMO-PPV, 0.45 - 3 µm for MEH-PPV, while 0.1 - 10 µm for P3HT. After aluminum anode evaporation, devices were encapsulated with glass under inert atmosphere. Measurements were performed in a closed-cycle Janis cryostat with Lake-Shore 330 temperature controller (140 to 330 K). Electric signals were generated by a wave function generator Agilent 33120A with a nitrogen Oriel laser, and measured through a current amplifier Stanford Research 570 coupled to an oscilloscope Tektronix 340A. Mobility approached 10-5 cm2/V.s with Poole-Frenkel behavior for PPV derivatives and 10-2 cm2/V.s for P3HT. The alternative integral-ToF for films as thin as 100 nm provided comparable results to current-ToF in micrometric layers and CELIV. Deviation due to P3HT film thickness variation could be the result of higher crystallinity and increased light absorption from casting. Work supported by the Brazilian agencies FAPESP (07/06064-0; 09/05589-7), CAPES (222/08) and CNPq (MCT/CT-INFO 17/2009, 142302/2010-4). [1] Tuladhar, S.M. et al. Adv. Funct. Mater. 15 (2005) 1171-1182. [2] Dennler, G. et al. Org. Electron. 7 (2006) 229-234. [3] Warta, W. and Karl, N. Phys. Rev. B 32 (1984) 1172-1182. [4] Juška, G. et al. Synth. Met. 109 (2000) 173-176. [5] Briseno, A.L. et al. Nature 444 (2006) 913-917.


U8.4
Low-Cost 13.56MHz Rectifier Based on Organic Diode.Hong Wang1,2, Zhuoyu Ji1, Liwei Shang1, Yingping Chen1,2, Congyan Lu1, Dongmei Li1, Yingquan Peng2 and Ming Liu1; 1Institute of Microelectronics, Chinese Academy of Sciences, Beijing, China; 2School of Physical Science and Technology, Lanzhou University, Lanzhou, China.

Organic electronics have gained considerable attention because of their potential for low-cost, large volume manufacturing. A possible application of the field is in radio-frequency identification (RFID) tags. To achieve large-scale application of organic RFID tags, one of the most important factors is to reduce the cost of the tags. Rectifiers as one of the most demanding components of RFID with rectification frequency exceeded 13.56 MHz have been reported. Most of high performance rectifiers based on organic diodes were with the gold (Au) anodes due to the energy level match with organic semiconductors. Unfortunately, the high cost of Au has overshadowed its applications in low-cost electronics. To reduce the cost of the rectifier, in this presentation, we describe a way to achieve high performance rectifiers based on organic diodes with low-cost copper (Cu) anodes. By modification of the Cu electrode/organic interface with self-assembling copper tetracyanoquinodimethane (CuTCNQ) layer, carrier injection barrier and interface contact between low-cost metal electrodes and the organic materials were significantly improved. Diodes with pentacene as electro-active functional materials with rectification ratio up to 2×106 at 5 V are obtained, which is two orders of magnitude higher than conventional Au based diodes. In addition, the AC rectification frequency of the rectifier circuits based on an organic diode with CuTCNQ modified bottom electrode was higher than 13.56 MHz. These results demonstrate that rectifier based on organic diode using low-cost Cu electrodes with CuTCNQ modification exhibit a promising application prospect in the coming flexible organic electronics


U8.5
Application of New Methodics for Contact Resistance Measurements of Zinc Phthalocyanine Nanolayers.Irena Kasparkova1, Premysl Fitl2, Josef Nahlik1 and Jan Vlcek2; 1Department of Solid State Engineering, The Institute of Chemical Technology in Prague, Prague, Czech Republic; 2Department of Physics and Measurements, The Institute of Chemical Technology in Prague, Prague, Czech Republic.

Metal phthalocyanines are widely-used in electronic devices such as organic solar cells, gas sensors, organic transistors and photodetectors. During investigating high-resistance organic nanolayers it was found out that the contact resistance should not be neglected. This work is focused on determining contact resistance between the organic layer and metal contact. New methodics enabled us to evaluate the contact resistance of high resistance materials. Zinc phthalocyanine has been studied as a promising material for organic transistors. In order to prepare organic nanolayers and also the contact electrodes in the form of stripe special masking system was created. Zinc phthalocyanine nanolayers were prepared by low thermal evaporation technique in high vacuum chamber (working pressure 10-4 Pa, deposition rate ~1nm/min, substrate temperature 20°C). Metal contacts were deposited by cathode sputtering (Balzers SCD 050 device). Two types of sample geometry were applied. The first sample geometry consisted of sputtered metal contacts beneath zinc phthalocyanine nanolayer. For the second type of geometry was first deposited phthalocyanine nanolayer and metal contacts were sputtered upon it. As contact metals were chosen noble metals, gold, platinum and palladium. The electro-physical properties of zinc phthalocyanine nanolayers were investigated. The electrical linearity was investigated by current-voltage characteristics. The resistance per square and volume resistivity was measured using electrometer Keithley 6517A. Gold was found to be the best metal used for contact electrodes because of the lowest contact resistance, but its contact resistance can not be neglected. Palladium is not recommended to be used as a contact metal. Lower contact resistance showed the samples with the first type of geometry.


U8.6
Semiconducting Arylacetylene: Insulating Polymer Blends for Organic-Based Electronic Devices. Pascal Wolfer1, Maria Laura Santarelli2, Luigi Vaccaro3, Alessandra Broggi2, Daniela Lanari3, Liyang Yu4, Thomas D. Anthopoulos4, Paul Smith1, Natalie Stingelin-Stutzmann4,1 and Assunta Marrocchi3; 1Department of Materials, ETH Zürich, Zürich, Switzerland; 2Dipartimento Ingegneria Chimica, Materiali Ambiente, University of Rome Sapienza, Rome, Italy; 3Department of Chemistry, Laboratory of Green Synthetic Organic Chemistry, University of Perugia, Perugia, Italy; 4Centre for Plastic Electronics, Imperial College London, London, United Kingdom.

There is considerable focus on increasing charge carrier mobilities in organic electronic devices to compete with current inorganic thin film technologies such as amorphous hydrogenated silicon. Blending of organic semiconductors with common bulk polymers has been shown to be a useful strategy to combine high carrier mobilities with features such as ease of processing, film uniformity, environmental stability and enhanced mechanical properties. Here, blends of chemically readily accessible, small-molecular arylacetylene derivatives with insulating poly(vinylidene fluoride) (PVDF) are presented that allow reliable solution processing of field-effect transistor (FET) architectures with electronic characteristics which are comparable to or better than those of the neat semiconductors. We demonstrate that having the chemical means and corresponding processing protocols to control solid-state microstructures by either adjusting the chemical nature of the organic semiconductor, blend composition or deposition temperature, permit straight-forward comparison between materials and allow probing if electronic characteristics are affected by the chemical structure of the organic semiconductor and/or selected processing protocols. It will be seen that it is possible to have arylacetylene concentrations as low as 30 wt% in the bulk but still have sufficient material to form a percolating network required for electronic transport.


U8.7
Structural, Optical and Electronic Properties of Precise Controlled Hetero-Molecular Multi-Layers Structures.Nobuya Hiroshiba1,2, Jonathan P. Hill2, Kaoruho Sakata2, Ryoma Hayakawa2, Toyohiro Chikyow2, Katsuhiko Ariga2, Kiyoto Matsuishi1 and Yutaka Wakayama2,3; 1Graduate School of Pure and Applied Sciences, Tsukuba University, Tsukuba, Japan; 2WPI-MANA, NIMS, Tsukuba, Japan; 3Department of Chemistry and Biochemistry Faculty of Engineering, Kyushu University, Tsukuba, Japan.

The final goal of this study is to explore novel molecular fuctionalities, e. g., Mott transition, lasering, giant magnetic resistance and other quantum properties. For this purpose, multi-layer structures (MLS) are probable candidates [1], since the physical properties can be controlled by choosing suitable combinations of molecules. Although preliminary experiments about MLS had been reported [2], the novel functionalities have not been realized yet. This is because that the growth conditions of MLS and the choice of molecules were not optimized in the previous reports. As the results, carrier transport and stimulated emission were disturbed due to the rough hetero interface and random molecular packing. Therefore, for the first step to achieve our goal, to establish a growth tecnique for well-difined thin film and to make a suitable choice of molecules are crucial. Here, we demonstrate the growth method of MLS which enables precise control of monolayer growth and well-defined molecular packing [3]. The most significant point is that this fabrication method is capable for adapting to various molecules such as pentacene, quaterrylene (QT), perylene tetracarboxylic di-imide derivatives (PTCDI), terylene tetracarboxylic di-imide derivatives (TTCDI), and α, ω-diperfluorohexyl-quaterthiophene (DFH-4T). Optical and electronic properties of the various MLS were investigated by using optical absorption, photoluminescence and photoelectron spectroscopies (UPS, XPS). Together with these fundamental characteristics, transistor properties of the MLS were examined. We will introduce the recent progresses, especially TTCDI/QT MLS and its properties, in this presentation. [References] [1] S.R. Forrest, Chem. Rev. 97, 1793 (1997). [2] M. Muccini et al., Nature Mater. 5, 605 (2006). [3] N. Hiroshiba et al., Org. Electron. 10, 1032, (2009).


U8.8
Stability of Low Voltage n-Type Organic Field Effect Transistors.Rizwan Ahmed1, Michael Sams2, Clemens Simbrunner1, Mujeeb Ullah1, Timm Ostermann2 and H. Sitter1; 1Institute of semiconductor and solid state physics JKU, Linz, Austria; 2Research Institute for Integrated Circuits JKU, Linz, Austria.

We are presenting a comprehensive study concerning the stability of n-type organic field effect transistors (OFET). Due to the fact that C60 is a high mobility n-type material the long term stability is investigated with C60 based OFETs. Alox and a thin layer of BCB is used as a dielectric material and C60 is deposited as an active layer using hot wall epitaxy. For source and drain contacts, LiF and Al are used. To show the long term stability a continuous bias stress for 300 hours inside the glove box has been applied. After this stress test, the OFET was stored in the glove box for another 600 hours, followed by again 300 hours of continuous stress. We report about the fluctuations in the device parameters comparing the transfer and output characteristics, as well as the on/off ratio for the whole period of the measurements. A shift in the threshold voltage (extracted from the Level 1 model of a transistor) during the continuous bias stress occurs, which recovers back to the initial value during the storage time in the glove box. A shift of the threshold voltage is also found in e.g. amorphous silicon thin-film transistors, which shows that transistors with such behaviour can still be used for circuit design. Additionally no appreciable change in the drain current has been measured during the whole period of investigations, thus the C60 based low voltage OFET proves excellent stability for an electronic circuit design.


U8.9
Synthesis of 3,7-dialkylthieno[2',3':4,5]thieno[3,2-b]thieno[2,3-d]thiophenes (DCnFT4) and Their Application in Semi-Conducting Polymers.Jieyu Hu, James R. Matthews and Mingqian He; Organic Technologies, Corning Incorporated, Corning, New York.

Recently much interest has been directed toward solution processable polymeric semi-conductor materials containing thiophene based moieties. In particular, dialkylated tetrathienoacene copolymers have been used as high mobility semiconductors in field-effect transistors with a field-effect hole mobility exceeding 0.3 cm2/Vs. Expansion of this class of materials to include new materials with either linear (DCnFT4) or branched (DbCnFT4) di-alkyl-substituted tetrathienoacenes has been achieved with the development of two distinct synthetic routes for ring formation and the introduction of the side-chains. DC17FT4 was prepared from tetrabromothieno[3,2-b]thiophene through a sequence of diketone formation, cyclization, hydrolysis and decarboxylation. Synthesis of the new compound di-2,4,4-trimethylpentyl-FT4 (DbC8FT4) has been accomplished in a more complex 8 step process, featuring a mono-ketone-ester as a key intermediate. DCnFT4 can be brominated in the 2- and 6-positions with NBS. The dibromide can then be used to form copolymers through cross coupling reactions. These polymers are solution processable semi-conductors. The polymers have been characterized by GPC and within OTFT devices.


U8.10
Nano-Imprinted Bulk Heterojuction in Inverted Structure Solar Cells.Xinhui Lu1, Htay Hlaing1,2, Danvers Johnston3, Chang-Yong Nam3, Kevin Yager3, Charles Black3 and Benjamin Ocko1; 1Condensed Matter Physics and Materials Science, Brookhaven National Lab, Upton, New York; 2Physics, Stony Brook University, Stony Brook, New York; 3Center for Functional Nanomaterial, Brookhaven National Laboratory, Upton, New York.

Carrier mobility measurements of organic electronic materials have shown that their mobility is very anisotropic[1], hence, their performance is in part determined by molecular packing and orientation factors. Modifying these characteristics provides the possibility of improved device performance. To alter the molecular packing in bulk heterojunction(BHJ) solar cells we have used nanoimprinting to transfer the nano-grating pattern from a silicon master stamp into P3HT/PCBM thin films. Devices were fabricated in an inverted structure where the following were successively deposited on an ITO/glass substrate; (1) TiO2 layer, (2) BHJ layer, followed by nano-imprinting, (3) PEDOT:PSS layer and (4) gold layer. The electrical properties of TiO2 assured that the electrons would be injected into the ITO/TiO2 layer where the device polarity is reversed compared with conventional OPV devices. We observed a consistent and substantial improved power conversion efficiency of the imprinted, inverted BHJ devices relative to similar devices made without nano-imprinting. Face-on oriented P3HT domains were induced by the imprinting process compared with the edge-on orientation that was found for the devices without imprinting .The improved efficiency in imprinted devices appears to be correlated with both the larger interfacial area for charge collection and the P3HT reorientation. The nano-imprinted inverted geometry might also improve the performance of thicker devices (increased light absorption) without the increased resistance which sometimes accompanies these devices[2]. [1] Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.; Herwig, P.; de Leeuw, D. M. Nature 1999, 401, 685-688. [2] J. B. Emah, R. J. Curry, and S. R. P. Silva, Appl. Phys. Lett. 93, 103301 (2008)


U8.11
Effect of Side Chain Length on Film Structure and Electron Mobility of Core-Unsubstituted Pyromellitic Diimides.Ming-Ling Yeh and Howard E. Katz; Materials Science and Engineering, Johns Hopkins University, Baltimore, Maryland.

Pyromellitic diimides (PyDIs) are π-conjugated electron-transport materials based on a small aromatic core (benzene). Different side chains can be introduced by reacting pyromellitic dianhydride with corresponding amines. The facile reaction and variety of side chain choices provide the ease for engineering solubility or volatility of the molecules, packing and morphology of the deposited film, and thus electrical performance of the device. We have synthesized PyDI derivatives with series of side chains and investigated their film structures and electrical performances in thin film transistors. Fluoroalkyl side chains likely protect the core from oxygen and moisture, and thus enable air-operable devices. While the 3-(perfluooctyl)propyl side chain, among the substituents with the same perfluorooctyl group attached to different alkylene chains ranging from one to four carbons, gives particularly high mobility on the larger naphthalene core, it does not provide the same beneficial effect on the benzene core, suggesting that smaller cores might need smaller side chains to achieve better electrical properties. The effect of the length of fluorinated segment in side chains attached to a common alkylene part is being examined. Among this series of PyDIs, shorter side chains are shown to exhibit higher mobilities. Mobility of 0.025 cm2/Vs was achieved with perfluorobutylmethyl side chain. The systematic comparison of different side chains not only provides clues to structure-property relationship but also enables a rational design of molecular structure for optimal electrical performance.


U8.12
Organic Field-Effect Transistors Using P3HT and PMMA as Dielectric.Alexandre C. Maciel and Roberto M. Faria; Departamento de Física e Ciência dos Materiais, Instituto de Fisica de São Carlos, São Carlos, Sao Paulo, Brazil.

We report a p-type organic field effect transistors made of poly(3-hexylthiophene) (P3HT) as channel material and poly(methyl methacrylate) (PMMA) as dielectric layer. We used spin coating technique to build both P3HT and PMMA thin films, and they are processed under nitrogen atmosphere. Using different device architectures we tested several parameters as the thickness of the layers, temperature of thermal annealing and different solvents. The thickness of the semiconductor layer varied from 20 nm to 150 nm, while the dielectric layer ranged from 100 nm to 800 nm. The solvents used in this paper were chloroform, toluene and dichlorobenzene for the P3HT and methyl ethil- ketone and ethyl acetate for the PMMA. Transistors with channel length ranging from 50 µm to 200 µm and W/L=13 showed electrical currents of the order of 1 µA and the hole mobility reached values close to 10-2 cm2/V.s. In order to discuss the charge carrier transport in these devices we applied the Vissenberg-Matters model [1], which provides a good understanding of the field-effect mobility. This approach allowed us to explain the characteristic curves of the fabricated transistors.


U8.13
P-Type Operation in Metal Base Organic Transistors.Ryotaro Akiba1, Ken-ichi Nakayama1,2 and Junji Kido1,2; 1Organic Device Engineering, Yamagata University, Yamagata, Japan; 2Research Center for Organic Electronics, Yamagata University, Yamagata, Japan.

The vertical-type organic transistor is a promising device structure that can make the channel length much shorter, leading to low voltage operation and high frequency response. Recently, we have reported a high performance vertical transistor having a simple layered structure composed of organic/metal/organic layers. This device was named a metal-base organic transistor (MBOT), because the inserted middle electrode behaves like a base layer in the bipolar transistors. MBOTs achieved very high current density modulation exceeding 100 mA/cm2 with low voltage operation of several volts. Thus far, the best performance of the MBOT has been obtained in the device using perylene bisimide and fullerene as organic semiconductors, and transistor behavior (current amplification) was observed only in the MBOT using n-type materials. In this study, we achieved good performance in p-type MBOT using pentacene that is the most typical organic semiconductor by introducing the heat-treatment method reported in n-type MBOT. The p-type MBOT showed similar behaviour to n-type MBOT except that main carriers are holes. The device was fabricated by vacuum deposition. The pentacene with a thickness of 400 nm was prepared on a cleaned indium tin oxide (ITO) glass substrate. After deposition of LiF/Al base electrode, the substrate was subjected to heat treatment for one hour at 150 oC under atmospheric condition. Then LiF layer and pentacene as emitter layer were deposited. At last, top Au emitter electrode was also evaporated thermally. The final device structure was ITO(collector) / Pentacene(400nm) / LiF(3nm) /Al(base, 10nm) / LiF(3nm) / Pentacene(100nm) / Au(emitter, 30nm). The device shows clearly modulation characteristics of collector current - base voltage curves. The output current was increased by small input base voltage of 3 V. The collector current and current amplification factor (hFE) reached 24 mAcm-2 and 566, respectively. This good performance was achieved by optimization of the base electrode thickness, and introducing the heat-treatment method as well as n-type MBOT. The modulation curves and current amplification behaviour was basically the same with that in n-type MBOT. The current amplification was interpreted as hole transmission through the base electrode like bipolar transistor. In addition, it was suggested that the LiF layer under and above the base electrode promoted hole transmission through the base electrode.


U8.14
Spin-Assembly Multilayered Light Emitting Organic Structures.Mike M. do Vale, Patricia B. Catandi, Roberto M. Faria and Francisco E. G. Guimaraes; Universidade de São Paulo - USP, São Carlos, São Paulo, Brazil.

We have combined spin-coating and spin-assisted layer-by-layer (SP-LbL) deposition techniques to construct multilayer polymeric structures with a precise control of the layer thickness, interfacial roughness and uniformity along the substrate surface and the deposition direction. We report the fabrication of organic multiple quantum well structures having as barrier a high HOMO-LUMO gap layer (Eg~3.2 eV) material, the Poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO) and as well-layer the poly(p-phenylene vinylene) (PPV, Eg~2.5 eV). 10 nm thick barriers of PFO diluted in toluene were spin-coated on SP-LbL bilayers (2-5 nm thick) consisting of a water soluble PPV precursor polyelectrolyte, the poly(xylyliden tetrahydrothiophenium chloride) (PTHT) and the long chain dodecylbenzenesulfonate ion (DBS) [1]. The PTHT was rapidly thermal converted into PPV at low temperatures (<100°C) under vacuum conditions after the SP-LbL deposition[1]. MQW structures with 25 periods were fabricated and small-angle x-ray diffraction was used to characterize the uniformity of the entire layer stack. We have observed the Bragg reflection with high order peaks at small angle corresponding to the signature of an artificial organic superlattice structure. Optical absorption, photoluminescence (PL) and excitation spectroscopy reveal an efficient excited state migration from the PFO barrier to the PPV well. The single excitonic PPV emission of the MQW structure demonstrates the ability to control the coverage of polyelectrolytes spin-assembled on a polymeric surface without layer intermixing and with hetero-deposition control at a monolayer level.


U8.15
Probing Carrier Injection in Au/ poly(3-hexylthiophene)/Polyimide/ITO Diodes Using Time Resolved Optical Second-Harmonic-Generation.Ryo Miyazawa, Dai Taguchi, Takaaki Manaka and Mitsumasa Iwamoto; Department of Physical electronics, Tokyo Institute of technology, Tokyo, Japan.

Organic electronics has demonstrated great potential in various applications. Taking advantage of the use of easy and low-cost preparation method, many organic devices have been proposed. Among them are organic field effect transistors (OFETs), and much effort has been exerted to efficiently derive OFETs by the use of novel organic materials, by the modification of gate-insulator surface and so forth [1]. Nevertheless understanding carrier behaviors in OFETs is still insufficient. Basically OFET is injection-type device, and we need to study carrier injection sufficiently. Injection carrier barrier height is a principal parameter, and is defined by using the difference between Fermi level of metal electrode and energy level of highest occupied molecular orbital (HOMO) of organic materials used as an active layer of OFET. However, owing to the formation of interfacial dipole layer and the presence of interfacial states and trapping states, the interface is rather complex and the carrier behavior is dependent on these as well as a space charge field caused by injected carriers. In our group, we have been developing a technique used for probing carrier motions in OFETs. The technique is electric-field-induced second harmonic generation (EFI-SHG) and time-resolved microscopic SHG (TRM-SHG) measurements [2,3]. In this study, we use these measurements to clarify transient carrier behaviors in the poly(3-hexylthiophene) (P3HT) layer of Au/P3HT/Polyimide/ITO diodes by directly probing the transient electric field generated across the P3HT layer. The transient electric fields shed light on the carrier injection process at the Au/P3HT interface and the carrier transport in the P3HT layer.The TRM-SHG evidently showed that carrier injection was strongly governed by the interface properties, and the relaxation time of the transient electric fields increased with decease of work function. We found that applying higher voltage caused additional relaxation process in the transient electric fields. The details will be discussed based on the TRM-SHG experiments, in terms of the C-V characteristics of the Au/P3HT/Polyimide/ITOdiodes. [1]Organic Electronics: Materials, Manufacturing, and Applications, ed. H. Klauk (Wiley-VCH, Weinheim, 2006) [2] T. Manaka, E. Lim, R. Tamura, and M. Iwamoto, Nat. Photonics, 1 581 (2007). [3]D. Taguchi, S. Inoue, Le Zhang, J. Li, M. Weis, T. Manaka, and M. Iwamoto, J. Phys. Chem. Lett, 1, 803 (2010).


U8.16
Electric Force Microscopy Used to Spectroscopically Image Chemical Charge Trapping in Pentacene Thin Films.Louisa Brown1, Vladimir Pozdin2, Justin Luria1, Chad Lewis1 and John A. Marohn1; 1Chemistry and Chemical Biology, Cornell University, Ithaca, New York; 2Materials Science and Engineering, Cornell University, Ithaca, New York.

Understanding and control of charge trapping in organic semiconducting materials is crucially important for both transistor and solar cell applications. However, there are very few systems in which the connection has been directly made between material degradation and decreased performance. Even in pentacene, an extremely well-established organic semiconductor, which of its many degradation products lead to charge trapping has been unclear. We find that trapped charge in aged pentacene films clears at different rates under variable-wavelength illumination and that the trap clearing rates are enhanced at wavelengths where pentacene does not absorb. We explain this finding with a new mechanism of photoinduced trap clearing involving excitation of the charged trap molecule followed by a neutralizing electron transfer from pentacene; measuring the charge detrapping rate versus wavelength by electric force microscopy thus enables the recording of electronic-absorption spectra of the charge-trapping molecule. Taken together, these findings suggest the presence of a chemical species as a source of the trapped charge. To further identify the molecule involved in charge trapping, we synthesized putative charge-trap precursors and deposited these trap precursors (~1 monolayer, 1 ML) onto very thin (4 ML) pentacene films. We studied the resulting charge trapping both spatially and with time- and wavelength-resolution using electric force microscopy. We find that doping with 6,13-pentacenequinone or with 6,13-dihydropentacene does not lead to charge trapping, but layering with pentacen-6(13H)-one does produce uniform charge trapping whose trap clearing spectrum agrees with theory and with the trap clearing spectrum observed in aged pentacene films. These findings implicate the pentacen-6(13H)-one cation as the active charge-trap species in aged films of pentacene.


U8.17
A Polymer Brush Organic Interlayer Improves the Overlying Pentacene Nanostructure and Organic Field-Effect Transistor Performance.Hwasung Lee, Chemical Engineering, Hanbat National Univ., Daejeon, Korea, Republic of.

We investigated the crystalline nanostructures and film morphologies of pentacene films deposited onto a polymer brush organic interlayer in high performance organic field-effect transistors (OFETs). Polymer brushes were grafted onto the oxide substrates by spin-coating and thermal annealing. Pentacene FETs fabricated on top of the polymer brushes showed excellent device performance, with a field-effect mobility of 0.82 cm2/Vs and an on/off current ratio of 107. These properties were superior to those of devices using typical surface modification techniques, such as octadecyltrichlorosilane (ODTS) and hexamethyldisilazane (HMDS). The improvements in OFET performance appeared to be due to the pentacene layer’s crystalline nanostructure and grain interconnectivity, which formed during the submonolayer stage of film growth. This stage of growth is strongly correlated with the surface energy, morphology, and viscoelastic properties of the resulting gate dielectrics. The inclusion of a polymer brush layer dielectric surface modification is a significant step toward optimizing the nanostructures of organic semiconductors, which are directly linked to device performance enhancement, by engineering the interfaces in OFETs.


U8.18
Structural Dependence of Conducting Polymer Sensor on VOC Detection.Hosang Ahn1, Seon-Bae Kim1, Sungkoo Lee2, Minseo Park3 and Dong-Joo Kim1; 1Materials Eng., Auburn Univ., Auburn, Alabama; 2Green Chemistry and Engineering, Korea Institute of Industrial Technology (KITECH), Cheonan, Chungnam, Korea, Republic of; 3Physics, Auburn University, Auburn, Alabama.

Conducting polymers such as P3HT, poly-DPOT, and PDDT have drawn a lot of interest in gas sensor applications due to its high versatility in structure modulation, low cost in production and room temperature sensing. In addition, chemical compositions of conducting polymers are similar with that of VOCs, which enables new sensing mechanisms based on the physical interactions between sensing materials and VOCs. Meanwhile, to recognize different VOCs are one of limitations of current sensing mechanism, reduction and oxidizing reactions between oxygen and VOC molecules because of similar reducing energies. Recent researches have been performed to investigate the role of structural changes in conducting polymers under popular VOCs such as acetone, ethanol, and toluene. However, no sufficient sensing data for other various VOCs are still available depending on types of conducting polymers. Among VOCs, gamma terpinene is reported to be a key volatile organic compound in biosynthesis related organic wastes and infected plants and applicable for environmental monitoring. In this study, six new conducting polymers based on PEDOT are synthesized to have different degree of randomness, length of side chain, type of side chain, and copolymers. For sensor fabrication, a 100nm thick platinum layer was deposited by DC sputtering on polyimide film. Six new conducting polymers were dissolved in chloroform in the mixing ratio of 20mg/ml. Approximately 500nm thick sensing layers were coated on interdigitated bottom electrode by a dip casting. Three VOCs, acetone, ethanol, and gamma terpinene were tested using six conducting polymer sensors. Obvious different sensing phenomena depending on structural changes in conducting polymers were observed in three VOCs detection. Conducting polymer with side chain, 2-thiophenylthiophene, revealed the slowest response to gamma terpinene. While, conducting polymer with an additional copolymer, NaSO3, showed the fastest response. Sensing mechanism to explain the distinct behavior in detection of acetone, ethanol, and gamma terpinene is proposed.


U8.19
ToF-SIMS (Time of Flight-Secondary Ion Mass Spectroscopy) Studies of Rubrene Single Crystals.Robert J. Thompson1,2, Sarah Fearn3, Steven Hudziak2, Christian L. Kloc4, Oleg Mitrofanov1,2 and Neil J. Curson1,2; 1Depatrment of Electronic & Electrical Engineering -UCL, UCL, London, United Kingdom; 2London Centre for Nanotechnology - UCL, UCL, London,, London, United Kingdom; 3Material Department, Imperial College, London, United Kingdom; 4Nanyang Technological University, Singapore, Singapore.

Rubrene (5,6,11,12-tetraphenyltetracene) is a promising material for many applications in the field of organic opto-electronic. Rubrene’s usefulness in this arena is due to several properties; it was recently suggested to have a very long exciton diffusion length, broad absorption in the visible light range and finally a very high field effect mobility of 10-40cm2/(Vs), making it an ideal candidate for Organic photovoltaics, Organic Field effect transistors and Organic Light emitting displays. As such, much of the research concerning rubrene has focused on its charge transport properties. However, an understanding of the surface chemistry and interfacial effects will lead to the ability to modify and control rubrene’s electronic properties, and therefore, its practical applications. In previous work we have studied the topographical evolution and associated conductive properties of the cleaved surface of rubrene using atomic force microscopy and conductive atomic force microscopy. Within that study the surface was seen to exhibit molecular reorganization resulting in the formation of nanoscale features aligned along molecular step edges. This only occurs in ambient environmental conditions implying the incorporation of environmental species into the molecular structure of the surface. With the AFM studies, the molecular makeup can only be eluded and inferred to by the variation in height of the surface features. In this work, ToF-SIMS (ION-TOF V) has been used to expand the information provided by the previous study. ToF-SIMS uses primary ion irradiation to ionize and liberate secondary ions from the surface of the sample, a time of flight analyzer then produces a mass spectrum of the molecules and fragments present in the sputtered secondary ions. Using this technique the surface of rubrene single crystals are characterized, and depth profiling of the crystal has also been carried out. In both cases it is important to understand the sputtering process of the organic crystal, and the ion beam damage occurring during the bombardment of the surface with an energetic ion beam. To this end the rubrene surface was exposed to various ion beams for a set dose density ranging from ‘static’ to ‘dynamic’ SIMS conditions. The bombarded surfaces were then studied by high resolution AFM to identify any sputter induced damage to the crystals surface. This method will allow analysis of the chemical composition of the nanoscale features and will allow investigations of the rubrene surface interactions with the environment.


U8.20
Two-Photon Spectroscopy of Rubrene Single Crystals.Pavel Irkhin, Aleksandr Ryasnyanskiy and Ivan Biaggio; Physics, Lehigh University, Bethlehem, Pennsylvania.

We investigate the electronic excited states that can be reached via two-photon excitation in rubrene single crystals. We report both the emission spectrum after two-photon absorption as well as the two-photon excitation spectrum for various illumination and detection conditions. We determine the two-photon absorption cross-section in the crystal from nonlinear optical transmission measurements and compare it to the same measurement performed in a rubrene solution. This allows the investigation of the separate contributions of localized molecular transitions, which explain crystal properties in an oriented gas model, and of transitions that correspond to electronic states delocalized over several neighboring molecules. These findings are confirmed by the observed anisotropy of the two-photon absorption coefficient measured in rubrene single crystals. The advantage of utilizing two-photon absorption processes is that it allows to access energy levels otherwise inaccessible via conventional light absorption and allows to excite deep in the bulk of highly absorbing materials, thus avoiding any possible surface effects and enabling the investigation of the intrinsic properties of a bulk sample. In particular, most photoluminescence investigations in rubrene have been done with a penetration depth of the excitation limited to less than ~10 micrometers. Our spectroscopy and anisotropy data provides experimental parameters for optimal application of two-photon absorption technique to the investigation of excitonic states in the bulk of rubrene, away from the influence of any possible surface defect states.


U8.21
n-Type Low Band-Gap Conjugated Polymers Based on Azadipyrromethenes.Lei Gao1, Genevieve Sauve1, Lei Zhu2, Saide Tang2 and Singer Kenneth3; 1Chemistry, Case Western Reserve University, Cleveland, Ohio; 2Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, Ohio; 3Physics, Case Western Reserve University, Cleveland, Ohio.

Low bandgap (<1.5 eV) conjugated polymers were synthesized via palladium-catalyzed Sonogashira coupling reactions. These copolymers incorporate red-light absorbing azadipyrromethenes (Aza-DIPY) within the main chain, allowing for enhanced conjugation and absorption in thin films up to ~1000 nm. These polymers displayed reversible reductions as ascertained by cyclic voltammetry. Reactions with trifluoroboron etherate yielded a new series of polymers having BF2 incorporated into each Aza-DIPY unit of the polymer backbone. These materials displayed increased solubility, red shifts (optical bandgap < 1.3 eV) and strongly stabilized LUMO energy levels. The resulting polymers thus have a unique combination of very low bandgap and high electron affinity. The absorption spectra of thin films red-shifted compared to those of the corresponding solutions,consistent with extended conjugation and higher degree of structural order in films. Synchrotron X-ray diffraction studies of annealed materials showed that substituents on the phenyl rings of aza-DIPYs and the BF2 chelation have strong effects on the molecular packing in the solid state, including π-π stacking of the main chain and lamellar ordering of the side chains. Charge transport measurements using time-of-flight will also be presented.


U8.22
Resonant Scattering with Polarized Soft X-Rays: A Probe for Nanoscopic Bond-Anisotropy in Low Z Matter.Brian A. Collins1, Hongping Yan1, Eliot Gann1, Justin E. Cochran2, Michael L. Chabinyc2, Christian Hub3, Rainer Fink3, Cheng Wang4, Christopher R. McNeill5 and Harald Ade1; 1Physics, NC State University, Raleigh, North Carolina; 2Materials, University of California Santa Barbara, Santa Barbara, California; 3Physikalische Chemie II and ICMM, Universität Erlangen-Nürnberg, Erlangen, Germany; 4Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California; 5Materials Engineering, Monash University, Clayton, Victoria, Australia.

We demonstrate that scattering with polarized soft x-rays (P-SoXS) has excellent sensitivity to the orientation of bonds in soft matter, which we use to characterize nanoscale and mesoscale morphologies and domains exhibiting differences in bond-orientation. The method is exemplified in three material systems, each having a unique morphology and bond orientation distribution. In an immediate application, we use P-SoXS to show that orientationally correlated domains affect device performance in polymeric thin film transistors. The anticipated practical measurement range of P-SoXS corresponds to features from ~2 nm to 6 μm. The method presented can be extended to any material primarily composed of elements with bonding configurations that would exhibit polarization dependent material properties in the soft x-ray range.


U8.23
Abstract Withdrawn


U8.24
Oligoaniline Crystals: Morphology Control, Hierarchical Assembly and Structure-Property Relationships.Yue Wang1,2, Henry D. Tran3, Jinglin Liu4, David C. Martin4 and Richard B. Kaner1,2; 1Chemistry and Biochemistry, University of California, Los Angeles, Los Angeles, California; 2California NanoSystems Institute, Los Angeles, California; 3Fibron Technologies, Inc., Inglewood, California; 4Materials Science and Engineering, University of Delaware, Newark, Delaware.

Short-chain oligomers of aniline are attractive semi-metallic materials for applications as organic electrodes or hole-transporting layers in organic photovoltaics. However, conventionally processed oligoanilines are often amorphous, which limits their conductivities and carrier transport mobilities. Here, we report a simple solvent-exchange method that can render a variety of oligoanilines and their derivatives into crystals of different shapes and dimensions, including 1-D fibers and wires, 2-D ribbons and plates, and high surface area 3-D hollow spheres, porous sheets, and flower-like structures. Dopant ions are also simultaneously incorporated into the crystals during self-assembly. Mechanistic studies suggest that the higher order crystals arise from the most primitive nanofiber morphology via hierarchical assembly, providing insights into a general approach to organic crystal morphology control. High-resolution electron microscopy studies reveal their single or polycrystalline nature. Different electrical transport, optical, and surface properties are observed for crystals of different shapes, demonstrating the explicit structure-property relationships, which have important implications in areas such as organic electronics and optoelectronics.


U8.25
Two-Dimensional Structural Mapping of OTFT Channels and Contacts: Microbeam Grazing Incidence Wide Angle X-Ray Scattering (μGIWAXS).Ruipeng Li1, Jeremy Ward2, Detlef Smilgies3, Oana Jurchescu2 and Aram Amassian1; 1Material Science and Engineering, KAUST, Thuwal, Saudi Arabia; 2Department of Phyics, Wake Forest University, Winston-Salem, North Carolina; 3CHESS, Cornell University, Ithaca, New York.

Solution-processed small molecule organic thin film transistors (OTFTs) have been receiving increasing attention due to their high charge carrier mobility and potential for low-cost processing. Among these materials, fluorinated 5,11-bis(triethylsilylethynl) anthradithiophene (diF-TES-ADT) shows the highest performance in bottom contact OTFT devices. The success of these devices depends heavily on the surface treatment of the Au contacts, which have been reported to control the texture of the polycrystalline molecular thin film in the channel of the device. Direct observation of structural differences in the channel or on the electrode of a given device is typically not possible using conventional grazing incidence wide angle X-ray scattering (GIWAXS). We have performed X-ray microbeam mapping of an array of OTFT devices, revealing differences in the crystallinity, texture and mosaicity of the polycrystalline this films in the channel, on the contact, or on the dielectric at a significant distance from contacts. In the case of diF-TES-ADT, we observed the pure (001) orientation on top of PFBT-treated gold electrodes as well as in the channel of the device, which means the (001) phase dominates the entire device. A second diF-TES-ADT phase, the (111) phase is observed 10-20 microns away from the edges of PFBT-treated Au electrodes. The Au (111) diffraction ring and large diffuse scattering from Au above the beamstop indicate the position of the microbeam on the 2D OTFT array unambiguously, providing extra validation of our device mapping technique.


U8.26
Correlation of Interfacial Width and Turn-on Voltage in All Polymer Thin Film Transistors Based on P(NDI2OD-T2).Hongping Yan1, Torben Schuettfort2, Chris R. McNeill3 and Harald W. Ade1; 1Physics, NC State University, Raleigh, North Carolina; 2Physics, University of Cambridge, Cambridge, United Kingdom; 3Materials Engineering, Monash University, Clayton, Victoria, Australia.

The interface between constituent semi-conductive polymers in organic electronic devices is critically important and a better understanding of the interfacial structure is required to better control the device performance. In thin-film transistors (TFTs), charge transport occurs essentially along a 1 nm deep accumulation layer in the semiconducting polymer at the interface with a dielectric layer. Besides molecular interactions, which determine the packing geometry of the semiconducting polymer, the charge transport performance is highly influenced by the microstructure of the film. For example, the mobility of crystalline organic semiconductors is highly sensitive to the roughness of the surface of inorganic dielectric layers. In order to characterize these films and interfaces, we use the unique technique Resonant Soft X-ray Reflectivity (R-SoXR). By tuning the soft X-ray energies, we are able to selectively affect the reflected intensity at the various interfaces and quantitatively characterize the interfacial width (root-mean-square roughness) and thicknesses of the films. The use of soft X-rays avoids the necessity for deuteration required for neutron reflectivity, yet provides much higher materials contrast and hence sensitivity to the buried polymer/polymer interface than is possible with the use of hard X-rays. In an effort to relate performance of TFTs to microstructure of interface, we have used R-SoXR to investigate the bilayer system containing the semiconducting polymer poly{[N,N9-bis(2-octyldodecyl)-naphtha-lene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5, 59-(2,29-bithiophene)}, (P(NDI2OD-T2), Polyera ActivInk N2200). The bilayer is made with polystyrene (PS) or poly(methyl methacrylate) (PMMA) or CYTOP (a fluorinated polymer) as the top dielectric layer and (P(NDI2OD-T2) as the channel supported on a Si substrate. Samples with different annealing conditions are investigated. Significantly, R-SoXR also yields excellent tunable contrast near the fluorine absorption edge. The measured interfacial widths depend on the dielectric polymer used, revealing that the deposition of the top layer changes the surface of the bottom layer and hence the interface thus created. The observed differences in interface structure and width could be the source of the differences in turn-on voltage observed for the three dielectrics, with the roughness correlating positively with the turn-on voltage. Our results suggest that further control over the smoothness of the interface might be required and possible for ultimate device performance and low turn-on voltage.


U8.27
Investigating the Distribution of Electronic Defects in BHJ Materials and Their Impact on Charge Transport and Device Behavior.Xin Jiang1, Sean Shaheen1, Alexandre Nardes2 and Nikos Kopidakis2; 1Physics, University of Denver, Denver, Colorado; 2National Renewable Energy Laboratory, Golden, Colorado.

Organic Photovoltaic devices are a promising low-cost technology that is approaching commercialization. However, the efficiency of Organic Photovoltaic devices is quite low compared to traditional device. Mobility would be an important factor to affect the power conversion capability. In this work, We develop a Monte Carlo simulation in order to simulate charge transport in a disordered organic semiconductor such as P3HT. we investigate the electrical transport properties of P3HT and P3HT:PCBM blends and the role of defects states with the help of time-of-flight (TOF) mobility measurements. The MC approach is coupled to non-adiabatic Marcus theory to calculate the mobility and mimic the photoconductivity transients from TOF. In Salleo Group’s work, they showed that an exponential tail of trap states follows the major Gaussian shape of density of states(DOS) in organic materials blends, while other group's experiments indicated that the trap states is Gaussian shape. To deeply understand band structure and electric field effects, we apply a Gaussian density of energy states with an exponential tail of energy states in MC simulation, compared to the modeling with two Gaussian shape distribution. The results show that Gaussian shape traps modeling have obvious effects on TOF simulation, which lead the long tail of the transient, while effects from exponential traps could be neglected. We also investigate the depletion region by adding non-uniform electric field into our MC modeling at the interface. At lower applied biases, it is interesting that a diffusional transient with a peak is observed, which correspond to experiment data showed in many TOF experiments. Moreover, different distributions of electronic defects and non-uniform electric field impacts on the device behavior is also present in our study.


U8.28
Synthesis of New Pyromellitic Diimide-Based n-Type Polymer for Organic Field-Effect Transistors.Srinivas Kola, Noah J. Tremblay and Howard E. Katz; Department of Materials Science and Engineering, Johns Hopkins University, Baltimore, Maryland.

Polymeric semiconductors are attractive for their excellent film-forming properties and compatibility with plastic substrates, thereby enabling mechanically flexible and large-area devices. Naphthalene/perylenetetracarboxylic diimide (NTCDI/PTCDI), moieties have been used as building blocks to develop polymeric n-type organic field-effect transistors (OFETs) because of their strong electron-withdrawing nature. Pyromellitic diimide (PyDI) also has high dimensional stability, good chemical resistance, and excellent thermal, mechanical, and reduction potential similar to NTCDI/PTCDI. Among diimides, PyDI is the smallest moiety with sufficient electron affinity for electron injection and transport, and can also interact with electron-rich π-systems. In our previous investigations, Some PyDI derivatives showed electron mobilities of up to 0.07 cm2 V-1 s-1 [1]. As a part of our continuing effort to develop materials based on PyDI, we have synthesized many polymeric and oligomeric materials which utilize different polymerization reactions. Thus far, we have successfully synthesized solution processible materials that exhibit electron transport. We are currently derivatizing the basic structure and optimizing device integration towards polymers with both dielectric and n-OFET semiconductor properties. Reference: [1] Zheng, Q.; Huang, J.; Sarjeant, A.; Katz, H.E. J. Am. Chem. Soc. 2008, 130, 14410.


U8.29
Imaging Stored Charge in Lateral Heterojunctions of Pentacene and Polystyrene.Thomas J. Dawidczyk1, Gary Johns2, Recep Ozgun3, Andreas G. Andreou3, Nina Markovic2 and Howard E. Katz1; 1Materials Science, Johns Hopkins University, Baltimore, Maryland; 2Physics, Johns Hopkins University, Baltimore, Maryland; 3Electrical Engineering, Johns Hopkins University, Baltimore, Maryland.

Threshold voltage tuning of organic semiconductor (OSC) transistors is critical for proper noise margins in CMOS circuits. One method used to tune the threshold voltage of organic field-effect transistors (OFETs) is to electrically charge the dielectric layer of the OFET. To investigate the charge transport and storage at the OSC/dielectric interface, lateral heterojunctions of pentacene/polystryene were created using a novel fabrication technique. This technique allowed the traditional vertical stack of the transistor to be placed horizontally for facile examination of the OSC/dielectric interface. The samples were analyzed using scanning Kelvin probe microscopy (SKPM), which provides surface potential of the substrate. Scans were taken as various electrical biases were applied to the heterojunction, showing the voltage drop across both the bulk and interface. To further study the effect of charging a dielectric, which leads to threshold voltage shifts in transistors, the dielectric had charges injected by applying a higher voltage (~200V) for a prolonged time. After charging, the heterojunction was exposed to the same applied voltages used before charging. While the uncharged samples showed an inversion in the SKPM scan as the bias was changed from positive to negative voltage, the charged sample kept the built in potential at the interface, showing no inversion. This offset potential can be either positive or negative depending on the polarity of the voltage applied during charging. The effect of charging time was also investigated, with short charge times showing a majority of the charge in the dielectric is still close to the interface, while a longer time allows for the charge to diffuse deeper into the dielectric. These results help demonstrate the nature of charge storage for devices where the threshold voltage was tuned by a charged dielectric.


U8.30
Abstract Withdrawn


U8.31
Abstract Withdrawn


U8.32
Structures of Bromoinated Oligothiophenes and Ethylenedioxythiophenes: A Combined Experimental and Theoretical Study.Mamoun M. Bader1 and Phuong-Truc T. Pham2; 1Chemistry, Pennsylvania State University, Hazleton, Pennsylvania; 2Chemistry, Pennsylvania State University, Worthington Scranton, Pennsylvania.

In this presentation, we report on the results of solid state structural study of a series of oligothiopehens and ethylenedioxythiophenens (EDOT) endowed with bromine atoms at different positions and in the presence/absence of the strong electron acceptor, the tricyanovinyl group (TCV). Five compounds were synthesized, crystallized and their single crystal structures were determined by X-ray diffraction. The impact of the presence of bromine atoms on solid state packing as viewed through analysis of intermolecular interactions will be presented. These results are compared with published structures on closely related molecules. Comparison of these experimentally determined structures is then made with computational chemistry results using density functional theory. Among the structural features observed were: competitive inter- and intra- molecular interactions involving the Br in the presence of TCV and EDOT. Distances shorter that the summation of the Van der Waals radii were observed to indicate the role of Br…NC and, S…S and Br…Br interactions in addition to pi stack formation and planarity of these compounds due to the presence of strong electron acceptors. Possible implications of how these structural features may impact charge transport in oligothiophenes will be discussed.


U8.33
In Situ Structural Study of Organic Semiconductor Thin Films.Takuya Hosokai1, Takeshi Watanabe1, Tomoyuki Koganezawa2, Jorg Ackermann3, Hugues Brisset3, Christine V. Ackermann3 and Noriyuki Yoshimoto1; 1Department of Material Science and Engineering, Iwate University, Morioka, Japan; 2Japan Synchrotron Radiation Research Institute, Sayo-cho, Japan; 3Centre Interdisciplinaire de Nanoscience de Marseille, Marseille, France.

Controlling film growth of organic semiconductors is essential to improve performance of organic semiconducting devices, such as organic field-effect transistors and organic photovoltaic cells. Owing to many growth parameters, e.g. substrate, surface roughness and deposition conditions (deposition rate and substrate temperature), organic molecules often show complicate film growth and structures. Moreover, it has been observed that organic molecules can migrate on a substrate surface after deposition in a wide range of time scale, showing dynamics of the structural change. Aging effects by gas exposure on the film structure have been also observed. Therefore, to consider those possible parameters toward establishing the detailed growth mechanism of organic semiconductor thin films, one should study the film growth and structure in situ and in real time. In this talk we would like to show our recent results of such a study employing a technique of X-ray diffraction. We have built a new apparatus being capable of the measurements of the diffraction during vacuum-deposition of organic molecules under a high vacuum pressure. Experiments were performed using synchrotron X-ray light in a beamline 19B2 at SPring-8 (Harima) combined with a two-dimensional hybrid pixel array detector, PILATUS detector. Focusing on distyryl-oligothiophenes derivatives/native SiO2/Si wafer systems, we will demonstrate how the side alkyl-chains influence on their film growth and structure of the derivatives on the surface. Furthermore, we will show the effect of air exposure to post-growth films of various organic molecules, e.g., resulting in the promotion of the crystallization for the case of pentacene films.


U8.34
Abstract Withdrawn 


U8.35
Low Driving Voltage Organic Transistor with High k Amorphous BST(Ba0.7Sr0.3TiO3) Thin Film Gate Dielectric.Zongrong Wang1, Xiaochen Ren1, Jianzhuo Xin2, Dennis Leung2 and Paddy Chan1,3; 1Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hong Kong, Hong Kong; 2Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong, Hong Kong; 3Department of Mechanical Engineering, The University of Hong Kong, Hong Kong, Hong Kong.

The high operating voltages of SiO2 based organic field effect transistors (OFETS) limit the applications requiring lower operation power devices such as portable electronics which is still dominant by the inorganic devices. To reduce the operating power in these OFETs, integration of suitable gate dielectric materials with high capacitance density is critical. In this work, the high quality of amorphous BST gate dielectric thin films with dielectric constant of about 25ε0 are fabricated by PLD (pulsed laser deposition) at low temperature 383K. Using BST, we fabricated bottom gate top contact transistor on silicon substrates with pentacene as organic semiconductor layer. The transistor demonstrated field effect at 2.5V, with a subthreshold swing of about 300mV per decade and the field effect mobility is larger than 3.5cm2V-1 s-1. Besides, the gold nanoparticles are introduced into the device for the OTFT based memory devices in both p-type and n-type semiconductors. This low fabrication temperature as well as low driving voltage has potential application in electronics circuit both on plastic/flexible and dedicate substrates.


U8.36
Abstract Withdrawn


U8.37
Carrier Transport Dependence on Phosphorescent Materials in Polymer Based OLEDs.Genichi Motomura, Mitsunori Suzuki, Takahisa Shimizu and Hideo Fujikake; Display & Functional Devices Research Division, NHK Science & Technology Research Laboratories, Tokyo, Japan.

The carrier transport properties in the emissive layer of phosphorescent polymer organic light-emitting-diodes (OLEDs) were observed by time-of-flight (TOF) mobility measurements. Mobility evaluation for organic semiconductors is a key theme in the development of highly efficient OLEDs. The emissive layer consists of phosphorescent Iridium complexes and carrier transport polymer made from 4,4’-bis[N,N’-(3-tolyl)amino]-3,3’-dimethyl biphenyl (HMTPD) for hole transport and tris(3,5-dimethylbiphenyl-4-yl) borane (TBPhB) for electron transport. These materials are soluble in common organic solvents and can be formed from solutions into uniform films by spin coating.The hole and electron mobilities in carrier transport polymer without Iridium complexes were measured with 3×10-4 cm2/Vs (hole) and 1×10-3 cm2/Vs (electron) at a bias field of 4.9×105 V/cm. TOF transient photocurrents indicated non-dispersive transport. The hole mobility in phosphorescent polymers with tris(2-(4’-tert-butylphenyl) pyridine) iridium (Ir(tBuppy)3), 1×10-6 cm2/Vs, was extremely lower than that of the carrier transport polymer. The electron mobility, 4×10-4 cm2/Vs, was a little lower than that of the carrier transport polymer. The effect of Iridium complex on mobility greatly varies with the Iridium complex contained in the phosphorescent polymer. When a bis(2-phenylpyridine)(acetylacetonate) iridium (Ir(ppy)2acac) structure was used for a green phosphorescent material, the hole mobility, 2×10-4 cm2/Vs, maintained a high mobility of non-dispersive transport. However, the electron transport properties decreased because the transient photocurrent in the electron mobility measurement turned into dispersive transport. When a tris(1-(4’-tert-butylphenyl)isoquinoline)iridium (Ir(tBupiq)3) structure was used for a red phosphorescent material, the hole mobility, 1×10-6 cm2/Vs, decreased as the phosphorescent polymer using Ir(tBuppy)3 and the transient photocurrent in the electron mobility measurement turned into dispersive transport. These results mean that the transport in the phosphorescent polymers was disturbed considerably by the Iridium complexes acting as a carrier trap. The HOMO levels were measured for each material by using photoemission yield spectroscopy in air and the LUMO levels were estimated by adding the energy of optical absorption to the HOMO levels. The HOMO level of the carrier transport polymer was 5.6 eV, and the LUMO level was 2.9 eV. The HOMO levels of the Iridium complexes were 5.4 eV [Ir(tBuppy)3], 5.6 eV [Ir(ppy)2acac], and 5.3 eV [Ir(tBupiq)3]. The LUMO levels were 2.7 eV [Ir(tBuppy)3], 3.0 eV [Ir(ppy)2acac], and 3.0 eV [Ir(tBupiq)3]. Therefore, the Iridium complexes acted as a trap in the phosphorescent polymers and decreased the transport properties of the polymers when the energy levels of the Iridium complexes were lower than that of the carrier transport polymer.


U8.38
Intermolecular Hybridization Governs Molecular Electrical Doping.Ingo Salzmann1, Georg Heimel1, Steffen Duhm2, Martin Oehzelt3,1, Patrick Pingel4, Ralf-Peter Blum1, Antje Vollmer3 and Norbert Koch1,3; 1Department of Physics, Humboldt Universität zu Berlin, Berlin, Germany; 2Graduate School of Advanced Integration Science, Chiba University, Chiba, Japan; 3BESSY II, Helmholtz Zentrum Berlin für Materialien und Energie, Berlin, Germany; 4Institut für Physik und Astronomie, Universität Potsdam, Potsdam, Germany.

Research efforts to apply strong organic donor/acceptor molecules as organic semiconductor (OSC) dopants are yet increasing in applied research. The admixture of a few mol-percent of a molecular acceptor with an electron affinity comparable to the ionization energy (IE) of the OSC is typically used for p-type doping. Electron transfer from the highest occupied molecular orbital (HOMO) of the OSC to the lowest unoccupied molecular orbital of the p-dopant is believed to lead to a localized electron on the p-dopant and a mobile hole in the OSC matrix, which accounts for the observed increase in the density of mobile charge carriers. Singly-occupied states (positive polarons) in the fundamental gap of the pristine OSC are thus expected, and could be observable as emission feature in ultraviolet photoelectron spectroscopy (UPS) experiments close to, or at the Fermi energy (EF). This, however, has not been observed to date. Here we present a combined experimental and theoretical study on the prototypical OSC/p-dopant pair pentacene (PEN) and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) up to very high dopant concentrations of 10% and 50% with the initial aim to observe such polaronic states at EF in UPS. The 10% doped PEN films exhibit slightly increased IE, and IE of 1:1 mixed (amorphous) films is even 0.85 eV higher than that of pure PEN. The HOMO of doped PEN films is always well below EF, which is in clear contrast to the expectation of PEN molecule ionization by the acceptor. To remedy these inconsistencies, we suggest intermolecular orbital hybridization between the OSC HOMO and the dopant LUMO, leading to the formation of a doubly occupied bonding and an empty anti-bonding supramolecular hybrid orbital with a reduced fundamental gap, which is corroborated by density-functional theory (DFT) calculations on PEN/F4-TCNQ complexes. Based on similar results for various OSCs, we propose molecular electrical doping to be governed by the presence of OSC/dopant hybrids within the OSC matrix exhibiting low-lying unoccupied states in the fundamental gap of the OSC. This model rationalizes why not every dopant molecule provides one free charge carrier: as available states are occupied following Fermi-Dirac statistics, only a fraction of the hybrids is ionized at room temperature, which further explains the considerably high doping concentrations of a few mol-percent usually needed to achieve high conductivity.


U8.39
Study on Work Function of Modified Indium Tin Oxide and Performance of Organic Electroluminescent Devices. Munkhbat Battulga, Qi Wei, Gendensuren Bolormaa, Davaasambuu Sarangerel and Chimed Ganzorig; Department of Chemical Technology, School of Chemistry and Chemical Engineering, Ulaanbaatar, Mongolia.

The work function of the indium tin oxide electrode (ITO) modified by a self-assembled monolayer (SAM), which is used as an electrode in organic electroluminescent (EL) devices, was investigated in this study. It is revealed that modification of ITO electrode by immersion in solution of the novel bicyclic organic compound as a SAM material with carboxyl binding group is caused to increase the work function of the ITO electrode. Through a self-assembly process, the transmittance of the ITO with a SAM was not changed. The electrochemical characterization of modified ITO with a SAM was measured by using cyclic voltammetry. Characteristics of EL devices were increased because the energy barrier was decreased in an interface between the ITO and an organic layer in the EL devices. We also investigated the correlation between the change in the work function and the performance of the chemically modified EL devices.


U8.40
Charge Carrier Storage and Delayed Recombination in White Organic Light-Emitting Diodes.Caroline Weichsel1, Sebastian Reineke1,2, Susanne Hintschich1, Bjoern Luessem1 and Karl Leo1; 1Institut für Angewandte Photophysik, Technische Universität Dresden, Dresden, Germany; 2Massachusetts Institute of Technology, Cambridge, Massachusetts.

White phosphorescent organic light-emitting diodes (OLED) are of strong interest as they can reach high efficiencies at good color quality and are therefore seen as potential replacement of currently available lighting technologies. However, although OLEDs reaching fluorescent tube efficiencies have been reported [1], many fundamental processes of light generation remain unknown. In this contribution, we present an OLED stack reaching warm white emission at CIE coordinates (0.444; 0.409), which is very close to the color point A. Besides high color rendering with a CRI of 82, this OLED obtains 10.0% external quantum efficiency and 17.4 lm/W luminous efficacy at a luminance of 1000 cd/m2. The device stack consists of state-of-the-art phosphorescent emitters, which are doped into separate thin layers. Using two matrix materials with opposite transport characteristics, it is possible to define the charge carrier recombination zone in the center of the emission layer. This so called double emission layer concept is well proven for layers with single emitters [2]. To investigate the charge carrier transport as well as exciton transfer processes, we performed time-resolved electroluminescence measurements. By applying voltage pulses with a length of 50 µs, we not only observe a transient decay on the microsecond time scale as expected for phosphorescent emitter molecules, but also an initial overshoot of the light intensity after the turn-off of the voltage pulse. This initial overshoot can be attributed to a delayed recombination of charge carriers stored within the emission layer under forward bias. Performing time and spectrally resolved measurements using a streak camera, we see that the delayed emission mainly originates from the red emitter. Therefore we can localize the charge carrier storage on the red emitting molecules. This storage occurs due to a distinct LUMO level alignment between the red emitter and the matrix molecules. This theory is verified by further transient measurements applying different electric fields and varying the hole blocking layer. We propose that this process negatively affects the external quantum efficiency of the OLED: Thus, by using other electron transporting matrix materials and hole blocking materials, it is possible to increase the efficiency of our OLED. It is possible that a similar effect also takes place in other OLED structures and we suggest that the methods presented here can help identifying delayed recombination as well as determining the position of charge carrier storage inside a complex emission layer. [1] S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lüssem and K. Leo: Nature 459 (2009), 234. [2] G. He, M. Pfeiffer, K. Leo, M Hofmann, J. Birnstock, R. Pudzich and J. Salbeck: APL 85 (2004), 17.


U8.41
Synthesis of Heteroacenes and Their Application to Organic Field-Effect Transistors.Koji Nakano1,2, Minh Anh Truong1, Natsuko Chayama1, Keiko Kawaguchi1 and Kyoko Nozaki1; 1The University of Tokyo, Tokyo, Japan; 2PRESTO-JST, Saitama, Japan.

Heteroacenes are promising materials as organic semiconductors in organic field-effect transistors (OFETs) because they demonstarte high charge-carrier mobility and higher stability to oxygen-involved oxidation than the corresponding hydrocarbon acenes such as pentacene. Recently, we have reported the synthesis of dibenzo[d,d′]benzo[1,2-b:4,5-b′]difurans (DBBDFs) by using the palladium-catalyzed carbon-oxygen bond-forming reaction. Here, we report the modified synthesis of DBBDF framework and the application of DBBDF derivatives to OFETs. 3-Alkyl-DBBDFs were synthesized via acid-mediated intramolecular cyclization of 3-(4-alkyl-2-hydroxyphenyl)-2-hydroxydibenzofurans in high yields. The cupper-mediated reaction with 4,4″-dialkyl-2′,5′-dibromo-2,2″-dihydroxy-p-terphenyls gave 3,9-dialkyl-DBBDFs in moderate yields. Both monoalkyl- and dialkyl-DBBDFs showed several endo- and exothermic peaks on differential scanning calorimetry (DSC). Based on DSC and polarized-optical-microscopy observation, they were found to possess liquid-crystal nature. Top-contact-type FET devices with a vacuum-deposited thin film of monoalkyl- and dialkyl-DBBDFs were fabricated on Si/SiO2 substrate with use of gold source-drain electrodes. In particular, OFET devices with 3-hexyl-DBBDF were found to give a high hole mobility of 0.25 cm2●V−1●s−1 under ambient condition. XRD measurement and AFM image demonstrated the formation of highly-ordered thin film where the DBBDF molecules were arranged with their longer molecular-axix perpendicular to the substrate.


U8.42
Development of the Field-Effect Mobility in Thin Films of F16PcCu Characterized by Electrical In Situ Measurements during Device Preparation. Christopher Keil and Derck Schlettwein; Institute of Applied Physics, Justus-Liebig-University Giessen, Giessen, Germany.

Films of perfluorinated copper phthalocyanine F16PcCu were prepared by physical vapor deposition under high vacuum conditions. They served as n-channel in organic field-effect transistors (OFET) and were characterized from the monolayer range up to 40 nm thickness [1]. The substrate consisted of a n++-doped Si wafer as gate electrode with a thermally grown SiO2 layer as gate dielectric and photolithographically structured films of gold or silver as source and drain electrodes. The charge carrier mobility in the saturation regime of the output characteristics and the threshold voltage as well as the on/off- ratio were determined during film preparation of F16PcCu. Electron mobility, threshold voltage and on/off ratio significantly changed with film thickness. A conductive channel was formed in the interface region at about 2 nm average film thickness. In the subsequent regime of layer formation up to about 8 nm a constant charge carrier mobility was established. The threshold voltage considerably decreased during this period of growth and reached a stable level. Subsequent island formation on top of this channel led to only minor changes in the transistor characteristics although the conductivity along the complete organic thin film decreased considerably. By such in-situ characterization different contributions of interfacial and bulk transport could be determined and used to optimize films to be used in organic field effect transistors. An ultrathin conductive layer was found optimum since parasitic islands were subsequently formed. Ultrathin films of F16PcCu further showed a considerably faster stabilization of the mobility upon exposure to air, the highest on/off ratios and therefore provide clear advantages for device applications. [1] C. Keil and D. Schlettwein, Org. Electron. 12, 1376-1382 (2011).


U8.43
Switching the Polarity of Charge Transport through Self-Assembled Monolayer Devices. David A. Egger1,2, Ferdinand Rissner1, Egbert Zojer1 and Georg Heimel2; 1Institute of Solid State Physics, Graz University of Technology, Graz, Austria; 2Institut für Physik, Humboldt-Universität zu Berlin, Berlin, Germany.

The miniaturization of electronic devices through top-down approaches has inherent limits. In search for a technological paradigm shift, bottom-up approaches based on individual functional molecules mark an appealing alternative in principle. In practice, however, such devices are technologically challenging to realize. More compatible with current manufacturing techniques and less sensitive to atomic-level details and fluctuations of the environment are devices based on self-assembled monolayers (SAMs) of organic molecules covalently linked to metallic electrodes. Also there, functionality is introduced through targeted chemical design of individual molecules. Clearly, the success of such a strategy critically hinges on understanding how the electronic properties of individual molecules translate into the transport characteristics of a molecular monolayer. Our present theoretical study highlights that establishing such a link is far from trivial. Density-functional theory calculations reveal that two SAM devices can exhibit fundamentally different charge-transport characteristics despite being composed of molecules with virtually identical chemical structure and orbital energies: Firstly, the total amount of current through representative model compounds can differ by up to one order of magnitude in the low-bias regime. Secondly, and even more intriguingly, the polarity of charge transport through such SAMs can switch between n-type and p-type, i.e., charge transport is established either via unoccupied or occupied electronic states. These findings are rationalized by the collective electrostatic action of local intramolecular dipoles in the SAMs. Understanding such physical phenomena at the molecular level thus emerges as valuable contribution to the chemical design of functional elements in future nanoscale electronics.


U8.44
Modeling Charge Transport in OFETs Based on a Measured Density of States.Christian Roelofs1,2, Simon Mathijssen2, Dago de Leeuw2, René Janssen1 and Martijn Kemerink1; 1Department of Applied physics, Eindhoven University of Technology, Eindhoven, Netherlands; 2Philips Research, Eindhoven, Netherlands.

The charge transport mechanism in organic field-effect transistors has been described by various models. Crucial in all models is the characteristic width of the density of states (DOS) [1,2]. Typically the width, given by a characteristic temperature T0, is indirectly obtained by fitting of measured transfer curves. Here, we present a direct determination of the DOS in transistors. We show for the first time that by using the measured DOS in a variable range hopping (VRH) model an excellent description of the transfer characteristics is obtained over a wide temperature range. The DOS is determined using scanning Kelvin probe microscopy, where the surface potential is probed while sweeping the gate voltage of the transistor [3]. This method can be applied as long as the temperature is lower than the characteristic temperature of the DOS. Crucial for an accurate determination is that the semiconductor thickness should be comparable to that of the accumulation layer. Therefore we investigate a self-assembled monolayer field-effect transistor [4]. The deep states of the measured DOS in the sub-threshold regime can be described by a single exponential distribution with a characteristic temperature of 1220 K. This value is in striking contrast to T0 values around 500 K that are obtained when one simply fits a VRH or mobility edge model to the transfer curves[1,2]. For voltages above the sub- threshold regime the measured DOS increases much stronger than the exponential distribution. In this regime the DOS is approximated by a Gaussian distribution with an approximate width of 0.7-0.8 eV. The measured DOS, consisting of an exponential tail below a Gaussian peak, has been used as input in mobility models to describe transfer curves of the transistor. A mobility edge model, based on band theory, is not able to describe the measured transfer curves with the found DOS. With the VRH-model, however, an excellent agreement over a wide temperature range is obtained. Such a good description cannot be obtained with a single exponential or a single Gaussian DOS. References: [1] Vissenberg, M.C.J.M. and M. Matters, Physical Review B, 1998. 57(20): p. 12964-12967. [2] Horowitz, G. and P. Delannoy, Journal of Applied Physics, 1991. 70(1): p. 469-475. [3] O. Tal, Y. Rosenwaks, Y. Preezant, N. Tessler, C. K. Chan and A. Kahn, Phys. Rev. Lett. 95 (25) (2005). [4]S. G. J. Mathijssen, E. C. P. Smits, P. A. van Hal, H. J. Wondergem, S. A. Ponomarenko, A. Moser, R. Resel, P. A. Bobbert, M. Kemerink, R. A. J. Janssen and D. M. de Leeuw, Nat. Nanotechnol. 4 (10), 674-680 (2009)


U8.45
Explorations of Effective Factors for Optimizing Light-Emitting Organic Field-Effect Transistors.Susumu Ikeda1, Kanagasekaran Thangavel2, Ryotaro Kumashiro2, Takuto Inoue2, Hui Shang2, Dairi Hirota2, Hidekazu Shimotani2 and Katsumi Tanigaki1,2; 1WPI Advanced Institute for Materials Research (WPI-AIMR), Tohoku University, Sendai, Japan; 2Graduate School of Science, Tohoku University, Sendai, Japan.

Organic semiconductors have attracted wide interest due to their flexibility, low energy consumption, low production costs, as well as large area electronics applications. Ambipolar carrier transport is also a unique property, generally unavailable in inorganic semiconductors, and enables simultaneous injection of electrons and holes into an organic crystal and leads to light emission. Based on this background, light-emitting organic field-effect transistors (LE-OFETs) have extensively been studied as new optical devices and one of the candidates to realize an electrically-driven organic laser [1-3]. For this purpose, many attempts have been made to tune and optimize the light-emission properties of LE-OFET devices. One important factor is the energy level alignment at organic/metal interfaces, that is, between the HOMO/LUMO levels and work functions of metal electrodes. Generally Au is used for hole injection and lower work function metals such as Ca and Mg are employed for electron injection. Another important factor is the surface treatment of substrates. It is known that hydroxyl groups on the SiO2 surface trap electrons, and electron transport cannot occur even when electrons are injected from low work function metals [4]. Therefore surface modification is necessary to prevent the trap of electrons when we use thermally-oxidized Si substrates in LE-OFETs. This presentation mainly focuses on the surface modification of the substrates; materials and processes for the modification. PMMA, parylene, amorphous fluoropolymer (CytopTM) and other insulators have been investigated. We evaluated the effect of the surface modification with these materials based on two experimental techniques; one is by fabricating and testing the LE-OFET (electrically-driven) devices and the other is by observing the optically-pumped stimulated emission (amplified spontaneous emission, ASE). We found that the surface modification influences not only carrier transport in FETs but also intensity and width of emission peaks of the optically-pumped ASE. We also investigated good selection of organic semiconductors. For LE-OFETs, there are two important factors which should be taken into account. The organic semiconductors for LE-OFETs have to satisfy both high carrier mobility (current density) and high luminescence efficiency. We will discuss organic semiconductors appropriate for LE-OFETs based on an idea suggested in collaboration with synthetic chemists. [1] J. Zaumseil and H. Sirringhaus, Chem. Rev. 107, 1296 (2007). [2] T. Takenobu, S.Z. Bisri, T. Takahashi, M. Yahiro, C. Adachi, Y. Iwasa, Phys. Rev. Lett. 100, 066601 (2008). [3] Y. Wang, D. Liu, S. Ikeda, R. Kumashiro, R. Nouchi, Y. Xu, H. Shang, Y. Ma, K. Tanigaki, Appl. Phys. Lett. 97, 033305 (2010). [4] L.L. Chua, J. Zaumseil, J.F. Chang, E.C.W. Ou, P.K.H. Ho, H. Sirringhaus, R.H. Friend, Nature 434, 194 (2005).


U8.46
Tuning the Surface Properties of Gold Electrodes in Organic Field-Effect Transistors Using Self-Assembled Monolayers.Janusz Schinke1,2, Sebastian Hietzschold2,3, Rebecca Saive2, Lars Mueller2,3, Manuel Hamburger2,4, Wolfgang Kowalsky1,2 and Michael Kroeger1,2; 1Institut für Hochfrequenztechnik, Technische Universität Braunschweig, Braunschweig, Germany; 2InnovationLab GmbH, Heidelberg, Germany; 3Kirchhoff-Institut für Physik, Universität Heidelberg, Heidelberg, Germany; 4Organisch-Chemisches Institut, Universität Heidelberg, Heidelberg, Germany.

In organic electronic devices, charge injection at the contacts is crucial for better electrical performance. In bottom-contact p-channel organic field-effect transistors (OFET), Au electrodes are very often used for drain and source contacts. A smart way of enhancing the device's performance is the use of self-assembled monolayers (SAMs) to tune the electrodes’ work function or substrate conditions for deposition of the organic semiconductor. We have studied the properties of SAM-treated gold surfaces via Atomic Force Microscopy (AFM), ambient Kelvin Probe (KP) and contact angle measurements. SAMs used for this work include fluorinated and non-fluorinated alkyl thiols and aryl-thiol-based molecules with amid or imid-functionalization. These molecueles were chosen in order to tune the work function of the gold surfaces to enhance charge injection in organic devices and to tune the wettability for the subsequent deposition of the functional polymer. The substrates were prepared by evaporating 100nm Au on Si, rinsing in acetone and isopropyl. Afterwards, plasma treatment was used to remove the impurities and contaminants from the surfaces. SAM solutions in ethanol were prepared, in which samples were immersed at room temperature for a period of 48h. Self-assembled monolayers were deposited the same way for AFM, Kelvin Probe and contact angle samples. The two materials with the greatest work function shift were investigated in detail first: Perfluorodecanethiol and Hexadecanthiol. The AFM measurements show a very smooth surface topography, which can be an indicator for ordered monolayers. The Kelvin Probe method is used to investigate the effect of the SAMs on the work function. For Hexadecanthiol we see a contact potential difference (CPD) of -500 mV compared to an untreated gold surface. In the case of Perfluorodecanethiol we measured a CPD of more than 1150 mV. The resulting change in the work function can be related to the molecular dipole orientation of the SAMs used. The concept of work function tuning will be applied to a new class of SAMs, aryl-thiol-based molecules, which we tune by different imid- or amid-functionalization. A difference in contact angle can be observed from 111 degrees for Hexadecanthiol to 112 degrees for Perfluorodecanethiol, which is measured using deionized water. This does impose problems with regard to further film deposition by solution coating or printing techniques, when polar solvents are used. We compare the characteristics of SAM-treated OFETs using TIPS-pentacene as an organic semiconductor to standard devices. For our bottom-gate, bottom-electrode FETs, an OTS-vapor treatment is used to functionalize the gate-dielectric. The gold electrode is subsequently treated by Perfluorodecanethiol to increase the effective work function and the device performance. Comparing to untreated OFETs, we see an enhancement of the mobility by two orders of magnitude and a significant reduction of the threshold voltage.


U8.47
Highly Efficient Blue Phosphorescent OLEDs Using 3,6-Diphenylcarbazole-Based Host Materials.Hisahiro Sasabe1,2, Tasuku Ishizaka1, Naoki Toyota1, Yong-Jin Pu1,2 and Junji Kido1,2; 1Organic Device Engineering, Yamagata univ, Yonezawa, Yamagata, Japan; 2Research Center for Organic Electronics, Yamagata univ., Yonezawa, Yamagata, Japan.

We designed and prepared a series of 3,6-diphenylcarbazole derivatives for a host material in blue phosphorescent OLEDs. A blue OLED with a structure of [ITO / TAPC / FIrpic-doped host / B3PyPB / LiF / Al] were fabricated. As a result, a high power efficiency of over 40 lm W-1 (40 cd A-1) was observed at 100 cd m-2 by using tetraphenylmethane-containing host. We also investigated the relationship between the device performances and the substituent on the carbazole moiety, and revealed an effective molecular design for realizing an efficient blue OLED.


U8.48
Morphology Dependence on Photoluminescence of Pentacene Nanostructures.Ji Eun Park and Hee Cheul Choi; Chemistry, Pohang university of Science and Technology (POSTECH), Pohang, Korea, Republic of.

Geometrically well-defined highly conjugated organic molecules have been investigated for soft electronic devices such as field effect transistor, solar cell, light emitting diode with contrasting advantages of low cost, large area, and flexible properties over inorganic semiconductors. Especially, planar aromatic molecules draw a special interest as high mobility characteristic could be achieved when they are systematically packed through π-π stacking. Moreover, such highly ordered conjugated molecules provide a chance to enhance fluorescence quantum yield compared to their ensemble state. In this presentation, we will discuss about our recent achievements in the selective synthesis of pentacene nanostructures into 1-dimensional (1D) wires and 2D disks by temperature controlled vaporization-condensation-recrystallization (VCR) process. Powder XRD data and SAED analysis have revealed that both morphologies are composed of an identical triclinic unit cell. Surprisingly, only 1D pentacene wires exhibit strong photoluminescence property while 2D pentacene disks are silent. The detail proposed mechanism of selective fluorescence by molecules orientation will be discussed.


U8.49
Printed OLEDs on Paper: Influence of Hole Injection Layer and Paper Quality.Wim Deferme1,2, Jeroen Stryckers1,2, Koen Gilissen1,2, Tim van Gerven2, Wouter Moons1 and Jean Manca1,3; 1Instituut voor Materiaalonderzoek, Hasselt University, Diepenbeek, Belgium; 2XIOS University College, Diepenbeek, Belgium; 3Division IMOMEC, IMEC vzw, Diepenbeek, Belgium.

Towards the development of ‘light emitting paper’, in this work organic LEDs (OLEDs) based on conjugated polymers are printed on various paper substrates. First, the physical properties of the paper, i.e. the roughness and surface energy are examined where after a first conductive layer which acts as hole injection layer is printed and examined. For this layer PEDOT:PSS and Ag are compared as conductive inks, eventually supported by an Al checkerboard structure to promote carrier transport. For the light-emitting polymer the well-known soluble phenylsubstituted poly(para-phenylene vinylene) (PPV) copolymer (“superyellow” from Merck/Covion)[1] was selected. As non-transparent paper is the substrate for printing, a top-emitting structure should be shaped, where a semi-transparent top contact is needed. A thin (20nm) Ca/Al anode is the choice which is made here. The resulting devices are electrically characterized by IV measurements, luminance efficiency, power efficiency and external quantum efficiency to elucidate the influence of paper quality and to require the best material and structure build-up for the hole injection layer. [1] H. Spreitzer, et. al., Adv. Mater. 1998, 10, 134


 

SESSION U9: Organic/Polymer Structure-Property
Chairs: Alex Briseno and Iain McCulloch
Thursday Morning, December 1, 2011
Room 311 (Hynes)


8:00 AM U9.1
``Self-Doped" Organic Nanowires - Charge Carrier Formation in Oligothiophene Nanofibrils.Holger Frauenrath1, Liangfei Tian1,2, Ruth Szilluweit1 and Stéphane Suarez1; 1Institute of Materials, Ecole Polytechnique Federale de Lausanne (EPFL), Lausanne, Switzerland; 2Department of Materials, ETH Zürich, Zürich, Switzerland.

We demonstrate that oligopeptide-polymer conjugates based on hydrogenated poly(isoprene) and oligo(alanine) are versatile scaffolds for the reproducible formation of one-dimensional aggregates of π-conjugated molecules. Thus, we determined the molecular parameters required to reliably obtain well-defined helical nanofibrils using diacetylenes as model compounds. These parameters were subsequently used in the design of the analogous oligothiophenes and perylene bisimides. The oligothiophene derivatives were found to reversibly self-assemble by hydrogen-bonding, giving rise to uniform nanofibrils with a diameter of a few nanometers and a length of several micrometers. These nanofibrils exhibited a core-shell architecture, with a linear stack of oligothiopehenes at their center. Upon the self-assembly of the nanofibrils, UV/vis/NIR as well as electron spin resonance (ESR) spectroscopy revealed the ``spontaneous" formation of radical cations with a polaron-like character, which could be quenched and reconstituted with reducing and oxidizing agents, respectively. The corresponding perylene bisimides gave rise to similar nanofibrils which could be processed into macroscopic fibers. Hence, the oligopeptide-polymer conjugates used in this investigation are structurally simple supramoelcular synthons for the preparation of self-assembled organic nanowires. Related hydrogen-bonded oligothiophene derivatives were investigated as the active component in organic field-effect transistors.


8:15 AM U9.2
Elucidating Processing-Structure-Function Relationships of Contorted Hexabenzocoronenes for Organic Electronic Applications.Anna M. Hiszpanski1, Jessica S. Saylors1, Arthur R. Woll2, Detlef Smilgies2, Colin Nuckolls3 and Yueh-Lin Loo1; 1Chemical and Biological Engineering, Princeton University, Princeton, New Jersey; 2Cornell High Energy Synchrotron Source, Ithaca, New York; 3Chemistry, Columbia University, New York, New York.

<p> The anisotropic nature of charge conduction in organic materials mandates that the performance of organic electronic devices is strongly influenced by the structuring and morphology of electrically active layers. The ability to control structural development of organic films is thus critical for optimal device characteristics. To this point, thermally evaporated films of contorted hexabenzocoronene (HBC) are amorphous, and field-effect transistors (FETs) comprising such films show no measurable mobility. We found several processing methods to controllably induce crystallization of these amorphous HBC films; devices comprising these crystalline films exhibit substantial improvement in electrical activity. -3 cm2/Vs. -2 cm2/Vs.

8:30 AM *U9.3
Controlling the Architecture of Semiconducting Polymers.Christine K. Luscombe, Materials Science and Engineering, University of Washington, Seattle, Washington.

Semiconducting polymers are actively under development for use in light-weight, flexible, disposable organic light-emitting diodes, and thin-film transistors. A key application which is currently attracting a lot of interest for semiconducting polymers is their use in organic photovoltaic devices (OPVs). The main drive for developing OPVs is the lower cost associated with their manufacturing, because of the fact that organic semiconducting polymers can be solution processed. Poly(3-hexylthiophene) (P3HT) remains one of the most commonly used polymers in organic photovoltaics due to its desirable electronic properties. Both the Yokozawa and the McCullough groups developed methods for the synthesis of highly regioregular P3HT with controlled molecular weights and narrow molecular weight distributions using the 1,3-bis(diphenylphosphino)propane nickel(II) chloride (NiCl2(dppp)) catalyzed polymerization of Grignard-type monomers. The drawback of this synthetic methodology is that it is not possible to initiate the polymerization from an external moiety. This is necessary for the synthesis of more complex polymer architectures such as brushes, star and block copolymers. Our work towards obtaining these complex architectures will be presented.


9:00 AM *U9.4
Surface-Confined Nickel Mediated Cross-Coupling Reactions: Characterization of Initiator Environment in Kumada Catalyst-Transfer Polycondensation.Jason Locklin, Department of Biological and Agricultural Engineering, University of Georgia - Athens, Athens, Georgia.

Kumada catalyst-transfer polycondensation (KCTP) has proven to be an excellent strategy toward the synthesis of well-defined conjugated polymers. We discuss the reaction of Ni(0) species with surface-bound aryl bromides to yield KCTP initiators of structure (aryl)Ni(II)-Br. Surface-confined Kumada reactions are carried out with a ferrocene functionalized Grignard reagent to quantify the of surface-confined nickel-mediated reactions in terms of initiator coverage, ligand exchange, and Kumada reaction kinetics. In addition, surface-initiated Kumada catalyst-transfer polycondensation (SI-KCTP) is carried out from the fabricated initiators to modify SiO2 and ITO surfaces. Uniform poly(3-methyl thiophene) films with thicknesses between 40 and 65 nm were characterized using a variety of spectroscopic and electrochemical techniques.


9:30 AM U9.5
Templating Highly Crystalline Organic Semiconductors Using Atomic Membranes of Graphene at the Anode/Organic Interface.Susmit Singha Roy, Dominick J. Bindl and Michael S. Arnold; Material Science and Engineering, University of Wisconsin - Madison, Madison, Wisconsin.

Charge and energy transport characteristics of organic semiconductors are typically highly anisotropic and strongly dependent on crystalline ordering. While there have been substantial efforts to control the ordering of monolayers of organic semiconductors to optimize the transverse charge transport mobility of organic field effect transistors (FETs), progress in controlling the ordering and crystallinity of thicker organic films for light emitting (OLED) and harvesting devices (OPVs) has been considerably slower. In such devices, light and charge injection/ extraction occur vertically, requiring that any template material be both optically transparent and electrically conductive. Here, we demonstrate a novel approach for templating highly crystalline organic semiconductors by using an atomic membrane of single monolayer graphene, which can be readily grown by chemical vapor deposition over large-areas and easily integrated onto arbitrary substrates such as glass or indium tin oxide (ITO). The single layer of graphene is > 97% transparent, highly conductive, and provides an ideal template for growing orientated and crystalline films of organic semiconductors. As a proof-of-principal, we have demonstrated that atomic membranes of graphene on glass or ITO are able to template highly crystalline films of α-copper phthalocyanine (α-CuPc). On graphene+glass, thermally evaporated films of CuPc (up to 800 Å in thickness) are preferentially orientated in a flat lying (312) crystallographic direction with lateral crystallite sizes of about 290 nm. In comparison, without graphene, the CuPc is randomly orientated with small domains of only 16 nm. Optical absorption also reveals a significant enhancement in the magnitude of the absorption coefficient in the Q band region of the orientated film on graphene with an 83% increase in the overall absorption. We believe the templating is driven by the substantially stronger attractive forces between CuPc and Graphene (graphite, -61 kcal/mol) as compared to the CuPc intermolecular attraction (-2.3 kcal/mol). This energy difference forces the first monolayer of CuPc to lie flat on the graphene. The first layer then templates subsequent crystal growth in the (312) direction which corresponds to a parallel orientation of the CuPc molecular plane with the growth substrate, as opposed to stacking with the CuPc long-axis perpendicular to the substrate (natural order on glass or ITO). This method is expected to serve as a foundation for developing future, high efficiency and mobility OLEDs and planar heterojunction OPVs with enhanced exciton transport, thicker active layers, and more efficient light absorption.


9:45 AM U9.6
Determining the Elastic Constants of Rubrene Single-Crystals.Marcos A. Reyes-Martinez1, Ashwin Ramasubramaniam2, Alejandro L. Briseno1 and Alfred J. Crosby1; 1Polymer Science & Engineering, University of Massachusetts, Amherst, Massachusetts; 2Organic single crystals have opened the doors to a new generation of high-performance organic electronic devices. Exceptional charge-transport properties combined with the advent of large-area pattern, University of Massachusetts, Amherst, Massachusetts.

Organic single crystals have opened the doors to a new generation of high-performance organic electronic devices. Exceptional charge-transport properties combined with the advent of large-area patterning techniques make organic single crystals excellent candidates for flexible electronics applications. However, in order to effectively employ organic single crystals on mechanically flexible arquitectures, their mechanical properties need to be understood and characterized. In this presentation, the mechanical properties of rubrene single-crystals are investigated. Given the limited dimensions of as-grown crystals and associated handling difficulty, the elastic buckling instability is chosen as a metrology tool for determining the in-plane elastic moduli. Our results show that ultrathin (200nm - 1μm) rubrene crystals exhibit anisotropic wrinkling wavelengths as a function of crystallographic direction, which can be correlated to the anisotropic nature of its molecular packing. Furthermore, an adaptive intermolecular reactive bond order potential (AIREBO) is employed to calculate the nine elastic constants corresponding to orthorhombic rubrene.


 

SESSION U10: Materials for Transport and Optoelectronics
Chairs: Alex Briseno and Antonio Facchetti
Thursday Morning, December 1, 2011
Room 311 (Hynes)


10:30 AM *U10.1
Oriented 2D Covalent Organic Framework Thin Films.William Dichtel, Chemistry and Chemical Biology, Cornell University, Ithaca, New York.

Covalent organic frameworks (COFs) offer a powerful means to assemble molecular building blocks predictably into robust, structurally precise networks, but they are formed as insoluble and unprocessable powders. We have overcome this limitation by growing two-dimensional (2D) COF films on single layer graphene (SLG) under operationally simple solvothermal conditions. The layered films stack normal to the SLG surface and show improved crystallinity compared to COF powders. SLG surfaces supported on Cu, SiC, and transparent fused silica (SiO2)substrates were employed, enabling optical spectroscopy of COFs in transmission mode. Efforts to expand the pore sizes of phthalocyanine COFs and measure their interlayer charge transport properties will be presented.


11:00 AM *U10.2
Organic Semiconductor Design and Controlled Molecular Packing and Growth.Zhenan Bao, Stanford University, Stanford, California.

Organic semiconductor materials are interesting alternatives to inorganic semiconductors in applications where low cost, flexible or transparent substrates, and large area format is required. Currently they have been incorporated into organic thin-film transistors (OTFT), integrated display driver circuits, photovoltaics and radio frequency identification tags. In this talk, I will present recent results on material design, surface and interface control for achieving efficient charge carrier transport and large area patterning of organic semiconductors


11:30 AM U10.3
New Organic Semiconductors for Field-Effect Transistors and Solar Cells.Eilaf Ahmed1, Selvam Subramaniyan2, Felix S. Kim2, Hao Xin2 and Samson A. Jenekhe1,2; 1Chemistry, University of Washington, Seattle, Washington; 2Chemical Engineering, University of Washington, Seattle, Washington.

Organic semiconductors are currently of broad interest for applications in organic electronics and optoelectronics, including thin film field-effect transistors and photovoltaic cells. New donor-acceptor copolymers based on benzobisthiazole and various donor moieties (benzodithiophene, cyclopentadithiophene, carbazole, dithienosilole, dithienopyrrole, and bithiophene) were synthesized, characterized and used in field-effect transistors and solar cells. Incorporation of benzobisthiazole unit leads to enhanced intermolecular interactions and improved oxidative stability, charge transport, and photovoltaic properties. X-ray diffraction of benzobisthiazole-based copolymers showed highly crystalline structure with an interlayer d-spacing of 15.6-21.2 Å which is about 22-32% shorter than the interlayer d-spacing of poly(3-alkylthiophenes) (20.1-27.2 Å). Thin film field-effect transistors based on the new polybenzobisthiazoles show hole mobility as high as 0.011 cm2/Vs with high Ion/Ioff (> 106). Thin film transistors retained their charge transport properties for almost 2 years in ambient conditions. Bulk heterojunction solar cells made from benzobisthiazole-based copolymers have power conversion efficiency of up to 3.0%. These results demonstrate that conjugated polybenzobisthiazoles can exhibit high performance and reliable operation and long shelf-lifetime without encapsulation.


11:45 AM U10.4
High Efficiency Organic Solar Cells: Modeling the Effects of Charge Dissociation Limitations.Jonathan D. Servaites1,2, Brett M. Savoie3, Joseph Brink4, Tobin J. Marks3,1 and Mark A. Ratner3,1; 1Materials Science and Engineering, Northwestern University, Evanston, Illinois; 2Chemical Engineering, Stanford University, Stanford, California; 3Chemistry, Northwestern University, Evanston, Illinois; 4School of Engineering & Applied Science, Yale University, New Haven, Connecticut.

Taking a joint experimental-theoretical approach, we propose an analytical model for charge dissociation limitations in organic solar cells, showing how high power conversion efficiencies (>10%) can be achieved despite these constraints. The model uniquely provides an account of donor-acceptor energy offset dynamics, thereby addressing the physical picture in the organic solar cell charge dissociation process (replacing traditional Onsager-Braun theory). With this model we accurately simulate behavior across an array of record performing organic solar cells in the literature. Furthermore, our findings help address an active debate in the field regarding the role of the internal electric field in charge dissociation; conflicting experimental results support or reject the importance of field-assisted dissociation. We find that donor-acceptor energy offsets play a fundamental role in determining when field-assisted dissociation is significant - i.e., electric field is increasingly influential at lower offsets. Additionally, a traditional design rule in organic solar cells is to match the donor-acceptor energy offset with the exciton binding energy. We demonstrate why this rule does not hold true in current state-of-the-art cells and show how these energy offsets should be optimized, with the optimal offset at a level higher than the exciton binding energy. We conclude proposing specific design strategies for finally overcoming the important 10% threshold in organic solar cell efficiencies.


 

SESSION U11: Single-Crystalline Semiconductors
Chairs: Antonio Facchetti and Iain McCulloch
Thursday Afternoon, December 1, 2011
Room 311 (Hynes)


1:30 PM U11.1
Photoluminescence Spectroscopy of Rubrene Single Crystals.Pavel Irkhin, Aleksandr Ryasnyanskiy and Ivan Biaggio; Physics, Lehigh University, Bethlehem, Pennsylvania.

We detail how the photoluminescence spectrum emitted by rubrene single crystals varies as a function of photoexcitation conditions (light intensity, wavelength, polarization, illumination geometry), detection conditions (polarization, detection geometry), and crystal surface quality and orientation. All these experimental parameters are responsible for strong changes in the detected photoluminescence emission spectrum, and selective control of all of them allowed us to resolve several apparently contradicting results that have been reported in the literature up to now. We relate our results to the absorption and emission properties of the rubrene molecule as can be observed by investigating differently oriented crystal surfaces as well as rubrene solutions, and distinguish between contributions to the photoluminescence and absorption spectrum that can be assigned to localized molecular transitions, basically undisturbed by the crystal lattice, and contributions that are unique to the crystal.


1:45 PM U11.2
Ionic Liquid-Assisted Growth of Pentacene Single Crystals in Vacuum.Yuji Matsumoto, Shinji Mantoku, Yoko Takeyama and Shingo Maruyama; Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama, Kanagawa, Japan.

The use of high-quality single crystalline organic semiconductors has huge potential to drastically improve performance of organic devices such as light emitting diodes, field effect transistors and solar cells, as expected from the analogy to inorganic semiconductor devices. However, in conventional solution growth processes, organic single crystals are not so easy to grow up with a size enough not only for their device applications, but also for basic studies of their properties. In a thin film form, there has been no way for such a high-quality organic semiconductor crystal. Here, if limiting to the methods of vacuum deposition, which have been often employed to fabricate organic thin films, the non-equilibrium nature of the process characteristic should be overcome in order to essentially improve the crystallinity of organic thin films. The same thing holds true for oxide thin films grown by vacuum deposition, but in some cases the problem has been solved by introducing a new concept of “Flux-mediated epitaxy” that was originally proposed by our group. 1 Flux-mediated epitaxy is, to be brief, a solution growth in vacuum, in which a liquid, called as “flux”, assists the growth of the film via repeatable dissolution and recrystallization as in the way of the solution process, except that thin film precursors are supplied from the gas phase. The successful applications of this approach include high-Tc superconducting NdBa2Cu3O7-δ1 and ferroelectric Bi4Ti3O122 oxide single crystal films. On the basis of the above substantial demonstrations of the capability of flux-mediated “oxide epitaxy”, it would be natural to extend this concept to “organic epitaxy” or "organic crystal growth". Here one question may arise: is there any organic solvent that is stable in vacuum, if possible, even at high temperature? The answer is “yes”; ionic liquid, which has become more and more popular in chemistry, is the organic solvent that is very stable in vacuum even above 100 oC, and can be best used as a flux for this purpose. In this paper, we demonstrate the capability of the ionic liquid-assisted growth in vacuum for high-speed and high-quality pentacene single crystals. 3 This work was supported by the Incorporated Administrative Agency New Energy and Industrial Technology Development Organization (NEDO) under Ministry of Economy, Trade, and Industry (METI). The authors (Y.T. and S.M.) were supported by the Tokyo Institute of Technology Global COE program. References: 1. Y. Matsumoto et al, J. Cryst. Growth 275 325 (2005) 2. R. Takahashi, and Y. Matsumoto et al, Adv. Funct. Mater. 16 485 (2006) 3. Y. Takeyama and Y. Matsumoto et al, Cryst. Growth & Des. 11 2273 (2011)


2:00 PM U11.3
Negative Transconductance in Organic Single Crystal Transistors Gated by Room-Temperature Ionic Liquids.Junichi Ikeda1, Quynh T. Phan1, Susumu Ikeda2, Ryo Nouchi2, Yoichi Tanabe2, Hidekazu Shimotani1 and Katsumi Tanigaki1,2; 1Department of Physics, Graduate School of Science, Tohoku University, Sendai, Miyagi, Japan; 2WPI Advanced Institute for Materials Research (WPI-AIMR), Tohoku University, Sendai, Miyagi, Japan.

In recent years, electric-double-layer transistors (EDLTs) using ionic conductors for the gate-insulators have attracted a lot of attention as a new technique of the field-effect carrier doping. When a gate voltage is applied to the ionic conductor sandwiched between the gate electrode and the semiconductor, ions in the ionic conductor move to screen the electric-field and are accumulated on the surface of the semiconductor. These accumulated ions generate strong electric-field at the ion conductor-semiconductor interface. Thus, carriers are induced at the inner surface of the semiconductor by this strong electric-field, and these assembled ions and carriers form the ultrathin electric-double-layer (EDL) at the interface. Owing to this small layer thickness of the EDL, the capacitance of EDLs becomes inevitably larger than that of the solid-gate insulators in conventional organic field-effect-transistors (OFETs). Additionally, as a result of this large capacitance, EDLTs make it possible to tune the carrier density up to values exceeding 1014 cm-2 [1]. Unlike conventional solid-gated OFETs, an anomalous drain current peak at a certain gate voltage in the transfer characteristic has been observed in organic semiconductor EDLTs. It is suggested that this apparent negative transconductance is attributed to the increase of the contact resistance [2] and the strong interactions of ions in ionic liquids with charge carriers at the liquid-semiconductor interface [3]. Nevertheless, the origin of this unexpected behavior has not been completely elucidated yet, and therefore it is interesting to investigate whether this phenomenon originates from the intrinsic nature of organic semiconductors and is important for understanding a basic principle of EDLTs. In this study, we discuss the specificity in the transfer characteristic of the rubrene single-crystal EDLTs gated by room-temperature ionic liquids. To clarify the origin of the unexpected drain current reduction, we probed electric potential distribution at the inner surface of the rubrene single crystal and precisely estimated the accumulated charge carrier densities by localized hall-effect measurements as well as contact resistances. In addition, we discuss the contribution of various types of anions to the interactions between charge carriers in semiconductors and anions in ionic liquids. [1] K. Ueno et al., Nature Mater., 7, 855, 2008 [2] H. Shimotani et al., Jap. J. Appl. Phys., 46, 3613, 2007 [3] Y. Xia et al., Phys. Rev. Lett., 105, 036802, 2010


2:15 PM U11.4
Controlling Carrier Recombination Line Width in Organic Single Crystal Light Emitting Transistor Having Split-Gate Electrodes.Hayato Kakizoe1, Kenichi Goushi1,2, Tomohiko Edura2 and Chihaya Adachi1,2; 1Center for Future Chemistry, Kyushu Univ, Fukuoka, Japan; 2Center for Organic Photonics and Electronics Research, Kyushu Univ, Fukuoka, Japan.

Organic light-emitting field-effect transistors (OLETs) are a new device that combines the function of light emission and switching properties. Ambipolar OLETs can inject both hole and electron simultaneously, resulted in intense electroluminescence by carrier recombination. Thus, artificial p-n junction was formed in a middle of the channel. Particularly, ambipolar OLETs based on a single crystal provide high carrier mobilities and high current density. In our previous work, we demonstrated that the single crystals of oligo(p-phenylenevinylene) (OPV) derivatives have excellent photoluminescence (PL) and ambipolar OFET characteristics. However, the external quantum efficiency (ηext) of single crystal based OFET was very low of <0.1 % and the carrier recombination characteristics were uncontrollable. Therefore, we focused on a split gate structure having two gate electrodes. In this study, we fabricated OFET having a split gate width of 1, 3 and 5 μm and investigated ηext depending on split gate width and their relationship between ηext and the applied gate voltages. First, we found that the split-gate OFET having a gate width of 3 μm showed high ηext compared to others with a gate width of 1 and 5 μm. When the gate width is 1 μm, ηext was very low since the gate width is narrower than the carrier recombination width, resulted in significant exciton annihilations by exciton-polaron interaction. When the gate width is 5 μm, ηext again showed a low value since a carrier recombination probability of the device with a gate width of 5 μm is lower than that with the gate width of 3 μm. We obtained the maximum ηext of 0.8 % at 53 A/cm2 in the device with the gate width of 3 μm, leading to dual injection and accumulation both electron and hole in the channel. Moreover, the split-gate structure realized ideal p-i-n junction and suppressed light propagation loss because the light emission position was far from S-D electrodes, resulted to high efficiency and high current density. Also, we were able to control carrier recombination line width and observed the straight line light emission.


2:30 PM U11.5
Dynamic of Triplet Excitons in Fluorescent Host/Guest Systems.Torsten Rabe1, Marcus Lehnhardt1, Thomas Riedl2 and Wolfgang Kowalsky1; 1Institut für Hochfrequenztechnik, Technische Universität Braunschweig, Braunschweig, Germany; 2Bergische Universität Wuppertal, Institute of Electronic Devices, Wuppertal, Germany.

Triplet excitons of fluorescent materials play an important role for the operation of organic optoelectronic devices. Due to their non-radiative character, they are difficult to investigate spectroscopically, especially at room temperature. In our study we investigate the triplet decay dynamics by a highly sensitive time-resolved measurement of the triplet state absorption via a pump and probe experiments within a waveguide configuration. The non-radiative triplet excitons were detected at room temperature by their absorption features. A variable time delay between pump and probe pulse allows to investigate the dynamic behavior of the excitons. In the first part we investigate the triplet dynamics of the polymer host/guest system F8BT/MEH-PPV as well as of the small molecule based host/guest system Alq3/DCM. We were able to identify triplet-triplet annihilation as a major decay mechanism of triplet states in the pristine amorphous organic materials at room temperature. On the contrary, the host/guest systems showed a significantly increased triplet lifetime caused by a reduced triplet-triplet annihilation rate which we attribute to a substantially lowered mobility of the localized triplet excitons. Our results are expected to be of paramount importance for organic light emitting devices based on fluorescent emitters (e.g. organic lasers and OLEDs) In a second experiment we investigate the influence of triplet excitons on the dynamic of organic thin film lasers based on the dye doped modified polyfluorene (BN-PFO/DPAVB). We demonstrate a dramatic decrease of the laser emission for excitation pulses longer than a few nanoseconds, whereas the undoped system (BN-PFO) offers a stable emission. By measuring the optical gain and the triplet absorption spectra we show that the spectral overlap of the two spectra leads to a dramatic increase of the laser threshold in the doped system due to a pile-up of long lived triplet states. On the contrary for pristine BN-PFO, optical gain and triplet absorption are found to be spectrally separated. As a result, accumulated triplet excitons hardly affect the laser emission.


2:45 PM U11.6
Photoluminescence Dynamics after Impulsive Excitation in Rubrene. Aleksandr Ryasnyanskiy and Ivan Biaggio; Physics, Lehigh University, Bethlehem, Pennsylvania.

We used picosecond laser pulses to create excitons in rubrene single crystals and determined the time-evolution of the photoluminescence intensity over 7 orders of magnitude and over more than 4 time decades after illumination. We observed both the unmistakable power-law decay of the photoluminescence that is due to triplet-triplet interaction as well as the later exponential decay corresponding to a triplet lifetime of 100 microseconds. The delayed luminescence observed at times longer than 10 ns after excitation is emitted by singlet excitons that are re-created by triplet-triplet fusion. We found that at higher illumination intensities the total amount of luminescence energy emitted through the fusion process, in the time period from 10 ns to 300 microseconds after excitation, can be more than 20 times larger than the luminescence emitted by radiative recombination of the singlet excitons initially photoxcited by the laser pulse. From this analysis we show that triplet collisions in rubrene must necessarily have a high probability of recreating singlet excitons, and that at the same singlet excitons must have a high probability of undergoing fission into triplet excitons. In fact, our data is consistent with the idea that the triplet exciton energy in rubrene is about half the singlet exciton energy, leading to very high quantum yields both for singlet exciton fission and triplet fusion upon triplet-triplet collision.


 

SESSION U12: Materials Patterning, Printing, Processing
Chairs: Antonio Facchetti and Vitaly Podzorov
Thursday Afternoon, December 1, 2011
Room 311 (Hynes)


3:30 PM *U12.1
Printed Integrated Flexible Systems Based on Organic Solution Processed Materials.Ana Claudia Arias1, T. N. Ng2, G. L. Whiting2, L. Lavery2, B. Russo2 and B. Krusor2; 1Electrical Engineering and Computer Sciences, University of California - Berkeley, Berkeley, California; 2Palo Alto Research Center, Palo Alto, California.

Ink-jet and gravure printing are desirable manufacturing technique for electronic devices as these additive methods should allow for integration of different electronic components over large substrate areas at low cost. In order to realize entirely printed devices, appropriate printed electrodes for organic semiconductor-based field effect transistors (FETs) must be chosen. In this report we will discuss the development electronics building blocks and systems output such as memory and displays. We have demonstrated all printed p-type TFTs with mobility of 1.6 cm2/Vs and n-type TFTs with mobility of 0.8 cm2/Vs. We have characterized the charge trapping rates for n- and p-channel devices and assessed the inverter gain and noise margin. All printed inverters showed a typical gain of 8 with VDD at 10V and -3dB cutoff at 150 kHz for a load of 0.02pF. Non-volatile memory devices for mechanically flexible electronics are being developed for short-term data storage in applications that require a maximum lifetime of only a few weeks. Inkjet printing was used to pattern ferroelectric field effect transistors (feFETs) and addressing thin film transistors (TFTs). The organic feFETs were fabricated with poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE) as the gate dielectric material. Application of an electric field across the ferroelectric insulator induces polarization of the dielectric layer. The remnant polarization alignment can be used as a record of the input voltage and thus functions as memory states. The integration of feFETs with addressing TFTs into an active-matrix memory array is needed in order to avoid pixel cross talking. The electrodes and addressing lines were composed of inkjet printed silver nanoparticles. A polythiophene derivative was used as semiconductor in the memory feFETs while a soluble small molecule was used as the semiconductor in the addressing TFTs. Memory feFETs were shown to retain 50% of output current over seven days. The transistor characteristics were monitored to understand the limiting factors to data retention time. We studied the degradation in the dielectric layer and in the semiconductor layer separately to understand the origins of possible device instabilities. Finally the flexible memory array was integrated with electrophoretic media to enable visualization of the working printed memory process.


4:00 PM *U12.2
Subfemtoliter Inkjets and Organic Transistor Integrated Circuits for Low Voltage, Large Gain, High-Speed Operation.Takao Someya1,2, Tsuyoshi Sekitani1,2, Tomoyuki Yokota2, Ute Zschieschang3 and Hagen Klauk3; 1JST, ERATO, Tokyo, Japan; 2Department of Electrical and Electronic Engineering and Information Systems, The University of Tokyo, Tokyo, Japan; 3Max Planck Institute for Solid State Research, Stuttgart, Germany.

In this talk, we report recent progress, remaining issues, and future prospects of organic thin-film transistors using self-assembled monolayer gate dielectrics. First, we have manufactured pseudo-CMOS integrated circuits operating at 2 V. The inverter gain of 302 is achieved at an operation voltage of 2 V. The oscillation frequency of a 5-stage ring oscillator comprising pseudo-CMOS inverters is 4.27 kHz at 2 V. Then, using a subfemtoliter inkjet, we have achieved a transconductance of 0.76 S/m for organic thin-film transistors with 4 V-operation. Silver electrodes with a linewidth of 1 µm and a channel length of 1 µm were printed directly onto a surface of organic semiconductor that was deposited on a self-assembled monolayer gate dielectric.


4:30 PM U12.3
Flexible Active Matrix OLED Display with Organic TFTs.Soeren Steudel1, Kris Myny1, Peter Vicca1, Steve Smout1, Sarah Schols1, Tripathi Ashutosh3, Bas van der Putten3, Martin van Neer3, Falk Schuetz4, Olaf Hild4, Erik van Veenendaal2, Marcel van Mil2, Jan-Laurens van der Steen3, Gerwin Gelinck3, Jan Genoe1 and Paul Heremans1; 1Large Area Electronics, IMEC, Leuven, Belgium; 2Polymer Vision, Eindhoven, Netherlands; 3HOLST center, TNO, Eindhoven, Netherlands; 4IPMS, Fraunhofer, Dresden, Germany.

Flexible AMOLED displays have attracted a lot of attention in the last 5 years. There are different routes how to achieve this goal. The first option is to rely on high-temperature foils (metal foil, polyimide) and use backplanes such as a:SiH or polysilicon or metal-oxides. A second option is to use low temperature foil (PET, PEN) and implement low temperature TFTs based on organic semiconductors or possibly low-temperature metal-oxide semiconductors. We will present a QQVGA top emitting monochrome AMOLED display with 72dpi resolution using an organic TFT backplane on PEN-foil. The process flow is based on a 7 layer photolithography process that yields a final mobility of the OTFT of 04.cm2/Vs. The aperture ratio of the top-emitting OLEDs is over 80%. We implemented the monochrome display using the three basic colors red, green and blue. For operation at 12 V supply voltage (VDD), the brightness of the display using red and blue OLEDs exceeds 200cd/m2, and using green OLEDs it exceeds 1000cd/ m2. We will furthermore discuss the relevant parameters that determine the performance of organic backplanes for OLED displays, in particular bias stress, output resistance, uniformity and current drive capability.


4:45 PM U12.4
Fabrication of Organic Field Effect Transistor by Directly Grown Poly(3 Hexylthiophene) Crystalline Nanowires on Carbon Nanotube Aligned Array Electrode.Biddut Sarker1,2, Jianhua Liu1,3, Lei Zhai1,3 and Saiful Khondaker1,2,4; 1Nanoscience Technology Center, University of Central Florida, Orlando, Florida; 2Department of Physics, University of Central Florida, Orlando, Florida; 3Department of Chemistry, University of Central Florida, Orlando, Florida; 4School of Electrical Engineering and Computer Science, University of Central Florida, Orlando, Florida.

We fabricated organic field effect transistors (OFETs) by directly growing poly (3-hexylthiophne) (P3HT) crystalline nanowires on solution processed aligned array single walled carbon nanotubes (SWNT) interdigitated electrodes by exploiting strong π-π interaction for both efficient charge injection and transport. We also compared the device properties of OFETs using SWNT electrodes with control OFETs of P3HT nanowires deposited on gold electrodes. Electron transport measurements on 28 devices showed that, compared to the OFETs with gold electrodes, the OFETs with SWNT electrodes have better mobility and better current on-off ratio with a maximum of 0.13 cm2/(V.s) and 3.1×105 respectively. The improved device characteristics with SWNT electrodes were also demonstrated by the improved charge injection and the absence of short channel effect which was dominant in gold electrode OFETs. The enhancement of the device performance can be attributed to the improved interfacial contact between SWNT electrodes and the crystalline P3HT nanowires as well as the improved morphology of P3HT due to one dimensional crystalline nanowire structure.


 

SESSION U13: Poster Session: Organic Devices/Materials II
Thursday Evening, December 1, 2011
8:00 PM
Exhibition Hall D (Hynes)


U13.1
Electronic Structure of Dopant Molecules in Liquid Organic Light-Emitting Diode Studied by Photoelectron Yield Spectroscopy.Takeshi Yamashita1, Yutaka Noguchi1,2, Yasuo Nakayama2 and Hisao Ishii1,2; 1Graduate School of Advanced Integration Science, Chiba University, Chiba, Japan; 2Center for Frontier Science, Chiba University, Chiba, Japan.

Very recently, an organic light emitting diode using liquid organic semiconductor, 9-(2-ethylhexyl)carbazole (EHCz), has been reported. In the device, 5,6,11,12-tera phenylnapthacene (rubrene) was doped as emitting material. Liquid OLEDs can open a way to perfectly flexible device, and have advantages such as easy replacement of active materials after degradation. From the viewpoint of device physics, there remain many issues to be clarified. Especially the operation mechanism including carrier injection and transport processes is not clear. To understand such points, we need the information about the electronic structures of the host and dopant materials. For example, the ionization energy (I) of each material is necessary to determine the location of HOMO to discuss hole injection and transport. In addition, the difference in HOMO energy between the two materials is important to consider the possible hole trap and release processes. As a first approximation, the difference in I between pure host and dopant materials corresponds to the relative energy difference. But, in the device, the dopant molecule is surrounded not by the identical molecules but by the host molecules. So, the polarization energy, which is the stabilization energy of ion state by polarization of the surrounding molecules, should be observed to precisely determine the relative location of HOMOs. In the first place, the ionization energies of liquid materials, and the difference between the solid and liquid states have been not well understood. Electronic structures are usually investigated by photoelectron spectroscopy (PES). But it is not easy to measure liquid materials with high vapor pressure because PES needs at least high vacuum environment during the measurements. In this study, we have investigated the electronic structure of EHCz doped with rubrene by photoelectron yield spectroscopy (PYS), which is capable of measuring in non-vacuum condition. Our PYS measurements revealed that the polarization energy of EHCz is comparable to that of rubrene (1.1eV). Although the molecular size is small, this relatively high polarization energy make the ionization energy of EHCz(liq) close to that of poly(vinyl-carbazole) (PVK) (I=5.8eV), which is well know as a good host material in dopant-dipersed polymer devices in solid phase. We developed the method to analyze the PYS spectrum of dopant-in-host system to separate the spectral contribution from the two components, and determined the relative energy difference in HOMO as 0.5 eV. In the presentation, the electric properties of the OLED device studied by displacement current measurement will be also reported. This research is granted by the Japan Society for the Promotion of Science (JSPS) through the “Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST Program),” initiated by the Council for Science and Technology Policy (CSTP).


U13.2
THz Absorption by Electric-Field-Induced Carriers in Pentacene FETs.Shiguang Li1,3, Toshio Matsusue2, Masatoshi Sakai1, Kazuhiro Kudo1, Ryosuke Matsubara3 and Masakazu Nakamura3; 1Graduate School of Engineering, Chiba University, Chiba, Japan; 2Graduate School of Advanced Integration Science, Chiba University, Chiba, Japan; 3Graduate School of Materials Science, Nara Institute of Science and Technology, Ikoma, Japan.

Terahertz (THz) electromagnetic radiation (0.1-10 THz), which lies in the boundary region between light and radio wave, has been attracting a great deal of attention in recent years due to its ability to achieve innovative sensing systems. Growing research activities using THz wave is driven by scientific and industrial applications such as defense, security, biology, or medical care. However, manufacturing THz sensing/imaging devices with low cost has been a difficult subject. We therefore propose utilization of organic field-effect transistors (OFETs) for this purpose owing to their applicability to flexible electronics with low fabrication cost. Our previous works revealed that the HOMO-band edge of a pentacene thin film is randomly fluctuated [1] due to the existence of crystallite boundaries [2]. The maximum amplitude of the fluctuations is around 30 meVp-p and the local barrier height against carrier transportation is 0.5-10 meV, which corresponds well with the photon energy of THz wave (0.41-41 meV). It is therefore highly probable that the irradiation of THz wave enhances the carrier transport in OFETs with pentacene thin films. In this presentation, we will report on the THz absorption in pentacene OFETs and its modulation by the gate electric field as the first stage of the study aiming at the THz sensing devices. Pentacene OFETs were fabricated on n-type silicon substrates covered with 300 nm-thick oxide layers. The substrate was served as a gate electrode to bias against interdigital-type source/drain electrodes on the oxide layer and the absorption spectra of the OFET was measured with THz time-domain spectroscopy (THz-TDS). Transmittance spectra were measured under ON-state gate bias (-30 V) of OFETs and they were normalized by OFF-state (+30 V) ones of the same OFETs. Reproducibility of the spectra was confirmed by measuring many spectra with many OFETs. Furthermore, similar experiments were performed with dummy samples that have the same structure except the absence of pentacene active layers. As a result, we found a broad absorption in the range less than 2.0 THz (8.3 meV) due to the accumulated carriers in pentacene films. We will also discuss on the possibility of the enhancement of carrier transport by the absorption of THz photon. References [1] N. Ohashi et al., Appl. Phys. Lett. 91, 162105 (2007). [2] R. Matsubara et al., Org. Electron. 12, 195 (2011).


U13.3
Synthesis of Novel Water-Soluble Semiconducting Polymers.Yeong-Beom Lee, Jongwon Park and Chulsung Bae; Department of Chemistry, University of Nevada Las Vegas, Las Vegas, Nevada.

Conjugated polymeric materials[1] are one of the most promising semiconducting materials because they can be fabricated over large areas by a low-cost solution processing. Thus, they offer significant economic advantages in applications of solar cells, light emitting diodes, field-effect transistors, and bio sensors compared to their inorganic material-based counterparts. Although many examples of high-performance, hole-transporting p-type conducting polymers have been developed over the three decades, lack of suitable n-type conducting polymers has prevented further applications of these promising semiconducting materials in optoelectronics.[2,3] Thus, the development of high-performance, solution-processible (especially soluble in environmentally-benign water) n-type conducting polymer is critical for successful commercialization of polymer material-based solar cells. Water-soluble conducting polymers have conjugated polymer backbone and ionic side chains. Water-soluble p-type conducting polymers have recently been used as materials in polymer light emitting diode and solar cell devices.[4,5] In contrast to well-studied p-type conducting polymers, however, there is a lack of stable n-type conducting polymers with strong electron-transporting properties. Furthermore, water-soluble n-type conducting polymer, although it has a great commercial potential for optoelectronic devices, has not been reported yet. We report a synthetic method that affords various water-soluble, n- or p-type conjugated polymers functionalized with alkyloxy or perfluoralkyl sulfonic acid groups. Those materials would have unique combination of electronic properties and solubility behavior. Electronically tunable conducting polymers could be prepared by modifying electronic properties of well-known conducting polymeric system (e.g., based on various electron donor-acceptor structures on the main backbone) with a strong electron-withdrawing group (e.g., -CF2CF2SO3Na) and an electron-donating group (e.g., -OCH2CH2SO3Na), respectively, for n-type and p-type applications. As first step, thiophene monomers functionalized with electro-withdrawing and electron-donating groups were prepared. Polymerization with different aromatic or heteroaromatic monomers was conducted using metal-catalyzed cross-coupling reactions, including Suzuki reaction. Structures and molecular weight properties of the polymers were characterized with spectroscopic methods (IR, UV, and NMR) and gel permeation chromatography. Solution behavior (especially in water) and thermal properties were also investigated. References 1. Burroughes, J. H.; Bradley D. D. C.; Brown A. R.; Marks R. N.; Mackay K.; Friend R. H.; Burns P. L.; Holmes A. B., Nature 1990, 347, 539. 2. Agrawal, A. K.; Jenekne, S. A. Chem. Mater. 1996, 8, 579. 3. Jakle F. Chem. Rev. 2010, 110, 3985. 4. Hoven, C. V.; Garcia, A.; Bazan, G. C.; Nguyen, T. -Q. Adv. Mater. 2008, 20, 3793. 5. Pu, K. -Y.; Liu, B. Adv. Funct. Mater. 2009, 19, 277.


U13.4
Molecular Stacking Engineering in the Highly Emissive Dicyanodistyrylbenzene Single Crystals for Organic Electronics: Tuning Optoelectronic Properties via Secondary Bonding Interactions.Seong-Jun Yoon and Soo Young Park; Department of Materials Science and Engineering, Seoul National University, Seoul, Korea, Republic of.

In the past decades, much attention has been paid on molecular orbital engineering of organic π-conjugated materials, because π-conjugation of organic semiconductors can be easily tuned by modification of their primary chemical structure. Recently, however, engineering of molecular stacking arrangement in addition to the molecular orbital control is getting more important, due to the development of practical solid state applications of them in optoelectronic devices. Especially, high-quality organic single crystals are one of the most promising materials to ensure high device performance and facilitate the study of structure-property relationships because of their purity and long-range ordered molecular packing. Herein, we have synthesized a new series of alkoxy-substituted dicyanodistyrylbenzene molecules (α-MODCS, α-MODBDCS, β-MODCS, β-MODBDCS). They organize to form fine single crystals with the regular supramolecular stacking architectures, assisted by various secondary bonding interactions such as multiple C-H N and C-H O hydrogen bond, local dipole interaction, donor-acceptor interaction, and π-π stacking to give rather unique optoelectronic features of the single crystals. Even though their molecular structures are very analogous to one another, a subtle molecular structural change significantly alters the molecular stacking arrangement to give completely different optoelectronic properties in their single crystals. Most uniquely, these molecular crystals exhibit very intense blue/green/orange/red fluorescence (λem: 495 nm (α-MODCS), 542 nm (α-MODBDCS), 576 nm (β-MODCS), 625 nm (β-MODBDCS)), indicating different intermolecular electronic interactions (e.g. excitonic coupling, excimeric coupling, and charge transfer complex) in their crystal stacking structures. In this presentation, we will discuss the optical, photophysical, and electrical charge carrier transport properties of the single crystals, as well as the fundamental organic single crystal growth, and single crystal stacking structures. Considering the concept of molecular stacking engineering, these methodical investigations of organic single crystals are expected to offer unambiguous structure-property relationships and a clue of high device performance for application in organic electronics.


U13.5
Oligothiophene Single-Crystalline Films.Lei Zhang, Nick Colella, Daniel E. Gonzalez, Justin Timmons, Jim Watkins and Alejandro L. Briseno; Polymer Science and Engineering, University of Massachusetts, Amherst, Massachusetts.

Polymer semiconductors such as the alkyl-substituted polythiophenes have been the focus of interest because of their solution-processable nature and application in devices such as transistors and solar cells. Their carrier mobilities, however, are significantly lower than small-molecule semiconductors such as pentacene and rubrene. The lower mobility is attributed to the structural disorder associated with entangled polymer chains that result in smaller and less-aligned crystals that form grain boundaries in thin films. In order for polymer semiconductors to outperform small-molecule semiconductors and make their way to real world applications, a protocol for fabricating highly crystalline films must first be developed. The objective of this proposal is to develop a crystallization protocol by employing low molecular weight polymers, or ‘oligomers’. Our intent is to develop a structure-property relationship from these materials and determine the electronic and molecular properties of this class of oligomers. Our approach to this problem is to synthesize a series of oligomers from the benchmark building block, didodecylquaterthiophene and then employ standard coupling chemistry to produce low molecular weight oligomers with well-controlled conjugation lengths and narrow polydispersities. We will also utilize oligomer single-crystalline films as test structures for measuring intrinsic carrier mobilities in field-effect transistors. The scientific outcome of the study will be a fundamental understanding of chain packing in low molecular weight polymers. This knowledge will help one to understand and ultimately control crystallization in higher molecular weight polymers. The impact of this work will also have a profound effect on how one will design polymers with controlled molecular weights and narrow polydispersities for growing highly crystalline films.


U13.6
Vertically Oriented Organic Single Crystal Nanostructures: The Ideal Architecture for Energy Harvesting.Richard Johnson, Lei Zhang, Timothy Mirabito and Alejandro J. Briseno; University of Massachusetts, Amherst, Massachusetts.

The exploration and understanding of the growth and the orientation of organic molecules is a very important feature in fundamental as well as applied research worldwide. This study is important because to date, there has been little success in growing large-area arrays of vertically oriented organic nanostructures. This is because the aromatic molecules employed prefer to crystallize parallel to the substrates instead of vertical. Our approach for growing vertically oriented single-crystalline nanostructures is to employ physical vapor transport to crystallize small molecules onto graphene surfaces. This method is simple, high-throughput, and can be used to grow both p- and n-type vertically oriented nanostructures. Finally, we describe the molecular packing and growth transitions at the graphene-organic interface.


U13.7
Abstract Withdrawn


U13.8
Scanning Tunneling Spectroscopy of Monolayer-Scale Pentacene Thin Films on Modified Si.Soonjoo Seo1, Guowen Peng2, Manos Mavrikakis2, Rose E. Ruther3, Robert J. Hamers3, Paul G. Evans4 and Hee Jae Kang1; 1Physics, Chungbuk National University, Cheongju, Korea, Republic of; 2Chemical and Biological Engineering, University of Wisconsin, Madison, Wisconsin; 3Chemistry, University of Wisconsin, Madison, Wisconsin; 4Materials Science and Engineering, University of Wisconsin, Madison, Wisconsin.

A dipolar interlayer can cause dramatic changes in the device characteristics of organic field-effect transistors (OFETs) or photovoltaics. A shift in the threshold voltage, for example, has been observed in an OFET where the organic semiconductor active layer is deposited on SiO2 modified with a dipolar monolayer. Dipolar molecules can similarly be used to change the current-voltage characteristics of organic-inorganic heterojunctions. We have conducted a series of experiments in which different molecular linkages are placed between a pentacene thin film and a silicon substrate. Tunneling spectra measurements were performed on submonolayer pentacene thin films on Si (001) surfaces modified with nitrobenzene and styrene using scanning tunneling spectroscopy (STS). We found in STS experiments that the molecular layers used to passivate Si (001) also modified the electronic property of pentacene thin films. Negative differential resistance (NDR) was observed in the tunneling spectra of both pentacene on nitrobenzene/Si (001) and on styrene/Si (001) for negative sample bias. Our results show that NDR can be produced by using the tunneling gap of STM and the molecular tunnel barrier between pentacene and the Si (001) substrate.


U13.9
Molecular Arrangement of Pentacene on Silica Surfaces from First-Principles Calculations.Hossein Hashemi, Xiao Ma and John Kieffer; Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan.

There has been a considerable number of experimental studies on the adsorption of pentacene on substrates for fabricating various electronic devices. The structure and electronic properties of a pentacene adsorbed on silica surfaces have been studied using density-functional theory calculations. First, we investigate the bulk properties of silicon and silica and pure Si(111) and SiO2(111) surfaces. The structural relaxation, work function, and surface energy for the surfaces, as a function of thickness, will be discussed. The properties of pentacene adsorbed on the silica surfaces will be compared to those of an isolated pentacene molecule. Factors affecting the molecular orientation, such as interface dipole and different binding configurations will be identified. The understanding of the molecule-molecule interplay and molecule-substrate interactions derived from our calculations provides guidance for substrate preparation and the control of pentacene film growth.


U13.10
Abstract Withdrawn


U13.11
Iodine Doping of Pentacene and Its Electrical Properties.Rahim Abdur, Young-Kyu Lee, Chiyoung Lee and Jaegab Lee; School of Advanced Materials Engineering, Kookmin University, Seoul, Korea, Republic of.

Organic thin film transistors (OTFTs) have been attracting considerable attention because of their potential use in low-cost, large area, electronic devices such as flexible displays, biochemical sensors, and smart cards. In past several years, gold/pentacene has been frequently used in OTFTs because of the high mobility of pentacene and the high work function of gold.To improve the performance of the OTFTs contact area doping of pentacene with p-doping materials are well known. In this work we demonstrated iodine doping of pentacene thin film and investigated the surface morphology as well as conductivity of the film with respect to doping time. The doping of Pentacene film showed more than five hundred times increase in conductivity. The iodine doping was confirmed by Raman spectra. We also carried out iodine doped contact structure, for the first time, to fabricate effectively contact area doped gold top contact pentacene based OTFTs. This doping of contact structure increased the saturation mobility by more than 50%. We fabricate all devices using octadecyltrichlorosilane (OTS) SAMs on the insulating oxide layer. The effectively contact area iodine doped pentacene based OTFT showed 0.78 cm2/Vs saturation mobility, with the on/off ratio of 4.06×105, threshold voltage of -1.90 V and the sub- threshold slope of 0.75 V/decade. Whereas the without doped pentacene based OTFT showed 0.51 cm2/Vs saturation mobility, with the on/off ratio of 5.44×105, threshold voltage of -2.23 V and the sub- threshold slope of 0.79 V/decade. We also determined the contact resistance between gold and both doped and undoped pentacene films using TLM pattern, which showed almost 50% decrease of contact resistance in doped pentacene TLM compared with that of undoped pentacene TLM.


U13.12
High Performance Flexible Organic Thin Film Transistors with Conducting Polymer Electrodes and Al2O3/PVP/ Al2O3 Multilayer Insulator.Young-Kyu Lee, Rahim Abdur, Chiyoung Lee and Jaegab Lee; Shool of Advanced Materials Engineering, Kookmin university, Seoul, Korea, Republic of.

Three issues such as mechanical stability, electrical performance ( mobility, leakage currents, operating voltage), and air stability encountered in the flexible organic transistors have been successfully addressed in this study with the use of poly(3,4-ethylenedioxythiophene)( PEDOT ) conducting polymer electrode, poly(4-vinylphenol)( PVP ) multilayer insulator, and encapsulation layer. We have fabricated flexible pentacene organic transistors on a polyethersulphone( PES ) substrate with PEDOT conducting polymer(conductivity ≈ 650 S/cm) electrode and Al2O3(30 nm)/PVP(63 nm)/ Al2O3(10nm) insulator, showing the high electrical performances such as threshold voltage of -3.9V, Ion/Ioff ratio of 8E+6, high mobility of 0.55 cm2V-1s-1, that operates under a strain of 2.0% with negligible degradation of hole mobility. In addition, we have measured the long-term stability on the device with an encapsulation layer on it, showing the excellent stability even after 1.5 month of air exposure. This excellent performance of flexible OTFS such as high mobility, low threshold voltage, low leakage current, high bendability, excellent air stability is attributed to the combination of high conductive PEDOT electrode, thin Al2O3/PVP/Al2O3 insulators, and encapsulation layer of tetratetracontane( TTC ).


U13.13
Dipolar Analyte Induced Polarization Effects on Charge Trapping in Organic Field Effect Transistor Based Sensors.Rohit Yadav, Davianne A. Duarte and Ananth Dodabalapur; Electrical and computer engineering, The University of Texas at Austin, Austin, Texas.

Various dipolar analytes were utilized to explore the polarization effects on the trapping mechanisms within organic field effect transistor (OFET) based sensors. It is well known that the origin of the electrical and optical properties of organic semiconductors comes from the presence of delocalized π-electron orbitals. Polarizability relates to the ability of the electrons to respond to a changing electric field. A non-polar molecule (organic semiconductor) such as pentacene may acquire a temporary induced dipole moment as result of the influence of a nearby charge or polar analyte which generates an electric field. This field distorts the electron distribution of the semiconductor leading to the induction of an electric dipole. A polar analyte (i.e. ethanol) can induce a dipole moment in a polarizable molecule because the partial charges produce an electric field that distorts the semiconductor. Generally, this involves the attraction of an induced dipole interacting with the permanent dipole but in terms of an OFET sensor this is related to the attraction between the permanent dipole of a polar analyte and the charge in the vicinity of a grain boundary in the organic semiconductor. We have tested various polar analytes with pentacene OFET sensors. The polarizability of pentacene has been thoroughly studied with a calculated polarization energy of ~1.5 eV. Based on an interaction potential equation, we found the strength of the attraction between a dipolar analyte and a charge carrier at a grain boundary has a direct relationship with the dipole moment, which becomes stronger in the presence of electron donating or withdrawing groups. The analytes with moderate to strong dipoles produced a shift in the output properties (decrease in mobility or current) with increases in the dipole moment producing more significant shifts.


U13.14
Spectroscopic Imaging of Surface Photovoltage and Localized Detection of Voltage Fluctuations in a Polymer-Blend Bulk Heterojunction Solar Cell.John A. Marohn1, Nikolas Hoepker2, Justin L. Luria1, Swapna Lekkala1 and Roger F. Loring1; 1Chemistry and Chemical Biology, Cornell University, Ithaca, New York; 2Physics, Cornell University, Ithaca, New York.

Surface photovoltage spectroscopy has proven a powerful tool for studying the sign of carriers and the optical band gap in inorganic semiconductors, yet there is little precedent for acquiring surface photovoltage spectra of organic semiconductors. Here we report 100 nm resolution images of surface photovoltage spectra of a PFB:F8BT polymer-blend heterojunction film, recorded under 350 nm to 750 nm low-intensity excitation in vacuum using frequency-modulated Kelvin probe microscopy. This sample was chosen because it is a nominally well-studied model system in which the role of the heterojunction interface in generating charge nevertheless remains controversial. To further explore the charge generation in the heterojunction film, we use cantilever frequency noise measurements to probe the low-frequency power spectrum of photo-induced voltage fluctuations. We introduce the idea of using cantilever frequency noise at selected points in the sample as a function of irradiation wavelength, tip voltage, height, and frequency to study voltage fluctuations arising from the motion of photo-induced charges. Our observed voltage-fluctuation spectra show a distinct wavelength dependence from the surface photovoltage data. The spectra indicate that the majority of dynamic charge, in both phases, is created from the optical absorption of F8BT. We find that the PFB/F8BT interface on PEDOT:PSS/ITO does not behave as a simple pn junction under illumination. Our results imply that the presence of the F8BT minority component in the PFB-rich phase has serious functional consequences in a bulk heterojunction solar cell. The nanoscale mixing leads to charge trapping and, probably more seriously, a surface photopotential that opposes exciton splitting at the interphase regions. A key parameter determining the efficiency of any solar cell is the open circuit voltage. Our data indicates that the minority component inevitably present in the bulk heterojunction solar cell leads to ``wasted'' voltage that does not result from free carriers --- a general efficiency-loss mechanism not before considered, to our knowledge.


U13.15
Large Scale Effects of a Static Electric Field on the Selective and Oriented Growth of Organic Crystals.Masatoshi Sakai1, Hiroshi Yamauchi1, Masaaki Iizuka1, Masakazu Nakamura2,1 and Kazuhiro Kudo1; 1Department of Electrical and Electronic Engineering, Chiba University, Chiba city, Chiba, Japan; 2Graduate School of Materials Science, Nara Institute of Science and Technology, Ikoma, Nara, Japan.

Many types of organic single crystalline devices have been extensively developed in the last decade and strong optical and electronic anisotropy of organic crystal had been revealed even in a two dimensional platelet crystals. For the industrial fabrication of a organic single crystal devices, selective and oriented growth on a substrate will be one of the key technologies. Although many oriented or selective growth techniques, including graphoepitaxy, dewetting etc., have been proposed yet, effects of external electric and magnetic field were recognized to be too weak. However, in some cases, we have demonstrated that the application of a static electric field during crystal growth is effective on the oriented growth and selective growth with no additional ionization of molecules and no additional physical templates. In our experiment, we used cylindrical furnace to make organic crystals. The cylindrical furnace was equipped with individually controlled three zone heaters and nitrogen gas flow system of which basic arrangement was similar to that reported Laudise et al. (J. Cryst. growth 187, 449, 1998) The source temperature, thermal gradient along the cylindrical axis, and flow rate of carrier gas are the variable parameters for crystal growth. In addition, we made wiring between the thin film Au/Cr electrode formed on the glass substrate and external DC power supply to investigate the effects of an electric field on the crystal growth under a quasi-thermal equilibrium condition. The first case is the crystal growth of copper phthalocyanine (CuPc). CuPc is a planar molecule which have a Cu ion in the center of gravity of the phthalocyanine ring. After the crystal growth under zero external electric field, uniform distribution of randomly grown needle-like CuPc crystals was observed. However, under the application of the external electric field, obviously oriented needle-like CuPc crystals grew over a wide area more than 500 micro meters. The observed orientation of the needle-like crystals agrees with the direction of electric lines of force calculated by the finite element method well. This oriented growth was explained by the anisotropic electrostatic energy of nuclei due to the uniaxial dielectric constant. The second case is the crystal growth of lead phthalocyanine (PbPc). PbPc is also one of the metallophthalocyanines but is the a shuttle-cock shaped molecule due to the large ion radius of Pb. In the PbPc case, a biased distribution of grown crystals toward the anode was observed; this biased distribution of grown crystals is explained by the drift motion of the diffusing molecules under the electric field due to the molecular dipole and quadrupole moment. On the other hand, effect on the oriented growth was not dominant in PbPc case because it is difficult to maintain quasi-thermal equilibrium conditions over a wide area due to the molecular redistribution.


U13.16
Investigation of the Influence of Domain Size on Carrier Transport in Bulk Heterojunction Solar Cells Using Monte Carlo Simulations. Chong Chen, Lance D. Cundy, Matt Biesecker, Jung-Han Kimn and Venkat Bommisetty; SDSU, Brookings, South Dakota.

Carrier transport in organic bulk heterojunction solar cells (BHJs), which constitute an interpenetrating network of the donor and acceptor materials, has attracted significant attention, in-part due to recent progress that helped high (>8%) power conversion efficiencies. Further improvement in the efficiency of these devices required a detailed investigation of carrier kinetics (generation, dissociation, transport and collection). However, the complex interpenetrating structure of BHJ solar cells, combined with spatially varying optical and electrical properties of donor and acceptor (DA) materials complicate precise analysis of carrier generation/collection processes. The factors that critically influence the device performance include: overall morphology of donor-acceptor network, interface structure, chemical properties of DA components, domain size etc. Among various factors, the domain size plays an important role in the formation of the network and thus has a strong influence on the charge generation and transport. Firstly, the interfacial surface area between the donor and acceptor depends on the domain size. Since the interfacial area serves as a dissociation site for the strongly bound photo-generated excitons created in polymer, the charge generation would be influenced by the domain size. Secondly, the internal electric field in the bulk heterojunction can be affected by the domain size and the charge transport in the domain may also be affected by the size of domain. In this study, a mesoscopic Monte Carlo technique was used to investigate the influence of the domain size, interface curvature that correlates to the morphological features measured using tomography and the properties of DA components. Initial simulations were targeted specifically to explain the charge generation and transport in prototypical P3HT:PCBM cells. The dependence of the dissociation efficiency of electron-hole pairs on domain size was calculated. The final current-voltage results were compared with the performance of the devices prepared with various domain sizes by altering process conditions.


U13.17
Interface Dipole Formation of Dithiocarbamate Monolayers on Noble Metal Surfaces.Philip Schulz1, Tobias Schaefer1, Dominik Meyer1, Riccardo Mazzarello2 and Matthias Wuttig1; 1I.Institute of physics (IA), RWTH Aachen, Aachen, NRW, Germany; 2Institute for Theoretical Solid State Physics, RWTH Aachen, Aachen, NRW, Germany.

Organic electronic devices like organic light emitting diodes (OLED), organic photovoltaic devices (OPV) and organic thin-film transistors (OTFT) are well-known for their potential in the field of optoelectronics. Yet significant improvements in the device functionality are expected if the organic/inorganic interface at the contact area of the metal electrode to the functional organic layer can be tailored [1]. In the past decades the application of self-assembled monolayers has been proposed for the modification of the electronic properties of metal contacts [2]. First successes of dithiocarbamate (DTC) monolayer formation yielding promising electronic properties have been reported [3]. Here, we present a study on dithiocarbamate (DTC) monolayers on various noble metal surfaces which were investigated by Photoemission Spectroscopy and Fourier-Transform Infrared Spectroscopy. We were able to achieve very low work functions down to 3.2 eV for the SAM covered gold substrates which can be explained by the formation of strong interface dipoles and a high packaging density of the double sulfur moiety of the DTC end group. This pronounced alignment of the electronic structure at the metal/SAM interface has also been reproduced theoretically by density functional theory. Thus, we present a promising and reliable route towards low work function metal contact interfaces for acceptor-type materials and ambipolar devices. [1] S.R. Forrest and M.E. Thompson, Chemical Reviews, 2007, Vol. 107, No. 4 925 [2] A. Aviram and M.A. Ratner, Chemical Physics Letters, 1974, Vol 29, 277; G. Heimel, F. Rissner and E.Zojer, Adv. Mater. 2010, 22, 2494-2513 [3] Y.Zhao, W. Pérez-Segarra, Q. Shi and A. Wie, J. Am. Chem. Soc., 2005, 127 (20), pp 7328-7329


U13.18
Ambipolar Organic Field-Effect Transistors Based on CuPc and F16CuPc: Impact of the Fine Microstructure at Organic-Organic Interface. Sebastien Nenon1, Daiki Kanehira2, Noriyuki Yoshimoto2, Frederic Fages1 and Christine Videlot-Ackermann1; 1CINaM, UPR CNRS 6118, Marseille, France; 2Graduate School of Engineering, Iwate University, Morioka, Japan.

The use of more complex structures than just one organic thin film deposited on a substrate needs the joint use of p-channel transistors combined with n-channel transistors to open new perspectives in the field of organic electronics. Such ambipolar OTFTs have been realized by three approaches: i) using two stacked layers of discrete hole- and electron-transporting semiconducting materials to form a p/n stacked heterojunction, ii) using a two-component layer comprising a blend of unipolar electron- and hole-transporting organic semiconductors listed as a p:n heterojunction, and iii) using a single-component layer with symmetric or asymmetric electrodes. Ambipolar organic thin film transistors were fabricated by using hexadecahydrogen copper phthalocyanine (CuPc) and hexadecafluoro copper phthalocyanine (F16CuPc) as p-and n-type semiconductors, respectively. Ambipolar transport was observed in thin films based on either heterojunction (CuPc/F16CuPc or F16CuPc/CuPc with CuPc and F16CuPc as first deposited layer, respectively) or blend (CuPc:F16CuPc) architectures [1]. Structure and morphology of thin films have been studied by atomic force microscopy (AFM) and X-ray diffraction (XRD). A careful study of the fine microstructure formed at the interface between CuPc and F16CuPc highlights the presence of three intermediate phase layers ensuring a continuously grown between highly ordered CuPc and F16CuPc polycrystalline thin films. Under the same conditions, Mori et al. described the existence of a single intermediate phase layer [2]. In the present study, a more detailed analysis of diffraction spectra highlights the fine microstructure of the interface between CuPc and F16CuPc by the presence of more complex intermediate phase layers. Theses intermediate phase layers originate from the interactions or relaxations between CuPc and F16CuPc at the interface. Due to a better distribution between the phases (CuPc, F16CuPc and the intermediate phase layers), CuPc/F16CuPc heterojunctions give rise to an optimized ambipolar transport. The initial interests associated to similar molecular shape and crystal packing structure as well as comparable performances turn out not to be factors of security for an efficient ambipolar transport. [1] S. Nénon, D. Kanehira, N. Yoshimoto, F. Fages, C. Videlot-Ackermann, Synthetic Metals, in press. [2] R. Ye, M. Baba, K. Suzuki, K. Mori. Appl. Sur. Sc. 254, 7885 (2008).


U13.19
Abstract Withdrawn


U13.20
New Organic Semiconducting Materials for Organic Photovoltaic and Organic Field Effect Transistor Applications.Peter J. Skabara and Jesuraj Inigo; Pure and Applied Chemistry, University of Strathclyde, Glasgow, United Kingdom.

We present the organic electronic device characterization of thiophene perfluorophenylene based materials for organic photovoltaics (OPV) and organic field effect transistors (OFET). The charge transport properties of thiophenes are expected to be modified by the addition of perfluorinated phenylene rings. The addition of such units to thiophene alters the lowest unoccupied molecular orbital (LUMO) energy levels of thiophenes, which in turn results in such materials having high electron affinity. The electron-rich/electron-deficient intermolecular interactions are sufficient to promote molecular π-π overlap. The presence of non-covalent interactions between thiophene and pefluorophenylene units also results in planar structures. These perfluorinated phenylene rings with thiophene units are also supposed to contribute high electron, hole mobilities and ambipolar mobilities in some cases. This further makes these materials strong candidates for OPV and OFET materials. We synthesized oligomers with thienyl and perfluorophenyl units and fully characterized it by cyclic voltammetry, UV/vis absorption, photoluminescence and X-ray crystallography. The X-ray diffraction data shows that these materials with flip stacking and thiophene-fluorine interactions. We will discuss the performance characteristics of OPV and OFET fabricated by thermal evaporation. Structure (bulk and surface) and charge carrier transport characteristics relationship will also be discussed in the light of thin film morphology characterization by tapping mode atomic force microscopy and X-ray diffraction measurements.


U13.21
Strong Air Stable n-Dopants Investigated by Photoelectron Spectroscopy and Scanning Tunneling Spectroscopy.Selina Olthof1, Yabing Qi1, Sang Bok Kim2, Swagat Mohapatra2, Tissa Sajoto2, S. Guo2, Steve Barlow2, Seth Marder2 and Antoine Kahn1; 1Department of Electrical Engineering, Princeton University, Princeton, New Jersey; 2School of Chemistry and Biochemistry and Center for Organic Electronics and Photonics, Georgia Institute of Technology, Atlanta, Georgia.

An understanding of charge transport in organic thin films is necessary in order to utilize organic materials in electronic and optoelectronic devices such as solar cells and light emitting diodes. The technique of doping can greatly increase the performance of organic devices by reducing energy barriers for charge transport and increasing the conductivity of the layer. Strong and efficient p-dopants have already been successfully applied, but the field of n-doping is still challenging due to the fact that an efficient n-dopant must have a low ionization potential, which makes these molecules reactive with oxygen and unstable in air [1]. In this presentation we introduce a new material class of strongly doping vacuum-evaporated sandwich compound n-dopants. These molecules are stable in air as only upon evaporation do the molecules dissociate and produce two reactive radicals. The strongest of these dopants is able to dope matrix materials with very low electron affinities down to approximately 2.7eV. The effect of doping is investigated in various small molecule matrices with varying electron affinities ranging from 2.6eV (hardly dopable) to 4eV (easily dopable). These n-doped layers are probed by photoemission and inverse photoemission spectroscopy (PES/IPES), as well as scanning tunneling spectroscopy (STM) and electrical measurements. Using STM, we are able to show a charge transfer from the dopant to the pentacene matrix molecules in highly ordered layers of a few monolayer thickness. PES measurements indicate a strong shift of the Fermi level upon doping the various transport materials, bringing the Fermi level in close proximity of the LUMO. At the same time, a significant increase in conductivity by several orders of magnitudes is observed, reaching values around 10-4 S/cm. From the measurements we conclude that the n-doped layers show a strongly reduced electron injection barrier and low Ohmic resistance which is necessary to achieve high performance in optoelectronic devices. [1] K. Walzer et al., Chem. Rev., 107 (2007) 1233


U13.22
Carbon Nanotube Enabled Vertical Field Effect Transistors with a Solution Processable Channel Layer.Evan P. Donoghue, Bo Liu, Mitchell A. McCarthy and Andrew G. Rinzler; Department of Physics, University of Florida, Gainesville, Florida.

The low mobility of organic semiconductors relative to inorganic semiconductors limits device performance in conventional lateral channel thin film transistors. The use of a carbon nanotube enabled vertical architecture in organic field effect transistors offers a means to achieve state-of-the-art current densities at low operating voltages from comparatively low mobility organic semiconductors. Here we extend the materials useful in this vertical architecture from evaporated small molecules (previously demonstrated) by utilizing a readily available, solution processable polymer: poly(3-hexylthiophene) (P3HT), as the channel layer. The P3HT-based carbon nanotube enabled vertical field effect transistors (CN-VFETs) achieve on-off current ratios of more than 104 for a gate voltage range of 5V, with the output current density exceeding 100 mA/cm2. This high level of performance from solution processable materials should facilitate inkjet printing of the devices, advancing the role it can play in large area organic displays.


U13.23
Dithiocarbamate-Metal Complexes and Their Impact on Charge Transport: From Monolayer Devices to Polymeric Interfaces.Florian von Wrochem, Deqing Gao, Frank Scholz, William E. Ford and Gabriele Nelles; Materials Science Laboratory, Sony Deutschland GmbH, Stuttgart, Germany.

As molecular electronics is emerging as an alternative to the conventional CMOS technology, the search for a convenient interface between organic molecules and metal electrodes became a key issue in the development of electronic devices. Here, the electronic properties and the binding chemistry of dithiocarbamates (DTCs) to metals are presented, showing that this anchor group provides a superior thermal stability and improved electrical contact compared to thiols on metals.2,3 Ultraviolet photoemission spectroscopy and density functional theory show a broad resonance at about 0.6 eV below the Fermi level of Au, which is characteristic for the DTC anchor group.3 Such surface states provide a conductance channel close to the Fermi energy of the metal, reducing the charge injection barrier at the metal-molecule interface. Electrical characterization using complementary test beds, such as Hg-drop junctions or nanoparticle films, demonstrate the impact of DTC anchor groups on electronic transport.3 The fabrication of asymmetric, rectifying junctions having two complementary anchor groups, e.g. thiols and DTCs, is further possible by the oriented growth of molecular linkers.4 Finally, based on the high intrinsic dipole moment of DTC groups, a systematic shift towards low work function values is obtained. This is exploited for the fabrication of polymer-diodes showing high current densities and rectification ratios tunable by synthesis.51 ITRS, International Technology Roadmap for Semiconductors, Emerging Research Materials (2007). 2Wessels, J. M.; Nothofer H.-G.; Ford W. E.; von Wrochem, F.; Scholz, F.; Vossmeyer, T.; Schroedter, A.; Weller, H.; Yasuda, A. J. Am. Chem. Soc. 2004, 126, 3349. 3von Wrochem, F.; Gao, D.; Scholz, F.; Nothofer, H. G.; Nelles, G.; Wessels, J. M. Nature Nanotech. 2010, 5, 618-624. 4Gao, D.; Scholz, F.; Nothofer H.-G.; Ford W. E.; Scherf, U.; Wessels, J. M.; Yasuda, A.; von Wrochem, F. J. Am. Chem. Soc. 2011, 133, 5921. 5Ford W. E.; Gao, D.; Scholz, F.; Karipidou, Z.; Rosselli, S.; Nelles, G.; von Wrochem, F. in preparation.


U13.24
Organic p-i-n Homojunction Device Operated in Reverse-Direction as Visible-Blind Photodiode. Sami Hamwi1, Thomas Riedl2, Wolfgang Kowalsky1 and Christian S. Weigel1; 1Institute of High-Frequency Technology, Technische Universität Braunschweig, Braunschweig, Germany; 2Institute of Electronic Devices, University of Wuppertal, Wuppertal, Germany.

The concept of p-i-n homojunction devices is well known in the world of inorganic semiconductors. However, reports on organic homojunction devices are rare due to the lack of appropriate ambipolar organic semiconductors which can be p- and n-type doped, respectively. On the other hand, the prospects of further simplification and cost-efficient production of organic devices like organic light emitting diodes and solar cells by the application of homojunctions call for a detailed study on this concept. We present an organic p-i-n homojunction device based on the ambipolar material 4,4’-Bis(carbazol-9-yl)-biphenyl (CBP) which is p- and n-type doped adjacent to the electrodes by MoO3 and Cs2CO3, respectively, and which functions both as ultra violet light emitting diode and visible-blind photodiode. Operating the device in forward direction we observe an emission of ultra violet light with peak wavelengths around 375 nm and 415 nm according to the photoluminescence spectrum of CBP distorted by microcavity effects. Operating the organic p-i-n homojunction in reverse-direction the device functions as visible-blind photodiode on the basis of the absorption spectrum of CBP. However, we observe an increasing deviation between the photocurrent and the absorption characteristics at wavelengths shorter than 340 nm. Studying the photocurrent characteristics as a function of the irradiated optical power densities in detail we observe a disproportionate relationship between both parameters. We attribute this superlinear relationship to an intensity-dependent photoconductivity of CBP.


U13.25
Organic Thin-Film Transistor-Based Inverters and Non-Volatile Memory Devices Fabricated on Flexible Plastic Substrates. Ji-Min Song and Jang-Sik Lee; School of Advanced Materials Engineering, Kookmin University, Seoul, Korea, Republic of.

Recently, active research has been carried out on organic electronic devices because of their advantages of low cost, simple fabrication processes, and flexibility in comparison to silicon-based electronic devices. Additionally, organic electronic devices can be applied to flexible electronic devices if processed at low temperatures. In this study, we fabricated organic electronic devices based on organic thin-film transistors (OTFTs). Flexible n- and p-channel OTFTs were fabricated by using F16CuPc and pentacene as organic semiconductor layers, respectively. Typical electrical properties were obtained for flexible OTFTs in terms of saturation mobility, subthreshold slope, and threshold voltages. We then fabricated CMOS inverters based on n- and p-channel OTFTs. Since the mobility was different for n- and p-channel OTFTs the channel width/length ratio was adjusted to get the high gain. In this study threshold voltage of 5 V and gain of 14 were obtained for flexible CMOS inverters. Finally, OTFT-based non-volatile memory devices were fabricated. For non-volatile memory devices the charge trapping elements, here, gold and aluminum nanoparticles, were inserted in the gate dielectric layer. Good programmable memory properties were obtained in terms of memory windows, operation voltages, and reliability. Overall, all organic electronic devices (OTFTs, inverters, and memory devices) were successfully fabricated on flexible plastic substrates. Detailed device fabrication processes, electrical characterization, and device operations will be discussed.


U13.26
Tuning Contact Recombination at the Metal Oxide/Organic Interface via Self-Assembled Monolayer Adsorption.He Wang1,2, Enrique D. Gomez1, Zelei Guan2, Cherno Jaye3, Daniel A. Fischer3, Antoine Kahn2 and Yueh-Lin (Lynn) Loo1; 1Department of Chemical and Biological Engineering, Princeton University, Princeton, New Jersey; 2Department of Electrical Engineering, Princeton University, Princeton, New Jersey; 3Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland.

The interface between metal or metal oxide electrodes and organic active layers plays an important role in organic electronic devices since this interface dictates energy level alignment, as well as the injection, extraction, and recombination of carriers. In this work, we used fluoro-alkyl and hydrogenated-alkyl phosphonic acid derivatives (FSAM and HSAM) to form self-assembled monolayers (SAMs) at the interface between indium tin oxide (ITO) and a bulk-heterojunction layer comprising poly(3-hexylthiophene), P3HT, and [6,6]-phenyl-C61-butyric acid methyl ester, PCBM. Polymer solar cells comprising such material stacks exhibit Vocs of 0.58 V, 0.48 V and 0.31 V with FSAM-, HSAM-modified and untreated interfaces at the anodes, respectively. By quantifying the slope of Voc as a function of illumination intensity, we have extracted the ideality factor (n) for each set of device characteristics. The ideality factor for devices with untreated interface at the anodes is 1.1, indicating that contact recombination dominates device behavior. For devices with FSAM- and HSAM-modified interface at the anodes, the ideality factors are 2.2 and 1.8, respectively, suggesting that contact recombination is suppressed, presumably because the adsorption of SAMs passivates the ITO surface. Such passivation decreases the recombination velocity at the anode/organic layer interface, ultimately increasing the Voc of these devices. The adsorption of FSAM and HSAM on ITO alters its work function from 4.3 eV to 5.5 eV, and to 4.0 eV. The fact that the Voc of devices with FSAM-treated anodes is higher than those with HSAM-treated anodes stems from a larger work function difference between the FSAM-treated anode and its cathode. This larger work function difference sets up an interfacial barrier that impedes electron transport to the anode. This interfacial barrier in turn results in a smaller minority carrier concentration at the anode, thereby decreasing contact recombination. Accordingly, we observe a higher Voc in polymer solar cells having FSAM-modified anodes compared to devices with HSAM- anodes.


U13.27
Highly Efficient OVPD-Processed Resonant-Cavity OLED. Manuel Boesing1, Florian Lindla1, Konrad Sell2, Dietrich Bertram2, Michael Heuken1,3, Holger Kalisch1 and Andrei Vescan1; 1Device Technology, RWTH Aachen (Technical University Aachen), Aachen, Germany; 2BCOL, Philips Technologie GmbH, Aachen, Germany; 3AIXTRON, Herzogenrath, Germany.

Even though OLED have developed at an impressive pace, their poor light outcoupling efficiency is still hampering their breakthrough in both, display and lighting applications. The approach of using (the comparably expensive) high refractive index substrates allows to achieve efficiencies of more than 100 lm/W [1]. However, this stands in clear contrast to the OLED industry’s request for low-cost substrates (especially if the use of flexible foils is projected). An extraordinarily cost-effective approach to improve the light outcoupling efficiency of a bottom-emitting OLED may be the introduction of a semitransparent silver layer at the anode side of the organic layer stack, thus forming an enhanced microcavity together with the reflective cathode [2]. In this work, we compare the potential of such resonant-cavity OLED (RC-OLED) comprising a semitransparent silver layer to those with a distributed Bragg reflector (DBR) as bottom reflector. All organic layers were processed by means of organic vapor phase deposition (OVPD). The DBRs (consisting of Nb2O5 and SiO2) as well the ITO anode were deposited by means of sputtering, whereas the silver layers were thermally evaporated. Phosphorescent red and green RC-OLED based on the triplet emitters Ir(MDQ)2(acac) (iridium(III)bis[2-methyldibenzo-(f,h)quinoxaline](acetylacetonate)) and Ir(ppy)3 (tris(2 phenylpyridin)iridium) as well as deep-blue fluorescent RC-OLED based on the singlet emitter SEB115 (MERCK OLED Materials) will be presented. In case of green and red OLED, the very simple approach of adding a silver layer was found to be a promising alternative to the more elaborate approach of a DBR. Efficiencies exceeding 100 lm/W (green) and 60 lm/W (red) were measured at the display-relevant brightness of 200 cd/m2, which roughly corresponds to an efficiency doubling compared to reference devices with a conventional structure. However, it was found that the employed silver layers can give rise to problems concerning the reliability and homogeneity of large area devices. These effects (which can partly be explained by a field-dependent migration of silver atoms within the device) will be discussed in detail. 1.) S. Reineke et al., White organic light-emitting diodes with fluorescent tube efficiency, Nature (2009), Vol 459 2.) R. Meerheim et al., High-efficiency monochrome organic light emitting diodes employing enhanced microcavities, Applied Physics Letters (2008), 93, 043310


U13.28
Strategies for Improving the Device Performance in All Printed High Performance Organic TFTs.Kanan Puntambekar, Lisa Stecker, Kurt Ulmer, Themistokles Afentakis and Steven Droes; Sharp Laboratories of America, Camas, Washington.

In this work, we show two strategies to improve the device performance of all printed organic transistors (OTFT). Optimization of the semiconductor (OSC) & source-drain (S/D) electrode interface is critical for improving device performance. In the case of OTFTs, this often involves coating of the metal S/D electrode surfaces with a surface treatment layer (typically a self-assembled thiol layer). Such surface treatments rely on pristine metal surfaces for being effective, which is easily achievable for evaporated electrodes. However, in the case of printed OTFTs, S/D layer is typically deposited using a solvent based ink (ex. metallic silver nanoparticle ink) with various additives that enable good printing. In such cases it is hard to avoid surface contamination post the ambient anneal processes, which can lead to formation of oxides/sulfides or leave behind residue of solvents/additives in the inks. This can lead to a non-ideal surface treatment and consequently result in poor contact properties. We demonstrate an effective strategy to address this issue by introducing a short forming gas plasma treatment prior to the surface treatment; we show two orders of magnitude decrease in the contact resistance as a result of this treatment. Further, for printed “small-molecule” or “small-molecule + polymer” (SM-OSC) blend OTFTs, optimization of the OSC morphology while simultaneous patterning it is a challenging problem. Incase of inkjet printing which is commonly used to print the OSC layer, the inability to precisely control the wetting properties can make it challenging to define well confined orthogonal patterns. Bank structures are commonly used to address this issue in polymeric OTFTs. Here we demonstrate bank structures for SM-OSC systems using an inkjet printed fluropolymer. In addition to containment, it is important to have control over the crystallization in the OSC layer as the solvent evaporates. In our case, if the OSC ink is deposited without any banks there is a large spread with large coffee stain rings due to favorable surface energetics regardless of the target print geometry. Also, the perimeter of the print area begins drying first nucleating the OSC grains at this edge and since more solvent is driven to the edges we see higher grain sizes at the edges and decreasing close to the center. Such a non-uniform and unpredictable grain growth is highly undesirable. We show that use of the fluropolymer banks for the OSC print in tandem with S/D metal electrodes can be used to drive an S/D mediated systematic grain growth in the channel region and reduce or eliminate the tendency of print edge nucleated grain growth. This leads to improved device performance as well as the ability to arbitrarily control the dimensions of the devices. Average device performance over 80 devices for all solution processed, printed OTFTs showed mobility 0.5 cm2/Vs, Vt -5.5V & On/Off > 106.


U13.29
Abstract Withdrawn


U13.30
Electrical Conductivity Property of Conductive Polymer Composites Base on Carbonaceous Fillers.Wilailak Chanklin and Felipe Chibante; Chemical Engineering, University of New Brunswick, Fredericton, New Brunswick, Canada.

Conductive polymer composites are composites where the electrical conductivity is dominated by conductive particles loaded into an insulative polymer matrix. These composites have attacked a great deal of interest because of their unique multifunctional properties such as the ability to tune physical and electrical properties. Structure and inherent conductivity of the fillers are important factors that determine the percolation threshold, the limit when the composite begins to conduct. However, in order to thoroughly understand the benefit of these composite materials, it is necessary to develop the fundamental knowledge of the controlled factors. Therefore, in this research, the electrical conductivity models will be proposed to explain and predict the conductivity behaviour of these conductive polymer composites. Through a parameter study, the effectiveness of literature models will be determined and the significant factors in each one will be identified. The models typically account for filler volume fraction, component conductivity, compatibility between filler and polymer matrix, filler aspect ratio, and filler orientation.


U13.31
Application of Hopping Theory for the Prediction of Charge Mobility in Amorphous Organic Materials.Karl Sohlberg1, Choongkeun Lee1 and Robert Waterland2; 1Chemistry, Drexel University, Philadelphia, Pennsylvania; 2Central Research & Development, E. I. du Pont de Nemours & Co., Inc., Wilmington, Delaware.

The application of hopping theory for the prediction of charge (hole) mobility in amorphous organic molecular materials is studied in detail. Application is made to amorphous cells of N,N’-diphenyl-N,N’-bis-(3-methylphenylene)-1,1’-diphenyl-4,4’-diamine (TPD), 1,1-bis-(4,4’-diethylaminophenyl)-4,4-diphenyl-1,3,butadinene (DEPB), N4,N4'-di(biphenyl-3-yl)-N4,N4'-diphenylbiphenyl-4,4'-diamine (mBPD), N1,N4-di(naphthalen-1-yl)-N1,N4-diphenylbenzene-1,4-diamine (NNP), and N,N’-bis[9,9-dimethyl-2-fluorenyl]-N,N’-diphenyl-9,9-dimethylfluorene-2,7-diamine (pFFA). Detailed analysis of the computation of each of the parameters in the equations for hopping rate is presented, including studies of their convergence with respect to various numerical approximations. Based on these convergence studies, the most robust practical methodology is applied to investigate the dependence of mobility on such parameters as the monomer reorganization energy, the monomer-monomer coupling and the material density. The results give insight into what factors should be controlled to develop materials with higher (or lower) charge (hole) mobility, and what will be required to improve the accuracy of predictions of mobility in amorphous organic materials.


U13.32
p-Type Doping of Small-Mmolecule Organic Semiconductors Using Organic Vapor Phase Deposition (OVPD).Michael Brast1, Florian Lindla1, Yuanzhou Gu1, Manuel Boesing1, Dietrich Bertram2, Michael Heuken1,3, Holger Kalisch1 and Andrei Vescan1; 1Device Technology, RWTH Aachen University, Aachen, Germany; 2Philips Technologie GmbH, Aachen, Germany; 3AIXTRON SE, Herzogenrath, Germany.

In the past few years, Organic Vapor Phase Deposition (OVPD) has been demonstrated to be a powerful deposition method for highly efficient monochrome and white OLED [1-4]. Especially for the production of OLED comprising a thick p-type doped hole transport layer, OVPD offers a high cost-saving potential due to its excellent material utilization efficiency and the high achievable deposition rates. An application of p-type doping is the improvement of hole injection either from the anode contact or from a charge generation layer in stacked OLED [5]. Nevertheless, no reports on p-type doping by OVPD can be found in literature, in part due to the limited thermal stability and high chemical sensitivity of available dopant molecules. In this work, p-type doping in an AIXTRON Gen-1 OVPD tool is examined and demonstrated. As p-type dopant, NDP2 (NOVALED) was employed to dope the organic hole-conducting host N,N‘-diphenyl-N,N‘-bis(1-naphthylphenyl)-1,1‘-biphenyl-4,4‘-diamine (α-NPD). First, p-type doped hole-only devices will be compared to conventional ones. Following, highly efficient monochrome OLED comprising a p-type doped hole transport layer will be presented. For a red p-type doped OLED, a current efficiency of 35 cd/A, a luminous efficiency of 36 lm/W and a driving voltage of 3.1 V (all values at 1000 cd/m^2) have been achieved (without improved light outcoupling). For comparison, reference devices using WO3 and MoO3 to improve the hole-injection are examined. In a second part, double tandem white OLED on the basis of NDP2 doping will be presented and compared to a stacked OLED using WO3. A voltage below 8,5 V could be reached by using NDP2. 1. F. Lindla, M. Boesing, C. Zimmermann et al., Appl. Phys. Lett. 95, 213305 (2009). 2. F. Lindla, M. Boesing, C. Zimmermann et al., J. Photon. Energy. 1, 011013 (2011). 3. T. X. Zhou, T. Ngo, J. J. Brown et al., Appl. Phys. Lett. 86, 021107 (2005). 4. R. R. Lunt, B. E. Lassiter, J. B. Benziger et al., Appl. Phys. Lett. 95, 233305 (2009). 5. H. Kanno, N. C. Giebink, Y. Sun et al., Appl. Phys. Lett. 89, 023503 (2006).


U13.33
Theoretical Characterization of Lithium-Endohedral Fullerene Complexes.Takeshi Matsushita1, Takashi Kato1, Kyouichi Tomita2 and Kenji Omote3; 1Chisso Petrochemical Corporation, Ichihara, Chiba, Japan; 2JNC Corporation, Chiyoda-ku, Tokyo, Japan; 3Idealstar, Sendai, Miyagi, Japan.

Since the first synthesis of a lithium endohedral fullerene Li@C60, much attentions are focused on the chemical and physical properties as well as its synthetic methodology. The parent C60 has been widely applied to the construction of organic photovoltanic devices (OPV) and field effect transistors (FET). Meanwhile, alkali metals are recognized as suitable for the purposes of lowering the injection barriers from cathode to the organic layers, because intermolecular interactions between metals and organic materials improve electron-accepting abilities. Thus, it is natural to consider that C60 based metallofullerenes are appropriate candidate for such electronic devices. Recently, the efficient synthesis of Li@C60 was succeeded by plasma shower method. But most characteristics of metallofullerenes have been still unknown. This is mainly because that the isolation of Li@C60 is difficult due to its strong attractive interaction with empty C60 in the mixture. We now report our findings regarding electronic structures and properties of C60 based metallofullerenes by using molecular orbital and density functional method. The geometrical structures were optimized using B3LYP method and the 6-31G(d) basis sets. Adiabatic electronic states were described by multi-configuration self-consistent field (MCSCF) method. Interestingly, the Li cation did not occupy the center of C60 cage with Ih symmetry, consistent with those reported by several experiment. The details of the electronic and structural characterization, as well as its usefulness as the industrial material will be reported at the symposium.


U13.34
New Oxidation-Resistant Organic Semiconductors for Thin Film Transistors: Synthesis and Characterizations of N,N′-Dialkylated Dihydrodiazapentacene Derivatives.Tetsuji Itoh1,2, Shigeru Aomori1,2, Masahito Oh-e1, Mitsuhiro Koden1, Katsumi Kondo1 and Yasuhiko Arakawa2; 1Materials & Devices Technology Laboratories, Sharp Corporation, Kashiwa-shi, Japan; 2Institute for Nano Quantum Information Electronics (NanoQUINE), The University of Tokyo, Meguro-ku, Japan.

We have successfully synthesized and characterized newly-designed oxidation-resistant N,N′-dialkylated dihydrodiazapentacene as stable semiconductor materials under ambient conditions for organic field-effect transistors (OFETs). Pentacene has been the subject of special attention due to its good mobility. The atmospheric instability, however, limits its practical applications. In fact, pentacene rapidly degrades under ambient conditions, because it is subject to oxidation at the 6 and 13 positions of the molecule owing to the π-electron localization. 6,13-dihydro-6,13-diazapentacene (DHDAP) with two N-H units replacing two carbon atoms of pentacene shows mobility of 0.45 cm2 V−1 s−1, comparable to that of pentacene OFET.1) The chemical stability of DHDAP is verified to be more stable than that of pentacene; however, it is still the case that DHDAP is decomposed, presumably oxidized, with amino hydrogens being released. With this problem in mind, we expect that alkylation of nitrogen sites, which replaces hydrogen atoms, makes it possible to prevent the oxidation. We report here newly-modified DHDAP derivatives with alkyl groups to achieve oxidation-resistant organic semiconductors. In this presentation, we describe how new organic semiconductors dihydrodiazapentacenes are synthesized and characterized, which bear alkyl chains on nitrogen atoms as protective groups (C6DHDAP: alkyl = hexyl, C18DHDAP: alkyl = stearyl). The alkylation of DHDAP is carried out by the reaction of DHDAP and sodium hydride, followed by the addition of alkyl bromide. These compounds are fairly stable and can be stored in ambient conditions without any appreciable decomposition in solid state and solution. The crystal structure analysis has allowed us to know that C6DHDAP has a face-to-face slipped π-stacking motif, which forms a one-dimensional column. XRD and AFM measurements reveal that the vacuum deposited films of C6DHDAP and C18DHDAP are similar crystalline phases with molecules being tilted from the substrate plane. The OFETs fabricated by vacuum deposition of these materials exhibit typical p-type FET performances and the highest mobility achieved is on the order of 10-3 cm2 V−1 s−1. The relatively smaller mobility compared to pentacene is discussed based on Marcus theory: the calculated hole mobilities using the crystal structure of C6DHDAP are one to two orders of magnitude smaller than those of DHDAP and pentacene, as reflected in the smaller transfer integral and larger reorganization energy. In addition, the strongly anisotropic one-dimensional stacking structure is considered unfavorable for charge transportation in the OFET channel. The storage tests of those OFETs in the presence of air for about 1 month have proven high device stability with no significant changes in mobilities and on/off ratios. 1) Tang, Q., Zhang, D. Q., Wang, S. L., Ke, N., Xu, J. B., Yu, J. C. and Miao, Q. Chem. Mater. 2009, 21, 1400.


U13.35
Charge Transport Enhancement of Organic Light Emitting Diodes Using ZnO/N-Doped Carbon Nanotube Nanocomposites.Ji Sun Park1, Ju Min Lee1, Sun Kak Hwang1, Sun Hwa Lee1, Hyun-Jung Lee2, Bo Ram Lee2, Ji-Seon Kim3,1 and Sang Ouk Kim1; 1KAIST, Daejeon, Korea, Republic of; 2UNIST, Ulsan, Korea, Republic of; 3Imperial College London, London, United Kingdom.

We demonstrate solution processible ZnO/N-doped CNT (N-CNT) nanocomposite charge transport layer with a remarkably enhanced electro-conductivity while maintaining the intrinsic energy levels of ZnO. The work function of CNTs could be systematically controlled by substitutional doping of electron deficient Boron (B) or electron rich Nitrogen (N). The chemically and thermally stable substitutional doping permanently alters the work function of CNTs and endures high temperature treatment involved in the spray-pyrolysis of ZnO layer. Among undoped or doped CNTs, N-CNT showed a work function that perfectly matches with the conduction band of ZnO to have an Ohmic contact for an electron transport. Consequently, the ZnO layer incorporated with finely dispersed but unpercolated N-CNTs demonstrated the five-fold enhancement of electron mobility and electro-conductivity, while maintaining the original work function value of ZnO. The electron transport of a hole dominating Organic Light Emitting Diodes (OLED) could be effectively promoted to achieve a well-balanced electron and hole injection with the ZnO/N-CNT electron transport layer. The resultant electroluminescence (EL) efficiency and the luminance could be more than two-fold improved up to 14.3 cd/A (at 13.6 V) and 46,100 cd/m2 (at 14.0 V) from 6.9 cd/A (at 13.4 V) and 21,000 cd/m2 (at 14.6 V) of reference device without CNTs, respectively. Owing to the facile work function tuning by substitutional doping, our CNT nanocomposite fabrication is generally useful to enhance the electric conductivity of various metal oxides and other charge transport materials having different energy levels without influencing their inherent band gap energy levels.


U13.36
High-Performance n-Type Organic Field-Effect Transistors Based on Push-Pull Styryl π-Conjugated Frameworks.Jong H. Kim1, Sun Woo Yun1, Seunghoon Shin1, Hoichang Yang2, Byeong-Kwan An3, Young Bin Lee4, Jong-Hyun Ahn4 and Soo Young Park1; 1Department of Materials Science and Engineering, Seoul National University, Seoul, Korea, Republic of; 2Department of Advanced Fiber Engineering, Inha University, Incheon, Korea, Republic of; 3Department of Chemistry, The Catholic University of Korea, Bucheon, Korea, Republic of; 4School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon, Korea, Republic of.

Recently, there has been increasing interest in π-conjugated organic semiconductors with self-assembling capabilities owing to their remarkable potential in nano/micro optoelectronic devices. Especially for realizing high-performance organic field-effect transistors (OFETs), organic semiconductors which favorably self-assemble into crystalline one dimensional (1D) or two dimensional (2D) structures are desirable for parallel intermolecular charge hopping on the substrate with in-plane source (S) and drain (D) electrodes. Therefore affording these semiconducting films a highly ordered stacking is a critical point for their applications as active materials in OFETs. In this presentation, we report a new class of push-pull styryl π-conjugated semiconductors (Hex-3,5-TFPTA (1) and Hex-4-TFPTA (2)) featuring well-organized 2D rod-type grains for the application of high-performance n-type OFETs. We demonstrate the outstanding n-type electrical properties of two molecules (mobility values up to 0.61 and 2.14 cm2 V-1s-1 for 1 and 2, respectively maintaining current on/off ratios >106), and elucidate the structural origin of these molecules with high field-effect electron mobility values. Finally, we also obtained an enhanced electron injection (i.e., improved contacts) of 2 with graphene S/D electrodes, attributed to the strong π-π interactions between 2 and graphene.


U13.37
Ambipolar Single Crystal Organic Field-Effect Transistors via Charge-Transfer Co-Crystal Formation.Sang Kyu Park1, Jong H. Kim1, Seong-Jun Yoon1, Byeong-Kwan An2 and Soo Young Park1; 1Department of Materials Science and Engineering, Seoul National University, Seoul, Korea, Republic of; 2Department of Chemistry, The Catholic Univeristy of Korea, Bucheon, Korea, Republic of.

For past decades, organic semiconducting materials have fascinated researchers due to their potential application to various optoelectronic devices such as organic field-effect transistors (OFETs) and organic photovoltaics (OPVs). In particular, for ambipolar OFETs applications, both precise control of self-assembly and molecular orbital matching are demanded to achieve efficient charge transport property. However synthetic difficulty as well as charge balance control remains a challenge to develop a high efficient ambipolar semiconductor. Herein we report on novel approach to fabricate ambipolar single crystal OFETs based on spontaneous co-assemblies of electron donor (4M-DSB) and acceptor (CN-TFPA) molecules. The photophysical properties on co-crystal are studied that showed unique optical properties of forming charge transfer complexes and exciplexes under ground and excited states, respectively. In addition, we could simply fabricate ambipolar co-crystal OFET devices using strong self-assemblies of 4M-DSB and CN-TFPA from solution process (solvent vapor annealing technique). OFETs based on single crystalline co-crystal showed hole and electron mobility values as high as 6.7×10-3 and 6.7×10-2 cm2V-1s-2 respectively.


U13.38
Electrical Properties of Organic Semiconductors n-Doped with Air-Stable Dopants.Yabing Qi1, Selina Olthof1, Sang-Bok Kim2, Swagat K. Mohapatra2, Tissa Sajoto2, Song Guo2, Stephen Barlow2, Seth R. Marder2, Adrian Grant1 and Antoine Kahn1; 1Department of Electrical Engineering, Princeton University, Princeton, New Jersey; 2School of Chemistry and Biochemistry and Center for Organic Electronics and Photonics, Georgia Institute of Technology, Atlanta, Georgia.

Doping is an efficient method to tackle key issues of conductivity and charge injection commonly encountered in organic electronics [1]. Several high performance air-stable p-type dopants have been reported [1-3]. On the other hand, the realization of strongly reducing air-stable n-type dopants remains challenging [1]. In this study, we report n-doping with a new class of effective, air-stable sandwich compounds used either via vacuum evaporation or in solution. Vacuum n-doping is achieved, for example, in the matrix of CuPc. Upon doping, the Fermi level measured by ultraviolet photoemission spectroscopy is seen to shift upward in the organic gap to only 0.15 eV below the LUMO. Charge transport measurements reveal an increase greater than 6 orders of magnitude in conductivity in the doped sample vs. the undoped one. Rutherford backscattering measurements demonstrate superior thermal stability of the n-dopant molecules in the CuPc matrix up to 130 degree C. N-doping is also implemented via solution process (e.g. spin coating) in the matrix of P(NDI2OD-T2), a very efficient solution processable electron-transport polymers [4]. Efficient n-doping is also confirmed by large Fermi level shift and significantly enhanced conductivity. Variable temperature current-voltage (VTIV) measurements show that the low-field conductivity of the n-doped (NDI2OD-T2) films is mainly due to thermally activated polaron hopping events [5]. The activation energy determined by VTIV measurements decreases dramatically from 1.19 eV for the intrinsic sample to 0.23 eV for the sample with a doping concentration of 2 wt.%, presumably due to the filling-up of deep traps. References: [1] K. Walzer, B. Maennig, M. Pfeiffer, and K. Leo, Chem. Rev. 107, 1233 (2007). [2] Y. Qi, T. Sajoto, S. Barlow, E.-G. Kim, J.-L. Brdas, S. R. Marder, and A. Kahn, J. Am. Chem. Soc. 131, 12530 (2009). [3] W. Gao, A. Kahn, J. Appl. Phys. 94, 359 (2003). [4] Yan et al, Nature 457, 679 (2009). [5] M. C. J. M. Vissenberg and M. Matters, Phys. Rev. B 57, 12964 (1998).


U13.39
Design Rules for Light-Emitting Electrochemical Cells.Stephan van Reenen1, Piotr Matyba2, Andrzej Dzwilewski1, Rene Janssen1, Ludvig Edman2 and Martijn Kemerink1; 1Applied Physics, University of Technology, Eindhoven, Noord-Brabant, Netherlands; 2Department of Physics, Umea University, Umea, Sweden.

Light-emitting electrochemical cells (LECs) offer plenty of attractive device properties compared to other organic lighting: The incorporation of a solid electrolyte enables dynamic doping of the organic semiconductor, facilitating both charge injection and transport. Here, we combine experiments and numerical simulations to obtain a deeper understanding of the complicated device physics of LECs. In particular, three parameters were found to set key properties like the electrochemical doping and the switch-on time: 1) Carrier injection, setting the operational mechanism; 2) the ion density, setting the final doping density and 3) the mobilities of the electronic and ionic carriers, setting the switch-on time of LECs. The results lead to design criteria for optimized LECs. Characterization techniques like scanning Kelvin probe microscopy and doping induced photoluminescence quenching on planar LECs with an interelectrode spacing of 100 µm were combined with drift-diffusion modeling to study the electrostatic potential and doping vs. time and bias. The ability to dope the active layer was found to depend on the ability to form ohmic contacts: Mobile ions move to the electrodes to make the bulk field free by formation of electric double layers (EDLs). These EDLs generate a large electric field, enabling the injection of charge carriers in the semiconductor over injection barriers up to 1-2 eV. Inability to form ohmic contacts e.g. due to the presence of an oxide tunneling barrier or due to side-reactions, strongly decreases injection and thereby frustrates electrochemical doping. In LECs with ohmic contacts, all ions are used to either form EDLs or dope the semiconductor. This makes the total amount of ions present in the LEC the determining factor for the final doping density. As doping results in enhanced carrier density and mobility, the ion density must be as large as possible. The relatively long switch-on time encountered in LECs is due to the relatively slow doping process. Optical experiments have proven that doping of the active layer does not occur gradually. Instead, doping fronts propagate shock-wave like through the active layer. Numerical modeling shows that such a doping process is related to the relative ionic and electronic mobilities: 1) the electronic mobility must be strongly enhanced by doping and 2) the ionic mobility must approximate the lower limit of the electronic mobility. Furthermore, it is found that both the ionic and the electronic mobility can limit the switch-on time. This work shows that for optimized operation and performance of LECs, the device must be able to form ohmic contacts and comprise as many ions as possible. To reduce the switch-on time, the ionic and electronic mobility must concertedly be enhanced.


U13.40
Blue Organic Light-Emitting Diodes Based on Carbazole Derivatives.Reza Saberi Moghaddam1, Dinesh Kabra1, Qasim Malik2, Harry J. Coles2 and Richard H. Friend1; 1Physics, Cambridge University, Cambridge, United Kingdom; 2Engineering, Cambridge University, Cambridge, United Kingdom.

Organic light emitting diodes (OLED) have been studied extensively due to their potential use in display and solid state lighting application. Recently, carbazole derivatives have been found to be very promising candidate due to their color stability in the blue region with high efficiency [1]. In this report, three different carbazole derivatives are investigated: 3, 6-diaryl-N-hexylcarbazole:p-acetophenyl(MOL. 1),3,6-diaryl-N-hexylcarbazole:pyrenyl(MOL. 2) and 3,6-diaryl-N-hexylcarbazole:p-cyanophenyl(MOL. 3). Device structures of the type ITO/PEDOT:PSS/MOL.X/Ca/Al gave efficiencies up to ~ 0.8 cd/A for MOL. 3. The efficiency of these devices can be enhanced by introducing a hole injecting interlayer. The ultra-thin layer of poly [(9, 9-dioctylfluorenyl-2, 7-diyl)-co-(4, 4’ (N-(4-sec-butylphenyl))) diphenylamine] (TFB) on top of PEDOT: PSS can improve holes injection. An alternative material that can access deep HOMO levels is Molybdenum trioxide (MoO3) [2] and it has been observed in hole-only devices based on carbazole derivatives. For electron injection, solution processible Cs2CO3 layer can be used as it was found to inject electron similar to Ca contact in conjugated polymer systems with the advantage of easy processing and air stability [3]. References: [1] Kim, S. H., Cho, I., Sim, M. K., Park, S. and Park, S. Y. J. Mater. Chem. 21, 9139 (2011) [2] Kabra, D., Lu, L., Song, M. H., Snaith, H. J. & Friend, R. H. Adv. Mater. 22, 3194 (2010) [3] Yana Vaynzof, Y., Kabra, D., Chua, L. L. and Friend, R. H. Appl. Phys. Lett. 98, 113306 (2011)


U13.41
Trap-Assisted Recombination in Disordered Organic Semiconductors. Martijn Kuik1, L. Jan Anton Koster1, Gert-Jan A. Wetzelaer1 and Paul W. Blom1,2; 1Molecular Electronics, Zernike Institute for Advanced Materials, University of Groningen, Groningen, Netherlands; 2Holst Centre, Eindhoven, Netherlands.

Recombination of electrons and holes in organic semiconductors is an important process as it determines the efficiency of organic light-emitting diodes and is an unwanted loss process in organic solar cells. If an organic semiconductor contains charge traps, trap-assisted recombination needs to be considered. In this contribution we study the role of charge trapping in recombination processes. By combining charge transport measurements, photovoltaic response at open-circuit, and device modeling, we demonstrate the importance of trap-assisted recombination and discuss its origin. The extracted capture coefficients of the trap-assisted recombination process are thermally activated with an activation energy that is identical to that of the hole mobility μp. We show that the rate limiting step for this mechanism is the diffusion of free holes towards trapped electrons in their mutual coulomb field, with the capture coefficient given by (q/ε)μp. As a result, both the bimolecular and trap-assisted recombination processes in organic semiconductors are governed by the charge carrier mobilities, paving the way for predictive modeling of organic devices.


U13.42
Molecular Design for Highly Ordered Smectic Liquid Crystals and Their Charge Carrier Transport Properties.Jun-ichi Hanna1,2, Takeo Kobori1,2, Takayuki Usui1,2, Yukiko Takayashiki1,2, Hiroaki Iino1,2 and Akira Ohno1,2; 1Imaging Science and Engineering Laboratory, Tokyo Institute of Technology, Yokohama, Kanagawa, Japan; 2CREST, JST, Yokohama, Japan.

We have proposed a new strategy of molecular design for highly ordered smectic (rod-like) liquid crystals exhibiting high charge carrier mobility, synthesized several model materials based on the present molecular design, and characterized their phase transition behaviors by differential scanning calorimetry, texture observation with a polarized microscope, and x-ray diffraction study, and charge carrier transport properties by time-of-flight experiments. We adopted anthracene and benzothienobenzothiophene (BTBT) moieties as a core part and synthesized their derivatives. We clarified that they exhibited a highly ordered smectic mesophase of SmE at a certain temperature range next to the crystallization temperature. We found that the charge carrier transport in the SmE phase was non-dispersive and ambipolar basically. We determined electron and hole mobilities to be 0.2cm2/Vs for the BTBT derivative, while the anthracene derivative exhibited very high hole mobility of 0.3 cm2/Vs. We discuss the availability of the new strategy of molecular design for highly ordered smectic mesophases exhibiting a high carrier mobility, and show their FET application in polycrystalline thin films.


U13.43
Abstract Withdrawn


U13.44
Noble Characteristics of Cesium Azide as an Electron Injection Layer in Organic Light-Emitting Devices.Jeihyun Lee, Hyunbok Lee, Pyungeun Jeon, Soohyung Park, Donggeun Shin, Younjoo Lee, Kwangho Jeong and Yeonjin Yi; Dept. of Physics, Yonsei Univ., Seoul, Korea, Republic of.

We investigated electron injection enhancement with a cesium azide (CsN3) layer inserted between tris(8-hydroxyquinoline) aluminum (Alq3) and aluminum (Al) which are a typical emission layer and a cathode in organic light-emitting devices (OLEDs), respectively. CsN3 showed to be decomposed into Cs and N2 during evaporation process in relatively low temperature (310 °C). Therefore only Cs atoms could be deposited on Alq3 and n-type doping was easily realized. OLEDs with CsN3 show highly improved current density-voltage (J-V) and luminance-voltage (L-V) characteristics compared to the control device. To understand the origin of the improvements, we carried out in-situ photoemission spectroscopy experiments. As CsN3 was deposited on Alq3, the highest occupied molecular orbital (HOMO) level shifted toward high binding energy and gap states emerged within the band gap of pristine Alq3. Combining all information obtained from spectral changes, we drew the energy level diagram of Alq3/CsN3/Al and Alq3/Al and compared them. It showed remarkable reduction of the electron injection barrier due to the n-type doping effect of CsN3. Collectively, CsN3 could be a potential candidate to dope Alq3 layer with alkali metal without getter source (alkali metal dispenser) which is difficult to control the deposition rate during the device fabrication.


U13.45
Highly Efficient MnO2 Electron Injection Layer for Both Normal and Inverted Organic Light-Emitting Devices.Hyunbok Lee, Jeihyun Lee, Kwangho Jeong and Yeonjin Yi; Institute of Physics and Applied Physics, Yonsei University, Seoul, Korea, Republic of.

Because the interfacial energetics plays a crucial role in organic light-emitting devices (OLEDs), it is highly required to introduce suitable charge injection layer. The controls of such energetics are one of the most important issues in both normal and inverted OLEDs. In this study, we adopted a highly efficient electron injection layer (EIL), manganese dioxide (MnO2), in normal and inverted OLED structure. For normal structure, current density-voltage (J-V) and luminance-voltage (L-V) characteristics were enhanced significantly with MnO2 EIL. To understand the origin of such enhancement, we investigated the interfacial electronic structures using in situ ultraviolet and x-ray photoemission spectroscopy. The change of each energy level was measured and complete energy level diagrams were evaluated. The electron injection barrier is dramatically reduced by inserting MnO2 of 1 nm thick. We also investigated the MnO2 layer deposited on ITO for inverted OLED structure. The work function of ITO was reduced by depositing the MnO2 layer, which renders notable enhancement in J-V characteristics. As a result, MnO2 could be a potential candidate as an EIL for both normal and inverted OLED at the same time.


U13.46
Variables Affecting Trapped Charge Lifetimes in Spun-Cast diF-TESADT OTFTs.Lucile C. Teague1 and John E. Anthony2; 1Savannah River National Laboratory, Aiken, South Carolina; 2Department of Chemistry, University of Kentucky, Lexington, Kentucky.

The amount of trapped charge and its subsequent dissipation in organic thin film devices is believed to be related to a variety of factors. These factors include, grain boundary density, charge traps at the dielectric interface, and chemical defects. Ultimately, this trapped charge has deleterious effects on organic thin-film transistor (OTFT) performance and stability, causing shifts in the threshold voltage over time. Numerous studies have focused on understanding these factors in a variety of organic semiconducting polymers and small-molecule semiconductors. In several of these studies, scanning Kelvin probe microscopy (SKPM) has been utilized as tool to probe charge transport in working OTFT devices, yielding structure—property relationships over multiple length scales with high spatial resolution.[1-4] SKPM also provides the opportunity to monitor charge trapping and dissipation over time. Herein, we will discuss time-dependent SKPM studies on charge trapping and dissipation in spun-cast difluoro bis(triethylsilylethynyl) anthradithiophene(diF-TESADT) OTFT devices as a function of both humidity and film structure and thickness. We will also compare and contrast our results with those of previous studies which utilized SKPM to investigate time evolution of the potential profile of the surface dielectric as a function of humidity.[5] [1]M. J. Jaquith, J. E. Anthony, and J. A. Marohn, J. Mater. Chem. 19, 6116 (2009). [2]J. L. Luria et al., Adv. Mater. 23, 624 (2011). [3]L. C. Teague et al., Adv. Mater. 20, 4513 (2008). [4]L. C. Teague et al., Appl. Phys. Lett. 96, 203305 (2010). [5]S. G. J. Mathijssen et al., Adv. Mater. 20, 975 (2008).


U13.47
Microwave Annealing: A Promising Step in the Roll-to-Roll Processing of Organic Electronics.Koen Gilissen1,2, Wim Deferme1,2, Jean Manca1,3 and Wouter Moons1; 1Physics, Hasselt University Institute for material research (IMO-IMOMEC), Diepenbeek, Limburg, Belgium; 2EMAP, Xios University College Limburg, Diepenbeek, Limburg, Belgium; 3Division IMOMEC, IMEC vzw, Diepenbeek, Limburg, Belgium.

One of the important aspects of organic electronics is its solution processability which allows the utilization of high throughput and low cost roll-to-roll processes for the preparation of large-area optoelectronic applications such as Organic Light Emitting Diodes (OLEDs) and Organic PhotoVoltaics (OPVs). To remove the solvent from the polymer layer after deposition, the sample is commonly annealed on a hot plate or oven, which is a rather time consuming process. Here, microwave annealing is introduced as an alternative, fast annealing technique in the roll-to-roll processing of organic electronics. In this study a systematic comparison is made between hot plate annealing and microwave annealing of the (screen) printed layers in an OLED. Morphological results obtained by scanning electron microscopy (SEM), have shown that there is no fundamental difference in topography between the thin films annealed on a hot plate or in a microwave but the annealing time is reduced significantly. Furthermore, electro-optical characterisation (IV measurements, luminance efficiency, power efficiency and external quantum efficiency) is performed to investigate the influence of the different drying techniques on the physical properties of the OLEDs. This study shows that microwave annealing is a faster annealing technique which is roll-to-roll compatible, yielding at least comparable results, related to important physical properties, compared to hot plate annealing.


U13.48
Abstract Withdrawn


U13.49
High Resolution Patterning of Organic Electronic Materials.Carol Newby1, Jin Kyun Lee2 and Christopher K. Ober1; 1Department of Materials Science and Engineering, Cornell University, Ithaca, New York; 2Department of Polymer Science and Engineering, Inha University, Incheon, Korea, Republic of.

With the expansion of the field of organic electronics there is a need for photoresists that do not use aggressive developing solvents which damage the active organic materials such as pentacene, P3HT and PEDOT:PSS. A series of Orthogonal photoresist systems which use non-damaging highly-fluorous developing solvents were recently published to fill this space [1, 2]. These resists used 365nm light to pattern organic materials down to dimensions on the order of a micron. In this work we have expanded this resist system by developing a similar highly-fluorinated material suitable for e-beam patterning at higher resolution. Issues including resist sensitivity, side wall profile (for lift-off) and etch resistance were characterized and optimized. In combination with the resists already available this new e-beam sensitive material allows for fabrication of smaller, more complex organic devices than previously possible. These enable further studies of organic electronic materials. As an example, we fabricate high resolution electrodes on top of organic semiconductors via lift off in order to be able to investigate the materials intrinsic transport properties. 1. A. A. Zakhidov, J.-K. Lee, H. H. Fong, J. A. DeFranco, M. Chatzichristidi, P. G. Taylor, C. K. Ober, G. G. Malliaras, Adv. Mater., 20, 348 (2008) 2. P.G. Taylor, J.-K. Lee, A.A. Zakhidov, M. Chatzichristidi, H.H. Fong, J. A. DeFranco, G. G. Malliaras, C.K. Ober,. PMSE Preprints 100, 521 (2009)


U13.50
Charge Transport in Amorphous Polythiophene-Fullerene Blends.Kiarash Vakhshouri1, Derek R. Kozub1 and Enrique D. Gomez1,2; 1Chemical Engineering, Pennsylvania State University, University Park, Pennsylvania; 2Materials Research Institute, Pennsylvania State University, University Park, Pennsylvania.

Energy-filtered transmission electron microscopy studies have revealed that fullerene-rich, homogenously mixed phases exist within mesostructured polythiophene/fullerene mixtures. However, the role of molecular mixing within the active layer of organic photovoltaic devices and its effect on the device performance is not fully understood. In this work, the field-effect electron mobilities within regiorandom poly(3-hexylthiophene)/[6,6]-phenyl-C61 butyric acid methyl ester blends have been studied as a model system for the fullerene-rich phase of the active layer of organic solar cells. The substrate surface energy was carefully chosen to prevent selective wetting at the dielectric interface. We find that the electron mobility is correlated with the miscibility of polythiophene/fullerene mixtures, which is determined by measuring the Flory-Huggins interaction parameter.


 

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