Symposium Organizers
Oliver Hayden, Siemens AG
Franky So, University of Florida
Paul Blom, TNO Holst Centre
Henning Richter, Nano-C, Inc.
Jongwook Park, Catholic University of Korea
Symposium Support
Nano-C, Inc.
National Science Foundation
Siemens AG
J4: Demonstrator Session
Session Chairs
Tuesday PM, April 10, 2012
Moscone West, Level 2, Room 2008
2:30 AM - J4
Prototypes of organic electronic devices will be demonstrated. After the demonstration the audience has the opportunity to discuss the prototypes with the experts.
Show AbstractJ5: Lighting I
Session Chairs
Tuesday PM, April 10, 2012
Moscone West, Level 2, Room 2008
4:30 AM - *J5.1
Outcoupling in Highly Efficient Organic LED
Karl Leo 1
1TU Dresden Dresden Germany
Show AbstractOrganic Light Emitting Diodes (OLED) have developed rapidly in the past years. For small mobile displays, they are already a commercially established technology. For OLED lighting, where white OLED with very high efficiency and stability are needed, a further improvement of parameters is still needed. I this talk, I will discuss some of our recent progress on highly efficient monochrome and white OLED. I will compare experimental results with a theoretical model which reveals how the outcoupling of the emitted photons proceeds and how the efficiency can be increased. For monochrome OLED, quantum efficiencies of well beyond 50% have been reached. Furthermore, I will discuss the difference of bottom- and top-emitting OLED, showing that the latter indeed show the better outcoupling properties as theoretically predicted. Furthermore, approaches to improve outcoupling by introducing hybrid inorganic-organic layers for wave guide mode scattering will be discussed.
5:00 AM - *J5.2
Molecular Orientation in OLEDs
Daisuke Yokoyama 1 2
1Yamagata University Yonezawa, Yamagata Japan2Yamagata University Yonezawa, Yamagata Japan
Show AbstractIn small-molecule OLEDs, vacuum-deposited amorphous organic films are commonly used, even though they generally have much lower carrier mobilities than those in other types of organic semiconductors such as single crystals and polycrystalline films. This is mainly because the amorphous films have many advantages in fabrication of OLEDs with a large area and a high uniformity: (1) nanometer-scale surface smoothness, (2) easy controllability of thickness, (3) no restriction in the choice of underlying layers, and (4) simple high-purity fabrication processes. These advantages make it possible to stack any materials having different properties orderly and also derive full potential and higher-order functionalities of organic materials with high stability and durability. When developing new OLED materials, we usually deduce the physical properties of films only from the chemical characteristics of the single molecules, and have not sufficiently considered how molecules are oriented and ordered in the bulk of the amorphous films. Since the beginning of research on OLEDs using vacuum-deposited thin films in 1987, it has been believed for around 20 years that molecular orientation in small-molecule OLEDs is generally random and isotropic. This simple assumption has been used unconsciously and contributed to the efficient development of new OLED materials since the early stage of OLED research. However, we have recently found that molecules in vacuum-deposited amorphous organic films are generally oriented depending on the anisotropy of the molecular shape. Linear and planar molecules are generally oriented along the substrate surface, whereas bulky and compact molecules are randomly oriented. Since this horizontal orientation of anisotropic molecules occurs on any underlying layer and even in doped films, we can effectively use it for applications in any structures of devices. This presentation will show an overview of recent studies on molecular orientation in small-molecule OLEDs. The molecular orientation of various kinds of OLED materials will be demonstrated, and the general characteristics of the orientation will be discussed. It will be also shown that the horizontal molecular orientation of charge transport materials and emitting materials can contribute to the enhancement of carrier mobilities and light outcoupling efficiency, respectively. Now, research on OLEDs has reached maturity, and the device performance is coming close to the theoretical limit. Thus, we have to understand the device physics and the underlying chemistry beyond the above simple assumption to further improve the performance and reliability of OLEDs. Through this study, we can find that the investigation of higher-order structures such as molecular orientation and ordering are very important to make fundamentals for the next step of research on organic optoelectronic devices. Reference: D. Yokoyama, J. Mater. Chem. (2011), in press, doi: 10.1039/c1jm13417e.
5:30 AM - *J5.3
High Efficient Printable Electrophosphorescence OLEDs
Hyeyeon Yang 1
1Samsung Electronics Yongin Republic of Korea
Show AbstractOur research provides a concept for increasing the performance of printable OLEDs. To achieve high efficiency of electrophosphorescence OLEDs (PhOLEDs), it is crucial to develop host materials for PhOLEDs and to develop a method for multilayer architecture based on crosslinking, which are challenges.
J6: Poster Session: Organic Field Effect Transistor, Materials and Other Devices
Session Chairs
Tuesday PM, April 10, 2012
Moscone West, Level 1, Exhibit Hall
6:00 AM - J6.1
Patternable Biopolymer Dielectrics from Chicken Albumen for Organic Electronics
Jer-Wei Chang 1 I-Tsung Chen 1 Chong-Yu Huang 1 Tzung-Da Tsai 1 Tzung-Fang Guo 1 Ten-Chin Wen 2
1National Cheng Kung University Tainan Taiwan2National Cheng Kung University Tainan Taiwan
Show AbstractPatternable protein polymers from chicken albumen were utilized as dielectrics for organic electronics. The solution of albumen was spin-coated and thermally stamped with a patterned mold, and then developed a patterned dielectric films. The patternability of albumen dielectrics is attributed majorly to the localized disulfide bond formation by which different protein molecules with thiol side chains are crosslinked and netted without adding any crosslinking agent. The patterned albumen dielectrics are water-insoluble and ready for next fabrication process, such as the vacuum- or solution-processed deposition of organic semiconductors, metals, or a second layer dielectric. This work demonstrated that patternable albumen dielectrics were deployed multifunctionally as dielectrics for capacitors or field-effect transistors, as insulators for inductors or multilayer circuits, and as materials for substrates or capsulators. It indicates that albumen is a practical material for the simplified fabrication of organic electronics.
6:00 AM - J6.10
High Performance Non-volatile Nano-floating Gate Organic Transistor Memory
Minji Kang 1 2 3 Kang-Jun Baeg 4 Dongyoon Khim 1 2 Dong-Yu Kim 1 2 3
1Gwangju Institute of Science and Technology Gwangju Republic of Korea2Gwangju Institute of Science and Technology Gwangju Republic of Korea3Gwangju Institute of Science and Technology Gwangju Republic of Korea4Electronics and Telecommunications Research Institute (ETRI) Daejeon Republic of Korea
Show Abstract
Non-volatile organic memory has emerged as a candidate for light-weight, flexible, and disposable charge storage media. To this end, a variety of approaches has been progressed including capacitor-type bistable devices and organic field-effect transistor (OFET)-based memories. Here we report on a solution-processed organic transistor memory using a n-channel (and potentially ambipolar) polymer semiconductor, poly{[N,Nâ?T-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarbo-ximide)-2,6-diyl]-alt-5,5â?T-(2,2â?T-bithiophene)} (P(NDI2OD-T2)), and bi-layered polymer gate di-electrics. High electron mobility (~0.3 cm2/Vs) and air-stability of the P(NDI2OD-T2) molecule enables to develop high performance organic flash memory element with large memory window (~39 V), high on/off-current ratio (~105), and long retention time more than ~106 s. These memory characteristics were achieved by proper selection of the dielectric materials and their orthogonal solvents, and systematically analyzed by embedding various metal nanoparticles, such as Au, Cu, Al, and Ag, in bi-layered gate dielectrics of the OFET.
6:00 AM - J6.11
Organic Thin-film Transistor with Short Channel Length Using Silver Electrodes by the Reverse Offset Printing Technology
Minseok Kim 1 2 Jae Bon Koo 1 Soon-Won Jung 1 Tae-Youb Kim 1 Yong Suk Yang 1 Byeong-Kwon Ju 2 In-Kyu You 1
1ETRI Daejeon Republic of Korea2Korea University Seoul Republic of Korea
Show AbstractPrinted electronics have attracted increasing attentions because of its realization of flexible, large-area, and low-cost for organic devices and circuits. Numerous printing techniques have been shown to realize simple devices or circuits such as solar-cell photovoltaic, memory, sensors, and RFID. Reverse offset printing (ROP) is one of candidates for printed electronics. Using ROP method is very attractive owing to a variety of advantages, such as very high resolution, high throughput, no limitation to substrate types or sizes, and ability to print fine and complicate patterns. In this study, Ag electrodes were printed to 15 μm of line width and to 3 μm of space width by ROP using the paste with Ag nanoparticles which were almost monodispersed with a mean particle size of ~ 5 nm confirmed by TEM and easily dispersible with common organic solvent such as octane, toluene, etc. It would be enable facile self-assembly of these particles into close packing when printed, thus greatly facilitating their coalescence into a continuous conductive film. After printing and sintering of Ag paste, microstructure of the Ag film was observed by SEM. From these results, we confirmed that the electrical resistivity was decreased with 1.6 μΩcm corresponding to densification of the printed film and growth of nanoparticles. Printed Ag electrodes were applied to source/drain of organic thin-film transistors (OTFTs) based on semiconducting polymer as poly(3-hexylthiophene-2,5-diyl) (P3HT) and poly(3,3â?Tâ?Tâ?T-dialkyl-quarterthiophene) (PQT-12). The OTFT with short channel of 3 μm showed reproducible characteristics. Field-effect mobility, threshold voltage, and on/off-current ratio were 0.01 cm2 V-1 s-1, 5 V, and ~104, respectively.
6:00 AM - J6.12
Effect of Exposure to Environmental Conditions on Charge Trapping Properties of Organic Thin-film Transistors with Gate Induced Bias-stress
Dongwook Kim 1 Jaehoon Park 2 Do-hyoung Lee 1 Sungwoo Lee 3 Youngmin Kim 4 Jong Sun Choi 4
1Hongik University Seoul Republic of Korea2Electronics and Telecommunications Research Institude Daejeon Republic of Korea3Hongik University Seoul Republic of Korea4Hongik University Seoul Republic of Korea
Show AbstractWe show that electrical bias-stress effects on pentacene-based organic thin-film transistors (OTFTs) can be used to verify the two different mechanisms of charge trapping in pentacene-gate dielectric interface and irreversible degradation of pentacene film. Generally, Thin-film transistors (TFTs) are suffering from the electrical instability by bias stress which is threshold voltage shift caused by the positive/negative voltage bias-stress through gate dielectric materials. These instabilities were found to be the results of both complex mechanisms in a charge trapping and mobility degradation. To clarify these mechanisms, the operating performance with bias stress of TFTs depended on the environmental conditions such as vacuum, air atmosphere, oxygen, etc. And no significant changes in threshold voltage shift measured in vacuum state. Furthermore, the metastable creation in the pentacene and gate-dielectric interface was not detected with the thermal accelerated degradation test. We attributed the external contents of oxygen gas or moisture to a trap-assisted charge trapping process in a conduction channel formed in the vicinity of the gate dielectric. We conclude that, exposure to environmental active gases such as oxygen, humidity generates trap-assisted charge trapping in channel when charge accumulated with gate-bias. And the charge trapping called reversible/irreversible threshold voltage shift due to the positive/negative bias stress effects are originated from temporally trapped charge in shallow-trap state, and the chemical reactions with gas ions attracted by strongly accumulated positive hole-carriers.
6:00 AM - J6.13
Air Stable Organic Light Emitting Field Effect Transistor with Gold Electrodes
Thangavel Kanagasekaran 1 Susumu Ikeda 1 Ryotaro Kumashiro 1 Hidekazu Shimotani 1 Katsumi Tanigaki 1
1Tohoku University Aoba Aramaki Aoba-ku, Sendai Japan
Show AbstractRecent advancements achieved in organic materials greatly promoted the development of devices such as organic light-emitting diodes (OLEDs), solar cells, ion batteries, organic memories, organic field-effect transistors (OFETs) and organic light emitting field effect transistors (OLFETs). In principle, pure organic semiconductors provide both electron and hole equivalent conduction, although organic semiconductors are typically classified as either hole-conducting (p-type) or electron-conducting (n-type) materials. This is known as the ambipolar action in organic semiconductors and is very different from that in inorganic semiconductors. However, in order to be accessible to such ideal ambipolar conduction, accurate control in both the electrode-semiconductor contact (charge injection process) and the semiconductor-dielectric gate interface (charge accumulation process) is crucial. If such ideal ambipolar conduction can be realized, electron and hole can simultaneously be injected into an intrinsic semiconductor layer and highly efficient OLFETs can be constructed without paying any attention to the complex p-n junctions, which are generally indispensable for making inorganic LFETs. We have carefully examined both injection and accumulation processes for the purpose of simultaneous gaining a large amount of electrons and holes. We have tested 2,5-biphenyl bithiphene (BP2T) at the present study because this compound is known to be a good candidate for OLFETs For improving the injection process a various metals with different work functions have been employed as the source/drain contacts. A good methodology has been searched by employing various methods, such as modification by polymer thin films, self assembled monolayers (SAMs) and deposited aliphatic molecular thin films. In this conference, we will demonstrate that a good ambipolar carrier injection can be realized even using gold (Au) electrodes for both source and drain, and therefore an air stable and high performance OLFET can be fabricated when both injection and accumulation conditions are optimized. The role of the injection barriers on the device performance will be dicussed in terms of the Bardeen limit and the Schottkey limit.
6:00 AM - J6.15
2,5-Bis(2-octyldodecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione -based Donor-acceptor Alternating Copolymers for Electronic and Optoelectronic Applications
Ji Ho Kim 1 Kyung Hwan Kim 1 Tae Wan Lee 1 Jicheol Shin 1 Dong Hoon Choi 1
1Korea University Seoul Republic of Korea
Show AbstractSubstantial progress has been made in the exploration of organic semiconductors as active elements in electronic devices i.e., field-effect transistors (FETs). Various conjugated polymers containing diketopyrrolopyrrole (DPP) in the repeating group were reported in the literature for showing their superb electronic device applicability. In the study, we successfully synthesized new low bandgap alternating copolymers containing quaterthiophene (QT), di(thiophen-2-yl)-2,2â?T-biselenophene (DTBSe), and dithienothiophene (DTT). Those copolymers were highly crystalline and provide very promising charge transport phenomenon. Thermal analaysis, cyclic voltammetry, absorption spectroscopy and photoluminescence spectroscopy were employed to investigate their intrinsic materials properties. Eventually, we fabricated field effect transistors and photovoltaic devices to study their semiconducting property. According to our past study, these polymers were found to exhibit quite high carrier mobilities of around 0.6~1.5 cm2V-1s-1.
6:00 AM - J6.16
Photophysics of Self Assembled Monolayer-field Effect Transistors(SAMFETs)
Fatemeh Gholamrezaie 1 2 Dago de Leeuw 1 Stefan Meskers 2
1Philips Eindhoven Netherlands2TUe (Eindhoven University) Eindhoven Netherlands
Show AbstractOrganic molecular materials and crystals can serve as active layer in a variety of optoelectronic devices, including solar cells, light emitting diodes and transistors. A comprehensive understanding of the opto-electronic properties of crystals is complicated by the occurrence of surface states, defects and disorder in crystals [1]. The problem may be simplified by studying two dimensional layers of organic molecule rather than the three-dimensional crystals. Recently, we reported closely packed, two-dimensional, self-assembled monolayers of quinque thiophene (5T). The layers are characterized by a high degree of order and are electronically active in a transistor. Self-assembled monolayer field effect transistor (SAMFET) can be grown on SiO2 or polymeric gate dielectrics and combined into integrated circuits. The SAMFETs feature bulk-like carrier mobility, large current modulation and high reproducibility. [2,3] Here we investigate the optical properties of the monolayer using fluorescence, excitation and extinction spectroscopy. [4] For partially covered substrates we find that the fluorescence is dominated by isolated molecules. Complete monolayers show optical properties that differ markedly from the isolated molecules. For the complete monolayer, we observed a red-shifted fluorescence spectrum combined with a blue shift of the absorption maximum. Hence the complete monolayer behaves as an H-type aggregate. The optical properties of the monolayers share many characteristics with those reported for oligothiophene crystals. Thus the optical properties of two dimensional, defect free oligothiophene monolayer are influenced by delocalized of the optical excitations over more than one molecule. Because of their fluorescence, the monolayers may use as active medium in light emitting devices. [1] Como, E. e t al. J. Am. Chem. Soc. 2006, 128, 4277. [2] Gholamrezaie, F.et al. in preperation. [3] Smits, E. C. P et al. Nature 2008, 455, 956 [4] Gholamrezaie, F.et al. Nano Letters, 2011, 10, 1998.
6:00 AM - J6.17
High Voltage Field Effect Transistors Based on Organic Semiconductors
Melissa Alyson Smith 1 3 Akintunde I Akinwande 2 3
1Massachusetts Institute of Technology Cambridge USA2Massachusetts Institute of Technology Cambridge USA3Massachusetts Institute of Technology Cambridge USA
Show AbstractA high voltage field effect transistor has been fabricated using a thin-film organic semiconductor technology. The transistors have an offset drain/source structure, which enables operation at drain-to-source voltages in excess of 450V with gate voltages no higher than 20V. Similar FET structures based on amorphous Si have been reported by other investigators. This work exploits the process advantages of organic materials to demonstrate high voltage devices capable of driving MEMS actuators or for applications requiring high voltages on flexible media. We believe this to be the first demonstration of integrated high voltage field effect transistors based on an organic semiconductor technology with a low temperature (� 95°C) and completely lithographic process that is compatible with both flexible and rigid substrates. One way to increase the drain-to-source driving voltage or electric field across an FET is to place a resistive structure between the drain or source electrode and the gated channel. In this work, high driving voltages are achieved by offsetting the drain or source electrode from the gate creating an ungated semiconductor region in series with a gated semiconductor region. The saturation current (IDsat) through the devices is controlled by the gated semiconductor region, limiting the voltage dropped across (and electric field) this region. The excess voltage is dropped across the ungated semiconductor enabling high voltage operation. There are many methods available to tune the threshold voltage (and transconductance) of these FETs as the gate insulator directly affects carrier transport in the device. Further, it is well documented that high-κ insulator materials are effective in reducing operating voltages (gate-to-source) in FETs while minimizing current leakage through the gate. We previously reported a process using 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 (~40-50), which can be deposited at room temperature through RF sputtering. We show that the threshold voltage of these pentacene-based high voltage FETs can be reduced from ~-9V to ~-0.5V when using a BZN/parylene-C insulator stack as opposed to an all organic gate insulator (parylene-C). Using the offset drain/source device structure and a dual stacked insulator system, these high voltage field effect transistors can be precisely matched for a multitude of MEMS applications requiring high voltages. The devices are fabricated with a low temperature and completely lithographic process allowing them to be readily integrated with high voltage applications. Electrical performance is evaluated through standard current-voltage and quasi-static capacitance-voltage measurements.
6:00 AM - J6.18
Air-flow Navigated Crystal Growth for TIPS Pentacene-based Organic Thin-film Transistors
Zhengran He 1 3 Jihua Chen 4 Zhenzhong Sun 1 Greg Szulczewski 2 3 Dawen Li 1 3
1The University of Alabama Tuscaloosa USA2The University of Alabama Tuscaloosa USA3The University of Alabama Tuscaloosa USA4Oak Ridge National Laboratory Oak Ridge USA
Show Abstract
6,13-bis(triisopropylsilylethynyl)pentacene (TIPS pentacene) is a promising active channel material for organic thin-film transistors (OTFTs) due to its solubility, air stability, and high mobility. However, the growth of TIPS pentacene crystal suffers from intrinsic crystal anisotropy, grain boundary limitation and â?ocoffee-ring effectâ?, which leads to poor performance and significant performance variation of OTFTs. In this work, air flow is utilized to effectively navigate crystal growth. Air flow facilitates uniform solvent evaporation by reducing the contact angle between solvent and silicon substrate, and pushes contact line and solute in solution forward along the crystal growth direction. Collectively these effects prevent solute diffusion from center to edge, resulting in uniform TIPS pentacene seed deposition for crystal growth. The crystallization of TIPS pentacene starts from front contact line, and moves in the same direction as air flow. As a result, air flow effectively navigates TIPS pentacene crystallization and reduces crystal anisotropy over the whole substrate. Under air flow navigation (AFN), TIPS pentacene has been observed to form thin films with improved crystal orientation and uniform coverage on substrate. In addition, the field-effect mobilities of OTFTs based on TIPS pentacene without AFN guidance vary from 7.2Ã-10-5 cm2/Vâ-s to 8.8Ã-10-1 cm2/Vâ-s, while AFN OTFTs consistently show mobilities around 0.1 cm2/Vâ-s, which indicates a great reduction in performance variation. Therefore, we conclude that AFN is an effective approach to reduce TIPS pentacene crystal anisotropy and improve performance of OTFTs. Key words: Air Flow Navigation, TIPS Pentacene, Crystal Anisotropy, Organic Thin-Film Transistors
6:00 AM - J6.19
Reliability Studies of Pentacene Based Thin Film Transistors
Romulald Ledru 2 Stephane Pleutin 2 Bruno Grouiez 2 Damien Zander 2 Habiba Bejbouji 1 Kamal Lmimouni 1 Dominique Vuillaume 1
1IEMN UMR CNRS 8520 Villeneuve d'Ascq Cedex France2CReSTIC Reims France
Show AbstractAs Organic Field-Effect Transistors (OFET) are now close to applications questions about their reliability under realistic atmospheric conditions become more and more important[1]. Electrical instabilities are evidenced, for instance, in the bias stress effect where under a prolonged application of a gate potential the device characteristics, such as the threshold voltage, evolve with time. Several mechanisms were proposed to understand these degradations of the device properties. An example of such mechanism was proposed recently based on proton production by electrochemical reactions involving water molecules adsorbed at the organic/oxide interface, followed by proton diffusion through the oxide[2,3] . In this work, we have considered the Si/SiO2/Pentacene/Au junctions that are the two terminal pendants of thin film transistors with the same layer structure. We study the dynamic electrical response of these junctions under ambient conditions over a large frequency window as function of a superimposed dc voltage. The complex admittance, Y(Ï?), was measured using a frequency response analyzer Solartron 1260/1296 that provides directly in general case, the parallel conductance and capacitance, G and C: Y(Ï?)=G(Ï?)+jÏ?C(Ï?). Compared to previous works, our frequency range is more extended, going down to 10-1Hz, and the analysis of our data insist on the crucial role played by the interactions of the defects with their surroundings which determine the shape of the measured response functions. At lower frequencies (100Hz) we observe anomalous Low Frequency Dispersion (LFD) that is consistent with the ionic diffusion current proposed in Refs. [2,3] but instead of normal diffusion considered in these works, our results evidence fractional diffusion to occur. The dynamical responses of the polarizable species (dipoles or ions) are strongly influenced by their surroundings. If they are markovian (without memory) we get a Debye response for the dipoles and a normal diffusion for the ions as shown in Si/SiO2/Au â?oreferenceâ? junctions (without the organic semiconductor layer). If they are non-markovian (with memory) the responses are more complex; in their simplest forms we get a Cole-Cole response for the dipoles and a fractional diffusion for the ions. Our data show anomalous behavior in presence of the organic layer. These observations could be the signature of particular structural organizations which can be located either, in the organic layer itself or, in the organic/oxide interface. [1]H. Sirringhaus, Adv. Mater. 2009, 21, 3859. [2]A. Sharma, S.G.J. Mathijssen, M. Kemerink, D.M. de Leeuw, and P.A. Bobbert, Appl. Phys. Lett. 2009, 95, 253305. [3]A. Sharma, S.G.J. Mathijssen, E.C.P. Smits, M. Kemerink, D.M. de Leeuw, and P.A. Bobbert, Phys. Rev. B. 2010, 82, 075322.
6:00 AM - J6.2
Hole Blocking in Pentacene-based Organic Field-effect Transistors with Aluminum Electrodes
Jer-Wei Chang 1 Cheng-Guang Wang 1 Chong-Yu Huang 1 Tzung-Fang Guo 1 Ten-Chin Wen 2 Yao-Jane Hsu 3
1National Cheng Kung University Tainan Taiwan2National Cheng Kung University Tainan Taiwan3National Synchrotron Radiation Research Center Hsinchu Taiwan
Show AbstractHole blocking effect of the interfaces of aluminum (Al) and pentacene in organic field-effect transistors (OFETs) was reported. Originally ambipolar pentacene-based OFETs of aluminum (Al) source-drain electrodes performed in n-type conduction only by inserting a coevaporated Al-pentacene layer. The highest occupied molecular orbital (HOMO) of the coevaporated Al-pentacene layer was moved downward 1.3 eV measured by ultraviolet-visible absorption spectroscopy and ultraviolet photoemission spectroscopy. The hole blocking effect was also observed in Al pentacene-based OFETs without the coevaporated layer for 24 hoursâ?T storage and verified by thermoionic emission model. This study highlights that the interaction of active metals and organic semiconductors impacts the performance of OFETs.
6:00 AM - J6.20
Conjugated Monomers and Oligomers Synthesised by Metathesis Route as Active Layer in Organic Field Effect Transistors
Nela Bejenaru 1 2 Kamal Lmimouni 1 Habiba Bejbouji 1 Dominique Vuillaume 1 Regis Gauvin 2 Andre Mortreux 2 Nicolas Cheval 3 Patrick Pale 3
1IEMN UMR CNRS 8520 USTL, BP 60069 avenue Poincareacute; 59652 Villeneuve d'Ascq Cedex France2UCCS (UMR CNRS 8181) ENSCL - Bacirc;t C7 BP 90108, 59652 Villeneuve d'Ascq Cedex France3LASYRO (UMR CNRS 7177) Institut de Chimie de Strasbourg- 1 rue Blaise Pascal 67000 Strasbourg Cedex France
Show AbstractIn this work, we describe the realization and the optimization of field effect transistors based on Ï?-conjugated PE monomer and OPE oligomers as active layers. The 1,4â?"dipropynyl benzene monomer PE was first prepared by Sonogashira coupling between 1.4-diodobenzene and propyne catalyzed by a Pd/PR3/basis/Cu system. The OPE was synthesized by an original route: the triple bond metathesis reaction of a para disubstituted alkyne. Metathesis polymerization of PE was done using the tungsten carbyne [(tBuO)3Wâ?¡CtBu] as catalyst.The obtained OPE was characterized by 1H and 13C NMR in solid and liquid states, IR and AFM. The average degree of polymerization is low (n = 4). The TFT devices were constructed on a highly doped n-type silicon substrate covered with 220nm-thick silicon oxide layer. The organic semiconductors were vacuum evaporated on patterned gold source and drain electrodes. The thickness of the active layers was ranging from 15 to 30nm and the channel length L from 1 to 50 μm. The TFT characteristics were measured both in Nitrogen atmosphere (Glove Box) and in air at room temperature. Both monomer PE and oligomer OPE based devices exhibited a field effect behaviour with motilities of around 10E-5cm2/V.s. These performances were improved in the case of OPE oligomer based devices. The mobility enhancements were obtained, either via functionnalization of the SiO2 substrate with Self Assembled Monolayers of perfluorodecyltrichlorosilane (µ= 10E-4 cm2/V.s), or via the control of the substrate temperature at 40°C during deposition (µ ~10E-3cm2/V.s). These results will be discussed according to the morphology and the structural modifications of the organic semiconductors induced by temperature and SAM treatments. Other conjugated oligomers synthesised by the metathesis route will also be presented in field effect transistor applications.
6:00 AM - J6.21
Stamping of a Self-assembled Monolayer for Organic Circuits with Spatially Controlled Threshold Voltages
Ikue Hirata 1 Ute Zschieschang 2 Frederik Ante 2 Tomoyuki Yokota 1 Kazunori Kuribara 1 Tatsuya Yamamoto 3 Kazuo Takimiya 3 Masaaki Ikeda 4 Hirokazu Kuwabara 4 Hagen Klauk 2 Tsuyoshi Sekitani 1 5 Takao Someya 1 5
1The University of Tokyo Tokyo Japan2Max Planck Institute for Solid State Research Stuttgart Germany3Hiroshima University Hiroshima Japan4Nippon Kayaku Co, Ltd. Tokyo Japan5JST/ERATO Tokyo Japan
Show Abstract
We have successfully developed the transfer technology of a few nanometer thick self-assembled monolayers (SAMs) using polydimethylsiloxane (PDMS) stamping, and fabricated organic thin-film transistors and circuits where two different SAMs were spatially transferred onto a same silicon substrate. Utilizing the SAM spatial-transfer method to the fabrication of gate dielectric layers, threshold voltages in organic transistors and inversion (switching) voltages in organic unipolar inverter have been systematically controlled. Furthermore, using transmission electron microscope (TEM), the formation of uniform SAMs was clearly observed, indicating the feasibility of the stamping that can transfer a single nanometer structure and that different SAMs can be spatially allocated on same substrates. The transfer method and transistor fabrication using two different SAMs (tetradecylphosphonic acid (HC14-PA) and pentadecylfluoro-octadecylphosphonic acid (FC18-PA)) have been reported in our previous work [1]; however, in this paper, we will report nanostructure analysis of transferred SAMs using TEM, and its relation to electrical performances on organic circuits. An organic semiconducting layer, DNTT [2], and electrodes of the organic circuit were fabricated using vacuum evaporation, while a SAM gate dielectric was formed using the stamping method. The stamped SAMs showed the equivalent insulating characteristics to conventional dipped SAMs [3], where the leakage current density was the order of 10-6 A/cm2. To improve transistor performances using stamping process, stamping time was systematically changed from 15 s to 10 min. As a result, the mobility exceeding 2.0 cm2/Vs was obtained when the time was more than 1 min. The uniformity of the stamped SAMs was observed using TEM, which was comparable to SAMs formed using conventional dipping process. This study was partially supported by JST/CREST and the Special Coordination Funds for Promoting and Technology. [1] I. Hirata, U. Zschieschang, F. Ante, T. Yokota, K. Kuribara, T. Yamamoto, K. Takimiya, M. Ikeda, H. Kuwabara, H. Klauk, T. Sekitani, T. Someya, MRS Communications (2011) 1-4. [2] T. Yamamoto, K. Takimiya, Journal of the American Chemical Society 129 (2007) 2224-5. [3] H. Klauk, U. Zschieschang, J. Pflaum, M. Halik, Nature 445 (2007) 745-8.
6:00 AM - J6.22
Organic Gate Dielectrics for Solution Processible Inorganic Semiconductor Thin-film Transistors
Jae-Won Ka 1 Hee Sun Kim 1 Min-Hye Seo 1 Kwang-Suk Jang 1 Mihye Yi 1
1Korea Research Institute of Chemical Technology Daejeon Republic of Korea
Show AbstractSolution processible inorganic semiconductors such as metal oxide precursors and nano-structured inorganic materials were widely investigated for high performance display backplanes, 3D-LCD display, smart-TV, and AM-OLED display. However, there have been some limitations for the utilization of organic gate dielectrics for flexible display with solution processible inorganic semiconductors such as high processing temperature and weak chemical resistance in basic precursor solution. In this study we designed and synthesized polybenzoxazole as organic gate dielectric for solution processible inorganic semiconductor TFT. The prepared polybenzoxazole thin-film showed remarkable chemical resistance and thermal stability. The insulating properties and chemical resistance results after treatment with inorganic semiconductor solution of prepared polybenzoxazole dielectric films will be discussed.
6:00 AM - J6.23
Influence of Self Assembled Monolayers on the Contact Resistance of Organic Thin Film Transistors
Marko Marinkovic 1 Dagmawi Belaineh 1 Anita Risteska 1 Veit Wagner 1 Dietmar Knipp 1
1Jacobs University Bremen Bremen Germany
Show AbstractA model was developed that describes the voltage dependence of the contact resistances of organic thin film transistors in staggered and coplanar contact configuration. The contact resistance of pentacene transistors in coplanar (bottom drain/source contacts) is approximately one order of magnitude higher than resistance of transistors in staggered configuration (top drain/source contacts). The contact resistance of transistors in the bottom drain/source contact configuration is higher due to the reduced lateral conductivity of the pentacene film in the contact area. Treating the gold drain and source contacts with a thiol based self assembled monolayer (SAMs) leads to an improved growth of the pentacene molecules on the electrodes and an increased lateral charge transport. As a consequence the contact resistance drops to values comparable to transistors with top drain/source contacts. The presented analytical model describes the influence of gate and drain voltages on the contact resistance of the thin film transistors. The model is in a good agreement with obtained experimental results. Both staggered and coplanar transistor configurations can be described by the model. Furthermore, the model can be used for explaining non-linear contact behavior (S-shape) which is very often observed in transistors with bottom drain/source electrical contacts.
6:00 AM - J6.24
Organic Thin Film Transistor Embedded by Streptavidin-biotin Binding with Au Nanoparticles as Memory Charging Element
Hunsang Jung 1 Sang-hyub Ha 1 Yo-Han Kim 1 SeWook Oh 1 Hyun-Ho Lee 1
1Myoung Ji University Yongin Republic of Korea
Show AbstractRecently, memory devices fabricated as organic thin film transistors (OTFTs) have been widely developed to facilitate organic electronics era on flexible substrates. To have an efficient monolayer of charging element in the memory OTFT, metallic or semiconductor nanoparticles (NPs) and surface of dielectric layers have been functionalized to accommodate peculiar binding mechanisms. In this study, using a strong streptavidin-biotin binding system, Au NPs were monolayered between pentacene and polyvinylalcohol (PVA) dielectric layer in OTFT device. The Au NPs were functionalized with streptavidin and the PVA surface was modified by 3-amino-propyltriethoxysilane(APTES) and N-hydroxysuccinimide (NHS)-biotin. Characteristics of memory charging effect were examined with threshold voltage shift (â-³Vth) of the pentacene OTFTs. The memory OTFT can be applicable to develop a new conceptual biosensor device.
6:00 AM - J6.25
High Performance and Highly Flexible Organic Thin Film Transistors with Conducting Polymer Source/Drain and Gate Electrodes
Young-Kyu Lee 1 Rahim Abdur 1 Hoesup Soh 1 Jaegab Lee 1
1Kookmin University Seoul Republic of Korea
Show Abstract
Flexible pentacene organic transistors are rapidly developing and have now reached mobility of 0.5-2.0 cm2V-1s-1, which is comparable with that of a â?" Si:H TFTs. Metals are often used for the gate or source/drain electrodes on a flexible substrate, but it has limited flexibility due to its plastic deformation and poor adhesion to pentacene semiconductor, and thus leading to the degraded electrical performance especially under a tensile strain. In this study, we have developed high conductive polymers, PEDOT:PTS(conductivity = 650 S/cm) which were used as gate electrode and source/drain electrodes in pentacene transistors. The use of poly(3,4-ethylenedioxythiophene)(PEDOT) electrodes combined with Al2O3/poly(4-vinyl phenol)(PVP) multilayer gate insulators and a polyethersulfone(PES) substrate allowed for the excellent mechanical stability and transparency. In addition, the addition of thin Al2O3 layer between PVP and PEDOT layers effectively suppressed the interaction of PVP with PEDOT film, thus leading to the high electrical performance of flexible organic thin film transitors(threshold voltage of -3V, threshold voltage shift of 1.12V, Ion/Ioff ratio of 1.5E+7, mobility of 0.41 cm2V-1s-1). Furthermore, the encapsulation of flexible organic transistors with Tetratetracontane(TTC) showed the long term stability of the device in terms of mobility, Ion/Ioff ratio, and threshold voltage upon exposure to air.
6:00 AM - J6.26
Molecular Aggregation-performance Relationship in the Design of Novel Cyclohexylethynyl End-capped Quaterthiophenes for Solution-processed Organic Transistors
TaeKyu An 1 Hyojung Cha 1 Lae H Kim 1 Yun-Hi Kim 2 Soon-Ki Kwon 3 Chan E Park 1
1POSTECH Pohang Republic of Korea2Department of Chemistry, Gyeongsang National University Jinju Republic of Korea3School of Materials Science and Engineering, Engineering Research Institute Jinju Republic of Korea
Show AbstractWe report here the synthesis and characterization of cyclohexylethynyl end-capped quaterthiophenes. Additionally, we describe an investigation of the performance of organic field-effect transistors (OFETs) based on these quaterthiophenes in view of the relationship between the solid-state (or aggregate) order and the electronic performance. UVâ?"vis absorption measurements revealed that the asymmetrically substituted cyclohexylethynyl end-group, CHE4T, induced formation of H-type aggregates whereas the symmetrically substituted cyclohexylethynyl end-groups, BCHE4T, favored formation of J-type aggregation. Two-dimensional grazing-incidence wide-angle X-ray scattering studies were performed to support the molecular structure-dependent packing (H or J) of the CHE4T and BCHE4T films. Solution-processed CHE4T and BCHE4T were tested as the active layers of p-type organic field-effect transistors (OTFT) with a bottom gate/top contact geometry. The field-effect mobility of devices that incorporated H-aggregated molecules, CHE4T, was quite high, exceeding 0.02 cm2/Vs, due to H-aggregation and good in-plane ordering. In contrast, the field-effect mobility of devices that incorporated J-aggregated molecules, BCHE4T, was low, above 5*10â?"4 cm2/(Vs), due to the formation of J-aggregates and poor in-plane ordering. A comparison of the symmetrical and asymmetrical quaterthiophene derivatives revealed that the molecular aggregation-dependent packing, determined by the cyclohexylethynyl end-groups, was responsible for influencing the OTFT performance.
6:00 AM - J6.28
Bias-dependent Degradation Characteristics of Pentacene Organic Field Effect Transistors (OFETs) in Hermetic Condition
Hoonsang Yoon 1 Youngmin Kim 1 Jongsun Choi 1 Hyunsuk Yang 1 Hyungtak Kim 1
1Hongik University Seoul Republic of Korea
Show AbstractIn this work, we present bias-dependent degradation characteristics of pentacene Organic Field Effect Transistors (OFETs) in Hermetic Condition. It has been reported that exposure to atmosphere led to performance degradation of OFETs. OFETs were affected by environmental factors such as humidity and air, etc. OFETs also degraded under electric bias stress test performed in atmosphere. However, electric-field induced degradation should be distinguished from the atmosphere-induced degradation to evaluate stability and reliability of OFETs properly. In order to investigate degradation characteristics varying electric field distributions preventing ambient-related degradation, we performed bias-stress tests with different biasing schemes in vacuum chamber. All measurements before and after stress tests were performed in vacuum condition and compared. First, we performed gate step stress test in the VDS = 0 state. In this test, VGS was stepped from 0V to -100V in -1V steps, and the devices were stressed for 1 minute at each bias. We monitored gate leakage current (IG) during the stress test. We found that there is a critical gate voltage which triggered sudden increase of IG. During the step stress test, IG started increasing significantly at near VG = -50V. IG of stressed OFETs was increased. The stress test with positive VGS did not increase IG. Second, OFETs were stressed under high channel current condition. VGS and VDS were simultaneously stepped from 0V to -100V in -1V steps, and the devices were stressed for 1 minute at each bias. Vertical Electric field between gate and source increased with gate bias stepping, with electric field between gate and drain maintained at zero. During the stress test, IG started increasing significantly at near VGS = VDS = -50V. IG of stressed OFETs was increased. The measurement with source and drain swapped revealed that the degradation mainly occurred at the source and gate overlap region. Third, OFETs were stressed under low channel current condition with stepping VDS from 0V to -100V in -1V steps at VGS = -10V. The devices were stressed for 1 minute at each bias. Sudden increase of IG was not observed during the stress test. Stressed OFETs did not show any symptom of significant degradation, accordingly. Vertical electric field across the gate is a dominant degradation agent compared to lateral field along the channel. Detailed analysis on the electric field-induced degradation characteristics of OFETs, with ambient effect eliminated, will be presented. Temperature dependence of field-induced degradation will also be discussed.
6:00 AM - J6.29
Threshold Voltage and Hysteresis of SAM-Modified Organic Thin-film Transistors: A Quantitative Analysis
Michael Walter Salinas Batallas 1 Abdesselam Jedaa 1 Christof M Jaeger 2 Alexander Ebel 3 Andreas Hirsch 3 Timothy Clark 2 Marcus Halik 1
1University Erlangen-Nuremberg Erlangen Germany2University Erlangen - Nuremberg Erlangen Germany3University Erlangen - Nuremberg Erlangen Germany
Show Abstract
Organic thin-film transistors (OTFTs) with hybrid dielectrics composed of thin oxide layers modified with a self-assembled monolayer (SAM) bear high potential for application in low-power integrated circuits (ICs). For a reliable operation of these integrated circuits it is crucial to have full control over the threshold voltage (VTH) of individual transistors which form the ICs.[1] Several groups have addressed this topic by showing that the turnâ?"on characteristics of transistors are strongly related to the dipole moment of the SAM-forming molecules, with the SAMs providing an additional electric field that either generates or withdraws mobile charge carriers in the semiconductor (SC) channel, depending on the direction of the dipole.[2-4] However, a quantitative correlation between VTH and dipole moment has not yet been established. We have investigated a set of functionalized n-alkane phosphonic acid molecules with different dipole moments (calculated by DFT method) that were used to form self-assembled monolayers on an aluminum oxide dielectric. We show that the shift of VTH correlates linearly with the dipole moment along the molecular axis for these transistors. This quantitative examination was validated for the semiconductors α,Ï?-dihexylsexithiophene, pentacene and C60. Our results enable a general approach to tune VTH for several organic semiconductors with dipolar monolayers and even predict VTH-values by knowledge of the z-component of the molecular dipole of the SAM molecules. A smart approach to fine-tune the threshold voltage of OTFTs is the use of Mixed-SAMs.[1] When redox active species, e.g. C60, are then introduced as one of the components, this additionally offers the possibility to realize non-volatile memory.[5] Such transistors show significant hysteresis during operation due to charging of the redox active species. The correlation between hysteresis and amount of redox active species in the SAM was investigated for Mixed-SAMs containing C60 modified phosphonic acids.[6] [1] U. Zschieschang , F. Ante , M. Schlörholz , M. Schmidt , K. Kern, and H. Klauk, Advanced Materials, 2010, 22, 4489â?"4493. [2] S. Kobayashi , T. Nishikawa, T. Takenobu, S. Mori, T. Shimoda, T. Mitani, H. Shimotani, N. Yoshimoto, S. Ogawa and Y. Iwasa, Nature Materials, 2004, 3, 317-322. [3] K. P. Pernstich, S. Haas, D. Oberhoff, C. Goldmann, D. J. Gundlach, and B. Batlogg, A. N. Rashid, G. Schitter, Journal of Applied Physics, 2004, 96(11), 6431-6438. [4] F.D. Fleischli, S. Suárez, M. Schaer and L. Zuppiroli, Langmuir, 2010, 26(18), 15044-15049. [5] M. Burkhardt, A. Jedaa, M. Novak, A. Ebel, K. Voitchovsky, F. Stellacci, A. Hirsch, and M. Halik, Advanced Materials, 2010, 22, 2525â?"2528. [6] A Jedaa, M. Salinas, C. M. Jäger, T. Clark, Alexander Ebel, Andreas Hirsch and Marcus Halik, Submitted to Applied Physics Letters.
6:00 AM - J6.3
High-performance Stable n-Type Indenofluorenedione Field-effect Transistors
Youngil Park 1 2 Joong Suk Lee 3 Beom Joon Kim 3 Beomjin Kim 2 Jaehyun Lee 2 Do Hwan Kim 4 Se-Young Oh 5 Cheng-Yu Kuo 1 Hsing-Lin Wang 1 Jeong Ho Cho 3 Jongwook Park 2
1Los Alamos National Laboratory Los Alamos USA2The Catholic University of Korea Bucheon Republic of Korea3Soongsil University Seoul Republic of Korea4Stanford University Stanford USA5Sogang University Seoul Republic of Korea
Show AbstractWe developed high-performance stable n-type organic field-effect transistors (OFETs) using indenofluorenediones with different numbers of fluorine substituents (MonoFIF-dione, DiF-IF-dione, and TriF-IF-dione). Top-contact OFETs were fabricated via the vacuum deposition of indenofluorenediones as the semiconducting channel material on polystyrenetreated SiO2/Si substrates. TriF-IF-dione FETs with Au source/drain contacts exhibited good device performances, with a field effect mobility of 0.16 cm2/(V s), an on/off current ratio of 106, and a threshold voltage of 9.2 V. We found that the electrical stability for OFETs based on indenofluorenedione improved with the number of fluorine substituents, which was attributed to higher activation energies for charge trap creation. Moreover, the TriF-IF-dione FETs yielded excellent environmental stability properties, because the LUMO energy levels were relatively low, compared with those of the MonoF-IF-dione FETs.
6:00 AM - J6.30
Low-operating Voltage and Stable Organic Field-effect Transistors with Low-k Polymer Gate Dielectrics
Ndubuisi Benjamin Ukah 1 Jimmy Granstrom 2 Suchi Guha 1
1University of Missouri, Columbia, MO 65211 Columbia USA2Georgia Institute of Technology, Atlanta, GA 30332 Atlanta USA
Show AbstractThe field of organic field-effect transistors (FETs) has seen an increasing usage in polymer dielectric materials and a progressively lower dependence on traditional oxide dielectrics. Appropriate gate dielectrics that are free of electron-trapping groups have allowed the demonstration of both p-channel and n-channel FETs. In order to achieve stable and low-operating voltage FETs, the dielectric surface should be hydrophobic and must provide a relatively high capacitance, which is often a challenge with many polymer dielectrics that have an inherently low dielectric constant (k). Our recent work demonstrates that FET characteristics significantly improve when the difference in the solubility parameters of the organic dielectric and solvent is high. [1] In this work we demonstrate low-operating voltage and stable pentacene field-effect transistors using low-k gate dielectric layers of poly (methyl methacrylate) (PMMA) and poly (4-vinyl phenol) (PVP). These devices exhibit field-effect (FET) hole motilities in the range of 0.1 â?" 1 cm2V-1s-1, threshold voltages of less than -1 V, and little off-regime current hysteresis. The smallest sub-threshold slope of 192 mV/dec and highest FET mobility of 1 cm2V-1s-1 were observed at a gate voltage of -2 V. Simultaneous capacitance-voltage and conductance-voltage measurements from accompanying metal-insulator-semiconductor diodes exhibit very low hysteresis and interface trap densities of the order of 1011 â?" to low 1012 eV-1cm-2. Solvents of varying dipole moments used to dissolve the polymer dielectrics were observed to play a significant role in improving the FET characteristics. Furthermore, the devices exhibited excellent operational and environmental stability with no shift and a small shift in threshold voltage after bias stress and days of air exposure, respectively. [1] N. B. Ukah, D. Adil, J. Granstrom, R. K. Gupta, K. Ghosh, and S. Guha, Org. Electron 12, 1580 (2011).
6:00 AM - J6.31
Solution-processed Organic Light Emitting Transistors Incorporating Conjugated Polyelectrolytes
Jung Hwa Seo 1 Andrea Gutacker 2 Ebinazar B Namdas 3 Alan J Heeger 2 Guillermo C Bazan 2
1Dong-A University Busan Republic of Korea2University of California Santa Barbara Santa Barbara USA3University of Queensland Brisbane Australia
Show AbstractImproved performance of p-type organic light emitting transistors (OLETs) is demonstrated by introducing a conjugated polyelectrolyte (CPE) layer and symmetric high work function (WF) source and drain metal electrodes. The OLET is compose of a tri-layer film consisting of a hole transporting layer, an emissive layer and a CPE layer as an electron injection layer. The thickness of the CPE layer is critical for achieving good performance and provides an important structural handle for consideration in future optimization studies. We have also demonstrated for the first time, high performance solution-processed blue OLETs. Overall, these results show that there is no need to evaporate two different metals as source and drain electrodes in order to inject holes and electrons into OLETs. With the benefits of the simplicity in device fabrication, the use of a CPE layer is beneficial for use in a variety of organic optoelectronics devices.
6:00 AM - J6.32
Well-orientated TIPS-Pentacene Crystal Growth via Thermal Gradient
Kyeiwaa Asare - Yeboah 1 Rachel Frazier 3 Dawen Li 1 2
1University of Alabama Tuscaloosa USA2University of Alabama Tuscaloosa USA3University of Alabama Tuscaloosa USA
Show AbstractIn this project, thermal gradient method was employed to produce well-orientated, high-quality 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS pentacene) crystals. TIPS pentacene, a solution processable organic semiconductor, has been widely studied as the active channel material for organic thin-film transistors (OTFTs) due to its excellent carrier transport and stability in air. In spite of all these advantages, TIPS-pentacene thin films are acutely anisotropic when they are grown from simple solution drop casting. This in turn gives rise to irregularities in OTFT device performance. The application of a thermal gradient to guide the TIPS-pentacene crystal growth alleviates the intrinsic anisotropy, allowing for a more controlled film morphology, uniformity and crystal orientation. A solution with specific concentrations of TIPS-pentacene in toluene and dimethyl formamide (DMF) was drop cast onto a substrate subjected to a thermal gradient. This leads to a difference in the solubility of the solute along the substrate and drives crystallization from the lower temperature end to the hotter end. Evaporation was controlled through the addition of a high boiling point solvent (DMF), which greatly improved the areal coverage on the substrate as well as the quality of the crystals. The resulting TIPS pentacene crystals were large with uniform morphology. To analyze the TIPS pentacene films, optical microscopy and X-ray diffraction were utilized. The successful improvement of the crystal orientation and concurrent increase in crystal coverage on the substrate, as seen from the results of the analysis, proved the efficiency of the thermal gradient approach.
6:00 AM - J6.33
High Performance of Pentacene Thin Film Transistors by the Doping of Iodine on Source/Drain Regions
Rahim Abdur 1 Young-Kyu Lee 1 Chiyoung Lee 1 Kyunghoon Jeong 1 Jaegab Lee 1
1Kookmin University Seoul Republic of Korea
Show Abstract
Organic pentacene thin film transistors have been of great interest because of their potential use as a low-cost alternative to the conventional amorphous silicon based transistors. Pentacene transistors show good carrier mobility. The OTFTs performance mainly depends on the structural properties of pentacene and the contact resistance. In addition, as the device dimensions decreases, the contact resistance as a part of the total device resistance dominates over the channel resistance, and therefore the speed of organic integrated circuits may be limited by the contact resistance, not by the intrinsic carrier mobility of the pentacene. As a result, the reduction of contact resistance is the central part to enhance the electrical performance. Iodine is an efficient acceptor for doping of pentacene, increasing the conductivity as well as hole mobility in this material. Iodine doping of pentacene can also modify the interface properties of source/drain and pentacene, thus resulting low contact resistance. We have studied on the iodine doping of pentacene film, which was confirmed by Raman spectroscopy, revealing the characteristic peaks of I3- and I5-. We also investigated the Iodine doping effect on the surface morphology, grain size, and conductivity of the pentacene films with doping time from 30 seconds to several hours using atomic force microscopy and four point probe respectively. Pentacene films grain sizes drastically increased with the doping time from an average value of 200 nm to 640 nm as a result of the facile movement of PEN+ cation radicals, generated by iodine anions. In addition, the XRD analysis showed both the bulk and thin film crystallinity increased with the low doping time while bulk peak disappears with high doping time. We also fabricated iodine doped source/drain contact pentacene TFTs, in which doping of iodine was limited to source/drain contacts using novel structures. This doping of contact area decreased the contact resistance to half of the devices without doping as well as increased the field effect mobility to 1.03 cm2/Vs, which is two times that of the devices without doping. We have fabricated all devices using octadecyltrichlorosilane (OTS) SAMs on the insulating oxide layer (100 nm SiO2). The effectively contact area iodine doped pentacene based OTFT showed mobility of 1.03 cm2/Vs with the on/off ratio of 3.86Ã-105 and threshold voltage of -2.13 V whereas the without doped pentacene based OTFT showed the mobility of 0.51 cm2/Vs with the on/off ratio of 3.55Ã-105 and threshold voltage of -4.10 V.
6:00 AM - J6.34
Control of Efficiency, Brightness, and Recombination Zone in Light-emitting Field Effect Transistors
Ben, Bang-Yu Hsu 1 Chunhui Duan 4 Ebinazar B Namdas 2 Andrea Gutacker 1 Jonathan D Yuen 1 Fei Huang 4 Yong Cao 4 Guillermo C Bazan 1 Ifor D. W. Samuel 3
1Center for Polymers and Organic Solids Santa Barbara USA2Centre for Organic Photonics and Electronics, The University of Queensland Brisbane Australia3Organic Semiconductor Centre, SUPA, School of Physics and Astronomy, University of St Andrews St Andrews United Kingdom4Institute of Polymer Optoelectronic Materials amp; Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology Guangzhou China
Show AbstractBalancing electron and hole transport lies at the heart of organic optoelectronics. It is very important in organic light emitting diodes (OLEDs), solar cells, and light emitting field effect transistors (LEFETs). LEFETs are of interest because they offer the combined advantage of an active channel from a thin film transistor (TFT) that controls the light emission from an OLED created in situ in the same device. In LEFETs, however, achieving balanced electron and hole transport and hence efficient electron-hole recombination is particularly challenging. Here we report an innovative strategy to actively and independently control the injection, transport and recombination of holes and electrons in the transistor channel by using the split-gate LEFET (SG-LEFET) architecture. A SG-LEFET is a 4-terminal device with Gate 1, Gate 2, Source, and Drain electrodes. Based on the operating voltages, (i.e. the bias polarities on Gate 1 and Gate 2), a split-gate field effect transistor (SG-FET) can be operated as a unipolar transistor, a bipolar transistor or a diode in different I-V quadrants. Because of the independent control of injection at Gate 1 and Gate 2, transport of specific carrier species can be enhanced or switched-off. Using the SG-LEFET, we have demonstrated that the largest injection current at the center of the transistor channel can be efficiently converted into light emission with maximum brightness of 609 cd/m2 at 1.27% EQE.
6:00 AM - J6.36
Mobility Enhancement of Pentacene-based Field-effect Transistors with Molecular Weight Dependent Mixture of Polystrene as Gate-dielectric
Sungwoo Lee 3 Jaehoon Park 2 Dongwook Kim 1 Woo sub Lee 1 Hyun Suk Yang 4 Jong Sun Choi 4
1Hongik University Seoul Republic of Korea2Electronics and Telecommunications Research Institute Daejeon Republic of Korea3Hongik University Seoul Republic of Korea4Hongik University Seoul Republic of Korea
Show AbstractEnhanced hole mobility in pentacene-based field-effect transistor is investigated experimentally as a function of composition of polystyrene mixture. First, we present measurements of the electrical performance with monotonously increased molecular weight of polystyrene as gate dielectric including the following: the discovery of high performance with higher molecular weight as gate dielectric, the morphological image of polystyrene surface, XRD diffraction of pentacene grain on the polystyrene, adhesiveness on glass substrate and aluminum electrode, and simple electrical characteristics such as capacitance, current density. Next, variable composition of polystyrene film fabricated that low (Mw 35,000) and high (Mw 980,000) molecular weights in the polystyrene film was mixed in the proportion range of 10:1 - 1:10, and largest mobility of 0.73 cm2/V-s is observed with composition of 1:10, which is higher when used single component of molecular weight. Furthermore, a mobility behavior is proposed based on the proportion of molecular weight materials to estimate the highest performance of fabricated devices. We attributed higher performance of device with less trap site in the vicinity of the polystyrene surface to the strongly conjugated polystyrene chains and enclosed packing by relatively short length of the polymer which of small molecular weight of polystyrene. Further details will be discussed.
6:00 AM - J6.38
Mobility Enhancement by Controlling Grain Boundary Direction of TIPS-pentacene TFTs
Seung-Hyeon Jeong 1 Chung-Kun Song 1
1Dong-A University Busan Republic of Korea
Show AbstractIn this paper, we figured out that the performance of OTFTs using TIPS-pentacene strongly depended on the grain boundary direction relative to the current flow between source and drain electrode. The OTFTs with the grain boundary direction parallel to the current flow exhibited the mobility (0.44±0.08 cm^2/ V.s) 2 times larger than those with the grain boundary direction angled to the current flow (0.21±0.03 cm^2/ V.s). It is meaning that the grain boundary seriously affected the carrier transport. We also found that when the center area of crystal, where the different grains met together, located in the channel, the mobility was seriously degraded to 0.09 cm^2/ V.s. Therefore, it was important to make the grain boundary direction parallel to the current flow and make the center area of crystal be out of channel for high performance. * This research was supported by a grant(F0004021-2011-34) from Information Display R&D Center, one of the Knowledge Economy Frontier R&D Program funded by the Ministry of Knowledge Economy of Korean government.
6:00 AM - J6.39
Effect of n-type Organic Semiconductor/Dielectric Interface on Its Stability: Bias Stress and Photo-electric Characteristic
Ji Hoon Park 1 Seongil Im 1
1Yonsei Univ. Seoul Republic of Korea
Show Abstract
In general, most of the organic thin-film transistors (OTFTs) operate in p-type accumulation mode, meaning that the semiconducting layer play a role as hole-transporting material. However, for development and application in many areas, such as CMOS-like logic element, not only high-performing but stable electron-transporting (n-type) materials are required as well. Recently n-type small molecule material, N,Nâ?T-ditridecyl-3,4,9,10- perylenetetracarboxylic diimide (PTCDI-C13), has attracted attentions for last few years, due to its air-stability better than C60-based device , high-performance comparable to p-type organic materials. For real-world application, it is someoneâ?Ts mission to research on device reliability and in-depth understanding of instability under ambient atmosphere. We fabricated three-types of PTCDI-C13 based OTFTs employing pristine, hexamethyldisilazane (HMDS)-treated, and fluoropolymer CYTOPTM -treated Al2O3+x dielectric operating at 7 V. We report on the behavior of hysteresis evolution (in transfer curve) in above-mentioned three types of TFTs. In case of pristine and HMDS-treated TFT, the hysteresis reduces exponentially (from 3.5 V to 0.02 V), meanwhile, CYTOP-interfacial TFT exhibits almost remained identical hysteresis (~0.07 V) consecutive measurement. We also discuss the evolution of threshold voltage (Vth) of the samples undergone positive or negative gate bias stress along with time-dependent analysis of sub-threshold swing. Under positive gate bias stress, in case of pristine and HMDS-treated TFT reveals that Vth shift occurs rapidly in early phase (500 s), meanwhile, for CYTOP-interfacial TFT, the evolution of Vth is described by the stretched exponential equation. These results lead us to conclude that semiconductor/dielectric interface difference leads to different behavior in stability (hysteresis, Vth shift). In order to understand in depth the trap behavior and its distribution in our TFTs, we performed bias stress test along with photo-excited charge collection spectroscopy (PECCS) measurement to elucidate interfacial trap charge density of state (DOS).
6:00 AM - J6.4
Pentacene Transistor and Photo-gating Inverter with Ambient-protecting Organic Light Transducer 6,13-pentacenequinone
Hee Sung Lee 1 Seongil Im 1
1Yonsei Univ. Seoul Republic of Korea
Show Abstract
Due to the distinct advantages and functionalities, organic devices on glass or flexible substrates have extensively been studied in such basic forms as thin-film transistors (TFTs) and light emitting diodes (LEDs). Organic pentacene TFTs are the most representative among many other organic devices. Since the pentacene-based and other organic TFTs have bottom gate device architecture opening their back channel surface, ambient protecting or passivation layers on the organic devices are necessary. Such passivation layers have thus been studied to keep those devices from damages or degradation under various ambiences involved with air, humidity, and ultraviolet light (UV). The adopted protection layers were inorganic oxide or polymers in general, however small molecule crystalline layer has rarely been chosen. Here, we chose a small molecule crystalline layer comprised of 6,13 pentacenequinone (PQ: oxidized pentacene) with the following two reasons. First of all, crystalline PQ/crystalline pentacene interface is quite analogous to that of SiO2/Si structure where SiO2 plays as natural protection layer over Si devices making a natural interface with Si. Because the hydrophobic PQ layer is an already-oxidized pentacene layer, diffusion of water and oxygen molecules into channel pentacene may not be easy when the PQ layer is well grown on pentacene making a good PQ/pentacene interface. Next, the PQ protects the pentacene from even high intensity UV light, since it converts or transduces the UV into green light as known. In the present work, we have fabricated a pentacene-based complementary photo-gating inverter devices to demonstrate the effects of the PQ layer, which was deposited on top of our pentacene channel. Our complementary inverter has a form of vertically-stacked TFT devices: bottom gated p-channel pentacene TFT on top-gated n-channel GaSnZn oxide (GTZO)-TFT with atomic layer deposited (ALD) Al2O3 and Al as common dielectric and gate. Our inverters with the crystalline-grown PQ on pentacene channel demonstrated strong ambient-endurance preserving good inverter characteristics for 60 days under quite a humid air ambience, and they also demonstrated dynamic photo-gating at a low voltage of 5 V without any photo-induced degradation.
6:00 AM - J6.40
Dielectric Properties of Single-walled Carbon Nanotube/Poly(dimethylsiloxane) Nanocomposites Gelated with Ionic Liquids
Kiwon Oh 1 2 Sang-Soo Lee 1 Daeheum Kim 2 Heesuk Kim 1
1KIST Seoul Republic of Korea2Kwangwoon University Seoul Republic of Korea
Show AbstractPolymer-based nanocomposites filled with inorganic fillers have been intensively studied due to their excellent mechanical, electrical, thermal and dielectric properties. Especially, the dielectric properties of polymer/inorganic nanocomposites have been examined for not only embedded-capacitor technology but also electroactive polymers (EAPs) that respond to the external electric field by changing their shape. Recently, a considerable amount of studies have focused on reducing the operating voltages of dielectric elastomer actuators (DEAs) in order to enhance their commercial feasibility and remove the dangers associated with high voltage. The basic method for reducing the operating voltage is to increase the dielectric constant of the elastomer films. A number of approaches have been explored for increasing the dielectric constant of elastomers introducing high-k ceramic fillers or conductive fillers such as metal nanoparticles and nanocarbons. In this study, we fabricated a dielectric elastomer films using single-walled carbon nanotubes (SWNTs) as a dielectric filler and polydimethylsiloxane (PDMS) as a polymer matrix. To achieve a high dielectric constant and low loss factor of SWNTs/PDMS nanocomposites, the SWNTs were highly exfoliated by using an ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide. We demonstrate that SWNTs/PDMS nanocomposites with ionic liquids show the higher dielectric constant and lower loss factor than nanocomposites without ionic liquids. We also characterize the dispersity of SWNTs in the nanocomposites by transmission electron microscopy (TEM) and scanning electron microscopy (SEM).
6:00 AM - J6.41
Conductive Single-molecular Wires
Lei Fang 1 Steve Park 1 Benjamin R Sveinbjornsson 2 Robert H Grubbs 2 Zhenan Bao 1
1Stanford University Stanford USA2California Institute of Technology Pasadena USA
Show AbstractSingle molecules provide ideal systems for the investigation of charge transport on the molecular scale, which is of great importance for both practical applications and fundamental understanding of molecular electronics. For example, in electronic devices based on organic semiconducting materials, intramolecular electron transport over distance of up to 100 nm between intermolecular transfers could significantly increase bulk charge mobilities. In order to study intramolecular charge transport on the length scale of tens to hundreds of nanometers, we designed a high molecular weight conductive polymer as a model. This polymer should (1) have a Contour length higher than 100 nm so that a single polymer chain is long enough to be placed between two electrodes for the investigation; (2) be rigid enough so that a single molecule does not fold to itself on the surface and (3) have a relatively large feature height when deposited onto surface so that it can be easily detected by atomic force microscopy. In order to achieve these goals, ruthenium catalyzed ring opening metathesis polymerization of norbornene units was chosen for the synthesis of such high polymer, because of the feasible and robust nature of this reaction. Polystyrene was employed as the solublizing group in the monomer on account of its compatibility to C60 unit and ease of synthesis. C60 unit was attached to the backbone as methanofullerene by mean of Bingel reaction. The ring opening metathesis polymerization reactions were carried out at a monomer/catalyst ration of >800/1 to give C60-containing polymers with molecular weights larger than one million. With these polymers in hand, we investigated the polymer chain distribution on surfaces using atomic force microscopy. Thin-film and single polymer chain field effect transistor devices, based on these polymers, have also been fabricated and investigated.
6:00 AM - J6.42
Stretchable Transparent Silver Nanowire-polymer Composite Electrodes
Weili Hu 1 2 Xiaofan Niu 1 Lu Li 1 Sungryul Yun 1 Zhibin Yu 1 Qibing Pei 1
1UCLA Los Angeles USA2Donghua University Shanghai China
Show AbstractElectrically conductive materials capable of substantial elastic stretching and bending are required in foldable displays, robotic skins, stretchable solar cells, implantable devices, and dielectric elastomer actuators. A wide variety of conductive polymer composites have been studied based on carbon nanotubes and grapheme, however, high stretchability and high conductivity cannot be simultaneously obtained. Here we present highly conductive, stretchable, and transparent hybrid composites comprised of silver nanowire (AgNW) network embedded in polymer matrix. The interpenetrating networks of AgNWs and the polymer matrix in the surface layer of the composites lead to high surface conductivity, high transparency and high mechanical compliance. A composite with 7.5 Ω/sq sheet resistance and 80% transmittance only experiences a 8 times increase in sheet resistance when it is stretched to 80% strain. Composites with higher conductivity exhibit better stretchability. Results of morphological studies will be presented to expound the conductivity changes during reversible stretching and relaxation.
6:00 AM - J6.43
UV Degradation of PMMA Components in Solar and Display Technologies
Minkyu Kim 1 Fernando D Novoa 1 Reinhold H Dauskardt 1
1Stanford University Stanford USA
Show AbstractPoly(methyl methacrylate) (PMMA) is ubiquitously used in optics and solar technologies, for example, as condenser lenses due to its high optical and UV transmission and is thought to have superior resistance against degradation. PMMA is also an attractive candidate as a polymer layer for ultra barrier coating systems for solar cells and display technologies. In these applications, PMMA layer or lens components may be exposed to high-fluence solar UV radiation. Little is known, however, about the coupled effects of mechanical stress, moisture, thermal cycling and particularly UV light on the degradation of PMMA in such environments and the role of environmental parameters on accelerating the growth of defects. Understanding those effects is critical for solar technologies that use PMMA components. In the present study, we measured the cohesive fracture energy of PMMA as well as investigated the adhesion of a PMMA layer with adjacent materials including oxides and organics. Bulk PMMA interfaces were fabricated by thermal bonding PMMA substrates at the PMMA glass transition temperature. The cohesive fracture energy was characterized and related to processing condition and the testing environment. In addition, samples were prepared by spin-coating a PMMA precursor solution onto a quartz substrate. Two substrates were subsequently bonded together to form a PMMA to quartz interface that is characteristic of solar application. For both samples, experiments to quantify the UV radiation effect on PMMA were conducted in an environmental chamber. This included both pre and in-situ UV exposure where the effect of UV light on significantly degrading cohesion and adhesion properties was quantified. Implication for the reliable use of PMMA in solar applications is discussed.
6:00 AM - J6.44
The Enhancement of Magnetoconductance Responses by Both Illumination and Induced Charge Transfer Complex States in Pentacene-based Diodes
Wei Shun Huang 1 Tzung Fang Guo 1 2
1National Cheng Kung University Tainan Taiwan2National Cheng Kung University Tainan Taiwan
Show AbstractIn this article, we investigate the magnetoconductance (MC) responses under specific conditions which including photocurrent, unipolar injection, and bipolar injection in pentacene-based diodes. Both photocurrent and bipolar injection contributed MC responses are resulted from triplet-polaron interaction (TPI). At unipolar injection regime the MC response is resulted from the charge transport through the bipolaron states. These results suggest that both single carrier and electron-hole pair contributed MC responses could be observed in pentacene-based devices. In addition, by blending 2,3,5,6-tetrafluoro-7,7,8,8 -tetracyanoquinodimethane (F4-TCNQ) into pentacene, the MC response at unipolar injection regime would be enhanced owing to the induced charge transfer complex states (CT complex states).
6:00 AM - J6.45
Various Fullerene - A2B6 Bulk Heterojunctions: Comparative Study of Electrophysical Properties
Viktor Ziminov 2 1 Irina B Zakharova 2 Andrey N Aleshin 1 Tatiana L Makarova 3 1
1Ioffe Physico- Technical Institute St. Petersburg Russian Federation2St. Petersburg State Polytechnic University St. Petersburg Russian Federation3Umea University Umea Sweden
Show AbstractResearch of the new composite nanomaterials and structures for electronics and photoelectronics based on organic acceptors and inorganic donors is relevant and important in case of the technical applications, as well as in terms of new knowledge in the field of organic semiconductors. Moreover, inorganic semiconductors have been investigated in search of a significant improvement of the lifetime for the organic devices [1, 2]. The properties of bulk heterostructures obtained by the different methods were investigated: 1. By the vacuum evaporation in quasi-equilibrium conditions 2. By the vacuum evaporation in non equilibrium conditions 3. By spin- coating method This types of samples have different structures of the composite film. In the first case polycrystalline films with about 300nm crystals were obtained. The samples obtained by the second method also shows a polycrystalline structure but with about 30nm crystals, and the third method of deposition leads to amorphous films with an inclusion of inorganic particles. The structures with composition uniformity through the thickness and with composition gradient through the thickness with a C60 - C60(1-x)CdS(x) â?" CdS structure were investigated. Morphology, structure and IVCs measured in the 'sandwich' geometry were analyzed. Structures with the composition gradient have IVCs with a rectification ratio up to 3 orders of magnitude. Raman spectroscopy of the C60- CdS structures was investigated, and the fullerene polymerization was shown, which is due to a charge transfer from the donor (CdS) to the acceptor component (C60) of the bulk heterojunction. References: 1. A. Briseno, T. Holcombe, A. Boukai et al., Oligo- and Polythiophene/ZnO Hybrid Nanowire Solar Cells, J. Nano Letters, Vol 10 (2010) p. 334-340 2. A. Takeda, T. Oku, A. Suzuki, K. Kikuchi, S.Kikuchi, Fabrication and characterization of inorganic-organic hybrid solar cells based on CuInS2, Journal of the Ceramic Society of Japan, Vol. 117 (2009) p.967-969
6:00 AM - J6.46
Synthesis and Characterization of Water-soluble Polyfluorene Derivatives for Organic Resistive Memory Devices
Seunghoon Lee 1 Tae-Wook Kim 2 Seung-Hwan Oh 3 Rira Kang 4 5 Juhwan Kim 4 Dong-Yu Kim 4 5 6
1Gwangju Institute of Science and Technology Gwangju Republic of Korea2Korea Institute of Science and Technology Jeollabuk-do Republic of Korea3Korea Atomic Energy Research Institute (KAERI) Jeollabuk-do Republic of Korea4Gwangju Institute of Science and Technology Gwangju Republic of Korea5Gwangju Institute of Science and Technology Gwangju Republic of Korea6Gwangju Institute of Science and Technology Gwangju Republic of Korea
Show AbstractAs information technology (IT) has been made scientific progress, organic memory has attracted great attention due to potential application for next generation information storage. There are many suggested switching mechanisms in organic memory, e.g. conformational change, charge transfer, space charge limited current (SCLC), and filamentary conduction. However, the switching mechanisms are still controversial issue. For this reason, exact understanding of switching mechanism is important for further improving memory performances. Especially, the investigation of relationships between chemical structure of the polymer and switching performances is required. Thus, the systemic studies by changing side chains of the polymer will contribute to verifying the performance-structure relationships in polyfluorene based organic memory devices. In our previous work, non-volatile memory devices fabricated using single component water-soluble polyfluorene-derivative showed good performance characteristics such as large on/off ratio, a long retention time, and thermally stable switching behavior. As the reasons for these good performances, we speculated that the SCLC with filamentary conduction contributed to the switching behavior of the memory device; in addition, side groups of polyfluorene derivatives also played crucial roles in device performances. Based on this motivation, we synthesized four kinds of polyfluorene derivatives incorporating different side chains and investigated organic resistor-type memory devices.
6:00 AM - J6.47
Determination of the Density of States on N-type Ptcdi-c13 Organic Thin-film Semiconductor
Joaquim Puigdollers 1 Albert Marsal 1 Sergi Galindo 1 Paz Carreras 2 Cristobal Voz 1 Joan Bertomeu 2 Ramon Alcubilla 1
1Universitat Politecnica Catalunya Barcelona Spain2Universitat de Barcelona BARCELONA Spain
Show Abstract
The optoelectronic properties of organic devices depend largely on the density of states (DOS) in the band gap of the active semiconductor layers. The DOS distribution determines the electrical transport, photosensitivity, doping efficiency and, at the end, impairs the device performance. The study of defects with states within the band gap is of great interest, since they act as traps and recombination centres with a strong influence in the electrical behaviour of the fabricated devices. Experimental information on the distribution of defects cannot be obtained in a straightforward way. Different techniques have been used for extracting information about the energy distribution of the DOS in thin-film semiconductors. Some of them are based on thermal emission measurements involving, in most cases, the effects of trapped charge in electronic states. On the other hand, the DOS can be also studied by measuring the weak subgap optical absorption related to electronic transitions involving defect states . In this paper we study the density of states in n-type N,Nâ?T-ditridecylperylene-3,4,9,10-tetracarboxylic diimide (PTCDI-C13) organic semiconductor using two different methods. The first one is based on the temperature dependence of the channel conductance in field-effect transistors. The second one is based on the subgap optical absorption coefficient measured using the Photothermal Deflection Spectroscopy technique. PTCDI-C13 thin-film transistors were fabricated on n-type crystalline silicon wafers with a 150 nm thick thermally grown silicon dioxide. A field-effect mobility of 0.12 cm2/Vs and a threshold voltage of 46 V were calculated. The transfer current-voltage characteristics was measured over a range of temperature (30 â?" 90°C) and the activation energy (EA) of the channel conductance was calculated as a function of the gate-source bias (Vgs). The variation of the activation energy of the conductance channel could be described in terms of a standard semiconductor model. The dependence of EA on Vgs corresponds to a gradual shift of the Fermi energy level (EF) toward the conduction band as more empty traps become filled due to the injected electrons from contacts. This dependence allows us to determine the DOS of the organic semiconductor. The density of localized states was around 1018 cm-3 eV-1 in the midgap and increases one order of magnitude towards the conduction band. DOS distribution could be described by the sum of two exponential contributions. We attribute the exponential regions in the DOS distribution to the presence of deep and band tail states. In addition, PDS measurements were performed to get information about the spectral distribution of the subgap optical absorption. PDS is a very sensitive technique that can measure very weak optical absorptions involving sub gap electronic transitions. In the case of PTCDI-C13, PDS measurements also revealed tail and deep distributions of localized states.
6:00 AM - J6.48
Synthesis of Organic-inorganic Hybrid Materials for Optical Waveguide Applications
Yong Ku Kwon 1 Jin Young Han 1 Mingyun Suk 1 Jina Chon 1 Kwon Bin Song 1 Kwang Hee Lee 1
1Inha University Incheon Republic of Korea
Show AbstractA series of novel organic-inorganic hybrid materials by using diol and alkoxysilane were through a non-hydrolytic sol-gel reaction. The diol compounds were reacted with inorganic precursors in the presence of a catalyst of barium hydroxide monohydrate. The amount of diol compounds was changed to 30 mol% and 60 mol% and that of catalyst was fixed at 0.1 mol% of the total silane compound. The mixed solution was transferred into a vacuum to remove the alcohol, a byproduct of the condensation reaction. The thin films of the organic-inorganic hybrids were obtained from spin-coating on a Si wafer. The spin-coated films were exposed to a UV lamp for crosslinking and annealed for additional hours at high temperature. The films exhibited excellent adhesion with the substrate. The surfaces of the films were flat and smooth. The films contained no crack and displayed uniform thickness. The film thickness increased with the relative amount of diol compounds. UV embossing was used in the fabrication of various optical waveguides and devices of diverse patterns with a PDMS stamp.
6:00 AM - J6.49
Simple Patterning Process of the Conducting Polymer Such as PEDOT:PSS for Organic Thin-film Transistors
Yunseok Jang 1
1Korea Institute of Machinery amp; Materials Daejeon Republic of Korea
Show AbstractIn this presentation, we will demonstrate a simple patterning method for the deposition of the conducting polymer such as poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) by using the difference in wettability between hydrophobic surfaces and hydrophilic surfaces on the substrate. And we will also demonstrate the influence of inner flow (such as convection flow and Marangoni flow) in the PEDOT/PSS droplet to achieve homogenous conducting polymer films. Finally, we will confirm the potential of our patterning method for polymer electrodes such as the PEDOT/PSS by fabricating pentacene thin-film transistors (TFTs) and measuring the electrical properties of the pentacene TFTs. **This study was supported by a grant (B551179-08-03-00/ B551179-10-01-00/ NK162D/ NK162H) from the cooperative R&D Program funded by the Korea Research Council Industrial Science and Technology, Republic of Korea.
6:00 AM - J6.50
Field Effects in Langmuir-Blodgett Networks of Single-walled Carbon Nanotubes
Mark Kieran Massey 1 Christopher Pearson 1 Mark C Rosamond 1 Dagou A Zeze 1 Mike C Petty 1
1Durham University Durham United Kingdom
Show AbstractThin films of carbon nanotubes are generating enormous interest as a potential new material for use in modern electronics [1]. Applications include thin film conductors and as the active layer in field effect transistors. Nanotube films can be deposited by direct growth methods and other high temperature techniques. However, solution-based processing methods are favoured as these lend themselves to coating a wide range of flexible substrates. This research builds on previous work using the Langmuir-Blodgett (LB) deposition technique as a method of forming thin networks of carbon nanotubes, in particular single-walled carbon nanotubes (SWCNTs). This technique allows precise control over the thickness of deposited networks. Furthermore, it is undertaken at ambient temperatures so is ideal for deposition onto flexible substrates. Generally, SWCNTs are a mixture of both semiconducting and metallic types. Both have been found to exhibit condensed isotherms at the air/water interface, and both can be transferred to solid supports using the LB technique and show anisotropy in the electrical conductivity, suggesting alignment during transfer [2]. This work now focuses on sorted semiconducting and metallic single-walled carbon nanotubes for improved and more predictable electrical properties. Electrical measurements have been taken for both types of nanotube network on a selection of electrode dimensions ranging from sub micron (~ 130 nm) to 1 mm separations and with several different metals including palladium, gold, silver and aluminium. The effect of temperature on the conductivity has been investigated between 77 K â?" 300 K and various electrical conductivity models have been used to fit to the resulting current-temperature data. Field effect transistor architectures have been built up on sub-micron and larger (50 micron) channel lengths using semiconducting nanotubes. Field effects have been observed in some of these devices and will be reported on in more detail. [1] L. Hu, D. S. Hecht, G. Grüner, Chem. Rev., 110(10), 2010, 5790-5844. [2] M. K. Massey, C. Pearson, D. A. Zeze, B. G. Mendis, M. C. Petty, Carbon, 49, 2011, 2424-2430.
6:00 AM - J6.51
Optical and Electrical Writing and Reading of a Multi-level Light-emitting Organic Memory Device
R. Clayton Shallcross 1 Peter O Koerner 1 Eduard Maibach 1 Philipp Zacharias 1 Anne Koehnen 1 Klaus Meerholz 1
1University of Cologne Cologne Germany
Show AbstractSolution-processable molecular switches are an intriguing option for implementation in nonvolatile memory elements due to the possibility for inexpensive fabrication over large areas on flexible substrates. Of the available molecular candidates, dithienylethenes (DTEs) are rivaled by few others in regard to their thermal stability (i.e. nonvolatile) and reversibility (re-writable). DTEs represent a class of photochromic molecular switches that undergo a change in both their UV-visible absorption and energy level position due to a photo- and/or electrically-induced ring-opening/-closing reaction. Here, we present a light-emitting organic memory (LE-OMEM) device platform employing a DTE transduction layer that offers the ability for both optical and electrical writing and readout. The memory effect in these diodes results from the significant difference in the highest occupied molecular orbital (HOMO) between the open and closed isomer of the DTE layer, which subsequently changes the hole injection barrier in the diode. Optimized LE-OMEM devices show impressive ON/OFF ratios (OORs) for both electroluminescence (EL) and current readout of ca. 10000 for optical switching and ca. 1000 for electrical switching. Both optical and electrical writing protocols demonstrate the ability to write an extraordinary number of grey levels (greater than 20 levels), which is promising for higher storage density memory. We elucidate the difference in the molecular-scale mechanisms that are responsible for the optically- and electrically-driven memory effect in these devices by in-situ monitoring of the fraction of closed molecules as a function of the external stimulus. The overall design and measurement principles discussed here provide a road map for the rational optimization of similar devices incorporating a range of photochromic compounds that may be used in nonvolatile memory devices.
6:00 AM - J6.52
Design and Synthesis of Thienylenevinylene-based Conjugated Polymers for High Performance Organic Electronic Devices
Juhwan Kim 1 Dongyoon Khim 1 Kang-Jun Baeg 3 Bogyu Lim 4 Seunghoon Lee 6 Yong-Young Noh 5 Dong-Yu Kim 1 2 6
1Gwangju Institute of Science and Technology Gwangju Republic of Korea2Gwangju Institute of Science and Technology Gwangju Republic of Korea3Electronics and Telecommunications Research Institute Daejeon Republic of Korea4Stanford University Stanford USA5Hanbat National University Daejeon Republic of Korea6Gwangju Institute of Science and Technology Gwangju Republic of Korea
Show Abstract
In recent years, conjugated polymers are emerging candidates for the construction of the active layers of organic thin film transistors (OTFTs) and organic photovoltaics (OPVs) because of comparable or even higher mobility with benchmark performance of amorphous silicon and their various flexible, large-area electronic applications through cost-effective graphic arts printing techniques. This class of conjugated polymer is considered on the basic of following design rules to obtain high performance. i) structural good planar Ï?-conjugated unit such as fused aromatic units for efficient electron and hole transport Ï?-electron delocalization and solid-state packing though favor intermolecular Ï?-Ï? stacking, ii) structural regularity, iii) alkyl side chains for solubility and processibility without steric hindrance of backbone, iv) decrease of lowest unoccupied molecular orbital (LUMO) energies by adopting an electron deficient unit or an electron-withdrawing substituent for achieving electron transport and air-stability. In general, newly synthesized conjugated polymers by aforementioned design rules are made with fused aromatic units. However, these fused aromatic units have some limitations such as poor solubility by rigid structure and complex synthesis. Therefore, the appearance of core unit which is different from fused aromatic unit is required to solve these limitations. In this presentation, we introduce thienylenevinylene unit for replacement of fused aromatic groups. Thienylenevinylene (TV) based materials can show a high charge-carrier mobility as a result of a high planarity by introduction of vinylene with double bond character between thiophene units and a large intermolecular attractive force by the thiophene. Therefore, the introduction of a vinylene moiety between the thiophene units resulting in a TV unit is an attractive building block for the active layer of OTFTs and OPVs. Previously mentioned design rules were selectively applied to TV-based conjugated polymers, and we synthesized two types of conjugated polymers which are predominant hole transport polymers (maximum hole mobility is exceeding 1 cm2/Vs) and ambipolar transport polymers (maximum hole and electron mobility are 0.75 and 0.06 cm2/Vs, respectively). These polymers were synthesized via Grignard, Kumada, and Stille coupling reactions. The basic material characterizations using NMR, DSC, TGA and GPC were investigated. In addition, optical and structural properties were analyzed using optical spectroscopy tools such as UV/Vis spectroscopy and X-ray diffraction. In addition, we investigated a significant morphological change by annealing process.
6:00 AM - J6.53
A Simplified Materials Approach toward RGB-sensitive Light-emitting Organic Memory
Eduard Maibach 1 Vincent Aubert 1 Peter O Koerner 1 Richard C Shallcross 1 Philipp Zacharias 1 Klaus Meerholz 1
1Institut fuuml;r Physikalische Chemie Kouml;ln Germany
Show Abstract
One of the biggest advantages of organic electronics is their solution-processability, which allows for easy and inexpensive manufacturing of devices (e.g. organic memory, organic light-emitting diodes, organic photovoltaics, organic field-effect transistors, etc.) over large areas and even on temperature-sensitive/flexible substrates. Optimized device layouts commonly employ multi-layer stacks, which requires either the use of orthogonal solvents for alternating layers or crosslinkable precursors in order to prevent dissolution of the previous layer. Here, we describe multi-layer solution-processed light-emitting organic memory (LE-OMEM) devices that employ crosslikable dithienylethene (DTE) molecules, which are intriguing for switch and memory applications due to their extraordinarily high thermal and fatigue stability. This class of photochromic molecules can be triggered optically (and/or electrically) between an open and closed form by irradiation with different wavelengths. The change in the highest occupied molecular orbital (HOMO) of the DTE layer upon isomerization corresponds to a change in charge injection barrier for holes, which is responsible for the modulation of the electroluminescence (EL) and current of our devices. LE-OMEM devices utilizing these DTE molecules have shown impressive ON/OFF ratios of 103-104 for both EL and current. We focus on the implementation of a variety of molecular switching molecules as transduction layers in LE-OMEM devices. An assortment of unique molecules is made possible by a simplified materials scheme where the absorption/transmission window (from blue to red color) and frontier orbital energies of the transduction layer can be systematically and predictably modified by variation of the chromophores attached to a central/crosslinkable DTE core. This enormous simplification (i.e. modification of a central crosslinkable core) makes this materials approach a convenient way to screen a variety of materials in solution-processable LE-OMEM devices.
6:00 AM - J6.54
Effects of Interfaces on Microstructure and Charge Transport in High Performance Polymer Semiconductors
Scott A Himmelberger 1 Jonathan Rivnay 1 Leslie H Jimison 1 Duc Duong 1 Michael F Toney 2 Alberto Salleo 1
1Stanford University Stanford USA2Stanford Linear Accelerator Center Stanford USA
Show Abstract
Semiconducting polymers have garnered significant interest for use in large-area, low-cost, and flexible electronic applications in which the thickness of the active semiconducting layer is on the order of tens to hundreds of nanometers. As polymer films approach these small dimensions, polymer-interface interactions become more significant and the film properties may begin to deviate significantly from those of the bulk. Therefore, as semiconducting polymers continue to increase in commercial significance, it becomes imperative to have a good understanding of how the material properties change in the confined geometry of ultrathin films. We study the effects of interfaces on microstructure and charge transport in thin films of the high performance, semicrystalline polymer, poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT), by systematically varying film thickness. We find that the interfaces disrupt the formation of crystallites but improve film texture and that the interplay of these two effects regulates the charge carrier mobility. Using x-ray diffraction (XRD) we quantify overall crystallinity and crystalline texture within the film and show that these vary significantly with film thickness and thermal processing. Crystallinity increases by an order of magnitude from the thinnest to thickest films and by a factor of 2-3 upon thermal annealing. Field effect transistor (FET) measurements are used to characterize the electrical properties of these films and charge carrier mobility was found to vary with film thickness, maximizing at values of 0.13 cm2 V-1 s-1 and 0.22 cm2 V-1 s-1 for as cast and annealed films respectively. Changes in device performance can be explained by the thickness induced variations in film morphology. We demonstrate how the observed trends in charge carrier mobility are controlled by overall crystallinity in the thinnest films and dominated by the effects of crystalline texture as the film is made thicker.
6:00 AM - J6.55
Increasing the Light Emission from TES-anthracene/Polymer Blends for Laser Applications
Kittiyaporn Singsumphan 1 John J Morrison 2 Stephen G Yeates 2 Maxim Shkunov 3 Joseph L Keddie 1
1University of Surrey Guildford United Kingdom2University of Manchester Manchester United Kingdom3University of Surrey Guildford United Kingdom
Show AbstractRecently, many high performance organic electronic devices, especially transistors, have been fabricated from blends of polymers with organic small molecules, such as acenes. The incorporation of acene within polymers has generated interest as a means to combine the functional properties of the small molecules with the processibility and stability of polymers. Whereas charge transport is the key requirement for transistors, this current research considers 9,10-bis(triethylsilylethynyl) anthracene (TES-anthracene)/polymer blends as candidate materials for lasers. Specifically, the research correlates the photoluminescence (PL) and quantum efficiency (QE) of the blends with the TES-anthracene crystal structure as it is influenced by important factors: the amount of TES-anthracene, the viscosity of the polymer, and film processing method. TES-anthracene and poly(styrene) were blended in a mutual organic solvent, 1,2,3,4-tetrahydronaphthalene (tetralin). Films were deposited by drop-casting (solvent-processed) or heated and cooled at controlled rates and times (melt-processed). The maximum temperature was above the polymerâ?Ts glass transition temperature and the aceneâ?Ts melting point. The blend films show a sharp absorption edge and exhibit high intensity PL peaks in the blue wavelength region. Notably, the materialâ?Ts QE is comparable to that of F8BT, a highly luminescent polymer. Differential scanning calorimetry revealed that acene crystallisation occurred in the solvent-processed films. The blend films with higher polymer viscosity re-crystallised when cooled from the melted state. On the other hand, films with lower polymer viscosity do not re-crystallise during cooling. However, re-crystallisation occurs slowly over 24 hours at room temperature. It was discovered that the polymer viscosity and the thermal processing conditions both have a pronounced impact on the TES-anthracene crystalline structure and the light emission of TES-anthracene/poly(styrene) blends. Melt-processing of blends with higher molecular weight (high viscosity) polymers results in needle-like acene crystals, and the QE is approximately the same value as in solvent-processed films. However, melt-processing of blends with lower molecular weight polymers results in a higher QE than in solvent-cast films. Melt-processing leads to more densely-packed TES-anthracene crystals, as was determined with emission microscopy. There is a strong correlation between the blendâ?Ts crystalline structure and the QE. The highest QE (ca. 20%) was obtained when the TES-anthracene was blended with a lower molecular weight polymer, and the resulting film was melt-processed. Thus, by increasing the light emission from the novel organic materials through control of processing and crystal morphology, our work is an important first step in the development of a new candidate for lasing applications.
6:00 AM - J6.56
Differences in the Electronic Structure of Highly-oriented Films of H2-, Fe-, Co-, and Cu-Phthalocyanines Revealed by NEXAFS Spectroscopy
T. M Willey 1 Michael Bagge-Hansen 1 J. R Lee 1 R. Call 1 2 L. Landt 1 3 T. van Buuren 1 C. Colesniuc 4 C. M Monton 4 I. Schuller 4
1Lawrence Livermore National Laboratory Livermore USA2Utah State University Logan USA3Technische Universitauml;t Berlin Berlin Germany4University of California, San Diego La Jolla USA
Show Abstract
Phthalocyanines are extensively studied as molecular semiconductor materials for chemical sensors, dye-sensitized solar cells, and other applications. Pthalocyanines offer high tunability through the choice of metal center atom; nearly all transition metals and many other heavier elements can reside at the relatively stable square planar center of the phthalocyanine. H2-, Fe-, Co-, and Cu-phthalocyanine molecules in films deposited on gold substrates show prostrate orientation, as opposed sapphire substrates, where phthalocyanines stand in a more upright conformation under deposition conditions used. Angular dependence in Near-Edge X-ray Absorption Fine Structure (NEXAFS) resonances, commonly attributable to pi* and sigma* resonances, in both carbon and nitrogen K-edges, quantify the orientational order. H2-phthalocyanine shows the cleanest angular dependence, with nearly no intensity in the pi* regime with normal beam incidence. Metal L-edges in prostrate films, on the other hand, have dramatic variation in angular dependence of resonances into empty states, particularly Cu-phthalocyanine. The combination of NEXAFS and StoBe DFT is used to explain and illuminate this unique electronic structure of Cu- compared to H2-, Fe-, and Co-phthalocyanines.
6:00 AM - J6.57
Organic Memory Device Using Aptamer-cojuated Au Nanoparticle Layer
SeWook Oh 1 Hunsang Jung 1 Yo-Han Kim 1 Hyun Ho Lee 1
1Myongji University Gyeonggi-do Republic of Korea
Show AbstractRecently, self assembly monolayer (SAM) of nanoparticles (NPs) for memory effect has drawn tremendous attentions for organic device fabrication. Meanwhile, various binding mechanisms for the formation of the SAM can be derived from specific intermolecular interactions, such as bio-inspired mechanisms. In this study, organic memory device structure using monolayer formation between aptamer conjugated gold nanoparticles (Au NPs) with prostate-specific membrane antigen (PSMA) protein hybridization was introduced and characterized. The capacitor type memory effect was examined by using pentacene and dielectric SiO2 layer intervened by the Au NPs layer. PSMA protein was immobilized onto the SiO2 layer combined by formation of amide bond. Capacitance-voltage(C-V) hysteresis loops and retention measurement of the device was characterized and discussed.
6:00 AM - J6.58
Fabrication of Ferroelectric Random-access-memory Cell with Two Switching- and Ferroelectric Organic Field-effect Transistors Using Transfer-printed Buffer Layer
Min-Hoi Kim 1 Gyu Jeoung Lee 1 Chang-Min Keum 1 Sin-Doo Lee 1
1Seoul National University Seoul Republic of Korea
Show AbstractRecently, ferroelectric organic field-effect transistors (OFETs) have attracted much attention for non-volatile memory applications such as a ferroelectric random-access-memory (FeRAM) due to simple fabrication and low cost. For FeRAM, two different types of the OFETs, one for the data selection by a conventional OFET and the other for the data storage in a ferroelectric OFET, should be integrated in an individual ferroelectric memory cell [1]. For the integration of two different OFETs, the patterns of two different insulators need to be prepared. A FeRAM with ferroelectric OFETs was demonstrated using inkjet printing [2]. However, the inkjet process has the difficulty in integrating memory cells into an array because of limited resolution. In this work, we present ferroelectric memory cells for the FeRAM in which a conventional OFET and a ferroelectric OFET are integrated by transfer-printing of a buffer layer on a ferroelectric insulator. The region with a pattern of the transfer-printed buffer layer acts as a dielectric surface for the conventional OFET while the region without a buffer layer provides a ferroelectric surface for a ferroelectric OFET. It was found that the transfer curves of the ferroelectric OFETs show hysteresis characteristics needed for memory applications but those of the conventional OFETs exhibit no hysteresis. The transfer-printing method of a buffer layer, in principle, provides high-resolution and thus enables to easily integrate two types of the OFETs into a unit memory cell for the FeRAM. References: [1] R. C. G. Naber, K. Asadi, P. W. M. Blom, D. M. de Leeuw, and B. de Boer, Adv. Mater. 22 (2010) 933. [2] T. Sekitani, K. Zaitsu, Y. Noguchi, K. Ishibe, M. Takamiya, T. Sakurai, and T. Someya, IEEE Trans. Electron. Dev. 56 (2009) 1027. Acknowledgement: This work was supported by the National Research Foundation of Korea under the Ministry of Education, Science and Technology of Korea through the grant 2011-0028422.
6:00 AM - J6.59
Structural Change of Ultralow Dielectrics by Ozone Treatment
Il-Yong Kang 1 Seung-Hyun Song 1 Hee-Woo Rhee 1
1Sogang University Seoul Republic of Korea
Show AbstractLow dielectric materials have been used as interlayer dielectrics for system IC over the years but there has not been much progress in the development of ultralow dielectrics required for less than 25 nm devices. Our group developed ultralow dielectrics (ULK) which had modulus of 9.1 GPa at dielectric constant of 2.12 by designing reactive porogens. We also treated ULK by either UV or ozone to optimize the structures of nanoporous ULK for higher mechanical strengths and better pore morphology. Recently we used ozone treatment which resulted in smaller pore size (less than 2 nm) and narrower pore distribution. Ozone treatment converted the alkoxy groups in the matrix and the reactive porogen to silanol groups and resulted in increased reactivity between the reactive porogen and the matrix. In addition, ozone treatment above 150 °C converted T structures of both the reactive porogen (right??) and the matrix to Q or SiO2-like structures, which are responsible for high mechanical strength as well as high dielectric constant. The structural changes partially came from the decrease in methyl groups of the copolymer of methyl trimethoxysilane (MTMS) and 1,2-bis(triethoxysilyl) ethane (BTESE) as the matrix. Therefore, in order to optimize ozone treatment condition, we studied the structural changes of reactive porogen and organosilicate matrix induced by ozone treatment as a function of time. FTIR and Si NMR were used to characterize their chemical and structural structures as a function of time and temperature.
6:00 AM - J6.6
Systematic Investigation of Thermal Treatment Effect on Solution Processed OSC/OGI Interfacial System and Its Transistor Application
Chang Bum Park 1
1LG Display Ramp;D Center Paju-si Republic of Korea
Show AbstractThe systematic study of the thermal treatment has been carried out on the solution processed organic semiconductor (OSC)/organic gate insulator (OGI) interface system and its application of soluble anthradithiophene deriative based organic field effect transistors (FETs). Here we show that the property of solution processed device is strongly attributed to the thermal treatment effect with resulting in the fundamental aspect of migration phenomenon at the OSC/OGI organic interfacial as well as the significant change of the crystalline polymorph and the film morphology. After the post-annealing of organic semiconductor close to a solid-isotropic (K-I) transition temperature at 140 °C, the solution processed device exhibits the noticeable performance improvement up to the carrier mobility of ~ 0.5 cm2/Vs, subthreshold swing of 1.1V/decade, and Ion/Ioff current ratio 9.6Ã-106 with the highly ordered moleculeâ?Ts structure in the film. It is also found that the polymorph of crystalline grains is significantly deteriorated after the post-annealing over a K-I temperature with suppressing the structural order of molecules. The depth profile of elemental mapping in the interfacial region clearly turns out that the decomposition of OSC and its migration toward the organic gate insulator are occurred during the post-annealing over a K-I temperature, resulting in the low electrical performance of the device.
6:00 AM - J6.60
Highly Crystalline Soluble Acene Crystal Arrays for Organic Transistors: Mechanism of Crystal Growth during Dip-coating
Jaeyoung Jang 1 2 Sooji Nam 1 2 Jong-Jin Park 2 John E Anthony 3 Chan Eon Park 1 2 Jong Min Kim 2
1Postech Pohang Republic of Korea2Samsung Advanced Institute of Technology (SAIT) Yongin Republic of Korea3University of Kentucky Lexington USA
Show AbstractThe preparation of uniform large-area highly crystalline organic semiconductor thin films that show outstanding carrier mobilities remains a challenge in the field of organic electronics, including organic field-effect transistors. Quantitative control over the drying speed during dip-coating permits optimization of the organic semiconductor film formation, although the kinetics of crystallization at the airâ?"solutionâ?"substrate contact line are still not well understood. Here, we report the facile one-step growth of self-aligning, highly crystalline soluble acene crystal arrays that exhibit excellent field-effect mobilities (up to 1.5 cm2Vâ?"1sâ?"1) via an optimized dip-coating process. We discover that optimized acene crystals grew at a particular substrate lifting-rate in the presence of low boiling point solvents, such as dichloromethane (b.p. of 40.0°C) or chloroform (b.p. of 60.4°C). Variable-temperature dip-coating experiments using various solvents and lift rates are performed to elucidate the crystallization behavior. This bottom-up study of soluble acene crystal growth during dip-coating provides conditions under which one may obtain uniform organic semiconductor crystal arrays with high crystallinity and mobilities over large substrate areas, regardless of the substrate geometry (wafer substrates or cylinder-shaped substrates).
6:00 AM - J6.61
Structural and Electronic Interaction in Hybrid Interface Based Resistive Memory Devices
Bharti Singh 1 B. R Mehta 2 Xinliang Feng 3 Klaus Mullen 4
1Indian Institute of Technology Delhi New Delhi India2Indian Institute of Technology Delhi New Delhi India3Max-Planck Institute for Polymer Research D-55128 Mainz Germany4Max-Planck Institute for Polymer Research D-55128 Mainz Germany
Show AbstractStudy of interfacial properties of hybrid junctions is scientifically challenging due to large dissimilarities in the structural and electronic nature of the two materials. Unique properties of hybrid interfaces can be utilized for modifying the characteristics of devices based on electron transport across the junction. In this work, bilayer junctions of Hexa-peri-hexabenzocoronene (HBC) or Graphene with copper oxide thin films have been studied for resistive memory applications. X-ray photoelectron spectroscopy and spectroscopy ellipsometry studies were carried out for studying the interfacial modification at the hybrid junction. In case of CuO-HBC based hybrid structure, it was observed that interfacial region has a number of distinct features in the absorption spectra of the interfacial layer obtained from optical modelling of the spectroscopic ellipsometric data. It was observed that Ti-CuO-HBC-Cu structure has resistive switching properties entirely different from the electrical properties of individual layers. In case of CuO-MLG bilayer structure, resistive switching characteristics are related to the electronic interaction at the interface as revealed by XPS studies. The resistive switching parameters are observed to be sensitive to the ambient conditions. Electrical characterization of the junction based on CuO, MLG and CuO-MLG layers shows that MLG plays an important role of oxygen ion storage and blocking layers and resistive switching is due to the two way migration of oxygen ions across the junction. Resistive switching properties of bilayer structures studied in the present study provide useful information about the nature of electrical and structural interaction at the hybrid interface. Keywords: Hybrid interface, resistive switching, Copper oxide, Hexa-peri-hexabenzocoronene, Graphene.
6:00 AM - J6.62
Growth of Organic Semiconductor Nanowires for the Application of Memory Devices
Dipak Kumar Goswami 1
1Indian Institute of Technology Guwahati Guwahati India
Show AbstractIn the last few years, the successful growth of organic semiconducting thin films and 1D structures in the form of wires, fibers and ribbons based on small conjugated molecules has been reported and their potential applications in nano scale organic field effect transistors (OTFTs) with improved charge carrier mobility has been demonstrated. We present here some of the recent results on the growth of organic nanowires grown by organic molecular beam deposition technique. Most of the cases, planer Ï?-conjugated molecules are used to grow wires where molecules are stacked by week Ï?â^'Ï? interactions. We have synthesized a Ï?-conjugated donor-acceptor molecule where the donor and the acceptor units are connected via a planar spacer unit and thus giving rise to a charge transfer character. These molecules form stacked layer structure held both by aromatic CH-Ï?-hydrogen bonding as well as Ï?-stacking between the donor unit in one layer and the acceptor aromatic unit of other in solid crystal. However, H-bonding is playing the prime role over Ï?â^'Ï? stacking for giving the layer structure and thus leading to the unique properties for 1D growth. It has been shown that hydrogen bonding along the perpendicular direction to the molecular plane is stronger than bonding within plane. Therefore, these molecules prefer to grow unidirectional with higher air stability. We have used this property of the molecules to grow nano wires with higher aspect ratio (typically 10 μm long and 100 nm width) on top of SiO2 surface. Detail growth mechanism of these wires has been studied. We have found very high persistent current that shows promises to be a very good candidate for designing optoelectronic memory units.
6:00 AM - J6.63
A Neuro-inspired Memristive Organic-nanoparticule Synapse-transistor (synapstor)
Fabien Alibart 1 Stephane Pleutin 1 David Guerin 1 Olivier Bichler 2 Weisheng Zhao 2 Christian Gamrat 2 Dominique Vuillaume 1
1Institut for Electronics, Microelectronics and Nanotechnology, CNRS Villeneuve d'Ascq France2CEA-LIST, Centre de Saclay Gif-Sur-Yvette France
Show AbstractA large effort is now devoted to the research of new computing paradigms associated to innovative nanotechnologies that should complement the classical Von Neumann/CMOS association. Among various propositions, spiking neural networks (SNN) seem a valid candidate. We demonstrated a memory transistor made of conjugated molecules and metal nanoparticles, which behaves as a biological spiking synapse [1], with a dynamic in the frequency 0.01-10 Hz depending on the NP size (5-20 nm) and the channel length (200 nm to 20 μm). This behavior is obtained by coupling the charging/discharging property of the nanoparticules when the gate voltage of the device is biased and the interaction into the gap of the transistor between the charges stored in the nanoparticules and the free carriers of the pentacen thin film. Here we demonstrate the implementation of the Spike Timing Dependent Plasticity (STDP) learning algorithm. First, we demonstrate that the NOMFET is a memristive device. Based on the formalism of Chua [2], we propose a physical model that describes the NOMFET behavior. Secondly, based on a theoretical proposition of STDP implementation with a memristor [3], we demonstrate a direct implementation of STDP using NOMFETs. The STDP allows tuning the weight of a synaptic connection depending on the timing between the input synapse spike (i.e the activity of the pre-synapse neuron) and the feedback pulse signal (due to the post-synapse neuron activity) [4]. As a demonstrator, we fabricated an original implementation of an associative learning neural network - inspired by the famous Pavlov's dogs experiment - using a single NOMFET to implement each synapse. We show how the physical properties of this dynamic memristive device can be used to perform zero-power write operations for the learning and implement short-term association using temporal coding. An electronic circuit was build to validate the proposed learning scheme with packaged devices, with very good reproducibility despite the complex synaptic-like dynamic of the NOMFET in pulse regime. [1] F. Alibart et al. Adv. Func. Mater. 20, 330 (2010). [2] L.O. Chua, IEEE Trans on Circuit Theory 18, 507 (1971). [3] B. Linares-Barranco et al.IEEE NANO 2009, pp. 601-604. [4] H. Markram et al., Science 275, 213 (1997); G. Bi and M. Poo, J. Neurosci. 18, 10464 (1998).
6:00 AM - J6.64
Deposition of Organic-Inorganic-hybrid Laminates Using Ozone Based Molecular Layer Deposition (MLD)
Jie Huang 1 Sunwoo Lee 1 Mingun Lee 1 Antonio T Lucero 1 Jiyoung Kim 1
1The University of Texas at Dallas Richardson USA
Show AbstractRecently, organic thin films have been attracting attention due to their large area, low cost, and flexibility for display applications and flexible electronics. Alternatively, inorganic thin films have several benefits over organic thin films such as high functionality (e.g. high conductance, high dielectric constant and polarization, etc.) Organic-inorganic hybridization is interesting because it widened the range of applications by not only offering new functions but also manipulating the surface properties. A novel technique has been reported using a modified atomic layer deposition (ALD) method, called molecular layer deposition (MLD), for building an organic and inorganic hybrid stack as designed by a manufacturer. MLD hybrid thin films minimized the formation of defects during the growth of the organic and inorganic layers because they are deposited by sequential, self-limiting, surface reactions similar to ALD process. In this study, we investigated the growth characteristics of organic-inorganic laminates, i.e. octenytrichlorosilane with 7 alkyl chains (7-OTS) and zinc-oxide (ZnO) hybrid thin film, using ozone based MLD at temperatures between 100°C and 150°C. 7-OTS is deposited by an exchanging reaction between functional group and water with growth rate of ~0.6 nm per cycle. The terminal methyl group (-CH3) of 7-OTS is converted into a carboxylic group (-COOH) through in-situ ozone (O3) activation. ZnO is deposited as a linker layer in-between each OTS layers using diethyl-zinc (DEZ) and DI water (H2O) as precursors with deposition rate of ~0.2 nm per cycle. Reference study using aluminum-oxide (Al2O3) and titanium-oxide (TiO2) as a linker layer will be also discussed. Materials characterization of organic-inorganic hybrid thin films have been extensively investigated using transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), Ellipsometer and Fourier transform infrared spectroscopy (FTIR). Electrical characteristics, including I-V and C-V, of the laminate structure will be analyzed. This research is partially funded through Korea-US collaboration R/D program by MKE-COSAR-KETI.
6:00 AM - J6.65
Oligothiophene-based Organic Thin-film Transistors: A Kelvin Probe Force Microscopy Study of Electronic Properties
Nasima Afsharimani 1 Bernard Nysten 1
1Universiteacute; catholique de Louvain(UCL) Louvain-la-Neuve Belgium
Show AbstractAbstract: It appeared in the past decades that semi-conducting organic liquid crystals could be used as the active layer in organic thin film transistors (OTFTs). They can be processed by simple methods such as inkjet printing, which paves the way to new applications for plastic electronics such as electronic tags, biosensors, flexible screens, â?¦ However, the measured field-effect mobility in OTFTs is relatively low compared to inorganic devices. Generally, such low field-effect mobility values result from extrinsic effects such as grain boundaries or imperfect interfaces with source and drain electrodes. It has been shown that reducing the number of grain boundaries between the source and drain electrodes improves the field effect mobility [1-3]. Therefore, it is important to understand the transport mechanisms by studying the structure of organic thin films and local electrical properties within the channel and interfaces with metal electrodes in order to improve the field-effect mobility in OTFTs. Kelvin probe force microscopy (KPFM) is an ideal tool to simultaneously investigate the local structure and the electrical potential distribution in electronic devices for the above mentioned purposes. In this work, we studied the structure and the electronic properties of OTFTs based on dioctylterthiophene (DOTT). The transistors were fabricated by spin-coating DOTT on the transistor structures with treated (silanized) and untreated channel oxide. The potential profiles across the channel and at the metal-electrode interfaces was measured by KPFM. The effect of surface treatment on electrical properties, charge trapping phenomenons and hysteresis effects was also studied. Keywords: Organic thin film transistor (OTFT), Oligothiophene, Kelvin probe force microscopy (KPFM) [1] G. Horowitz, M.E. Hajlaoui Synth. Met.122 (2001), 185. [2] T.W. Kelley, C.D. Frisbie J. Phys. Chem. B 105 (2001), 4538. [3] T. Minari, T. Nemoto, S. Isoda J. Appl.Phys. 96 (2004) 769.
6:00 AM - J6.66
Molecular Side Chain Substitution Effects in High Performance Field Effect Transistors based on 2,7-substituted-[1]benzothieno[3,2-b][1]benzothiophenes
Atefeh Yousefi Amin 1 Knud Reuter 2 Timo Meyer Friedrichsen 2 Marcus Halik 1
1Organic Materials and Devices Erlangen Germany2Heraeus Precious Metals GmbH amp; Co Leverkusen Germany
Show Abstract
Since the early researches in organic thin film transistors (TFTs), new organic semiconductors and materials optimization have pushed the field close to marked applications[1].The tuning of interfaces and thin film morphology has been highlighted during the last years due to the optimization of promising materials[2,3], and to possibility to adapt materials properties to electrical device properties as needed for application. A set of symmetric C13-BTBT-C13, and asymmetric C13-BTBT organic semiconductors[4] was synthesized and applied in thin film transistors (TFTs). The asymmetric substitution pattern is favourable, because preserving large charge carrier mobilities (up to 3.4 cm2/Vs) and easy processing but strongly reduced injection barrier as investigated in detailed studies of thin film morphologies, contact- and channel resistance. The top-contact bottom-gate structure of TFT allows us to study the interface characteristics related to the surface energy and interface functionalization. The formation of the first monolayer of BTBTs on different hybrid dielectrics was investigated with AFM. The high threshold/operation voltage motivated us to apply ultra thin hybrid dielectrics (AlOx/SAM) providing high capacitance, low leakage current, and low operation voltage while having the charge carrier mobility up to 3.6 cm2/Vs. Varying the SAM in hybrid dielectric, the threshold voltage can actively be tuned. For instance, using fluorinated SAM (FC18-PA) and C13-BTBT lead to devices with good charge charier mobility of 1.84 cm2/Vs and a Vth shift closer to zero ( -0.9 V), providing excellent characteristics for further integration.
References: [1] G. Gelinck, P. Heremans, K. Nomoto and T. D. Anthopoulos, Advanced Materials, 2010, 22, 3778 [2] A. Virkar, S. Mannsfeld, Z. Bao and N. Stingelin, Advanced Materials, 2010, 22, 3857. [3] H. Ma,H. L. Yip, F. Huang and A. K. Jen, Advanced Functional Materials, 2010, 20, 1371 [4] H. Ebata, I. Takafumi, M. Eigo, T. Kazuo, I. Masaaki, K. Hirokazu and Y. Tatsuto, Journal of the American Chemical Society, 2007, 129 (51), 15732.
6:00 AM - J6.67
Synthesis of New n-Type Polymer Based on Indenopyrazine for Opto-electronic Devices
Seungho Kim 1 Beomjin Kim 1 Jongwook Park 1
1Catholic University of Korea Bucheon-si Republic of Korea
Show AbstractMany research efforts have been focused on new organic compounds in n-Type materials due to the various applications such as OTFT and OPV. New n-type polymer of Poly-6,6,12,12-Tetrahexyl-2,8-bis-(5-methyl-thiophen-2-yl)-6,12-dihydro-diindeno[1,2-b;1',2'-e]pyrazine (PTIDP) was recently synthesized. UV-Visible absorption maximum values of the synthesized polymer were 499 and 497 nm in solution and film state. PL spectrum showed maximum values of 525 and 530 nm in solution and film state, respectively. In this presentation, the related physical property as well as the device data using PTIDP will be discussed.
6:00 AM - J6.68
A Phase-space Analysis to Understand Morphology Formation in Organic Thin Films
Olga Wodo 1 Baskar Ganapathysubramanian 1 2
1Iowa State University Ames USA2Iowa State University Ames USA
Show AbstractThe multiplicity of fabrication process variables (solvent type, solvent blends, evaporation profile, annealing, spinning rate, solvent atmosphere, blend ratio, substrate effect, additives) makes tailoring (solvent based) fabrication processes a challenging task. It is of particular interest to be able to comprehensively estimate the morphological features depending on particular combinations of various process and system variables. A phase-diagram that maps the phase space of processing conditions to various classes of morphology formed is an attractive method to understand the interplay between multiple thermodynamic and kinetics parameters involved in morphology evolution. Such phase-diagrams will be very useful as we seek to tailor fabrication conditions to achieve designer morphologies. We utilize a predictive computational framework to model morphology evolution during solvent-based fabrication of thin films. This method provides the flexibility to search the phase space of process and system variables in a very efficient way. A systematic study of the various processing conditions reveals disparate modes of morphology evolution. We concentrate on three main variables: evaporation rate, blend ratio and Flory Huggins interaction parameters. We construct a phase diagram and quantify the interplay between kinetic, thermodynamic and system variables (for model systems) that typically leads to various morphological features. Finally, we illustrate the utility of such approach by focusing on a specific polymeric system of interest to the OPV community.
6:00 AM - J6.7
Optimizing the Interface between Vapour-deposited n-octyl Phosphonic Acid and Pentacene for Low Voltage Organic Thin-film Transistors
Swati Gupta 1 Krishna C Chinnam 1 Helena Gleskova 1
1University of Strathclyde Glasgow United Kingdom
Show AbstractTo turn organic electronic circuits into viable technology platform, transistors require high field-effect mobility and operating voltages below 5 V. In addition, technology that can be adapted to roll-to-roll processing is desirable. Here, dry vacuum technologies provide an alternative path to wet printing of organic devices.
Our bottom-gate and top-contact transistor fabrication process employs dry fabrication procedures only. The gate dielectric consists of aluminium oxide (AlOx) prepared by UV/ozone exposure of thermally evaporated Al and a vapour-deposited monolayer of n-octyl phosphonic acid. The total thickness of the dielectric is ~ 20 nm, leading to capacitance value of 2.76 x 10-7 F/cm2, breakdown voltage in excess of 10 V, and the gate leakage current less than 4 x 10-7 A/cm2. To increase the field-effect mobility of the transistors, the interface between the gate dielectric and organic semiconductor must be optimized. We improved the transistor performance and device uniformity by using pentacene of higher purity and by choosing moderate pentacene evaporation rates. Transistors with one-time (TCI), three-time (Sigma-Aldrich), and four-time purified (in-house) pentacene were fabricated while the evaporation rate was varied from 0.12 to 4.2 nm/min. Transistors with four-time purified pentacene and moderate evaporation rate, achieved higher field-effect mobility and lower inverse subthreshold slope as compared to one-time and three-time purified pentacene. To date the best transistors have field-effect mobility of 0.09 cm2/Vs, threshold voltage of -1.1 V, and inverse subthreshold slope of 93 mV/decade. The increase in the field-effect mobility correlates with the decrease in the channel resistance (determined by the transmission line method) achieved for moderate evaporation rates of pentacene. Studies of surface roughness are underway to provide further optimization of the dielectric/semiconductor interface.
This research is supported by Scottish Funding Council through Glasgow Research Partnership in Engineering.
6:00 AM - J6.8
Conductivity Maxima on the Surface of Organic Single Crystals at Charge Densities above 1013 cm-2
Wei Xie 1 C. Daniel Frisbie 1
1University of Minnesota Minneapolis USA
Show Abstract
We have previously studied the transport properties of rubrene electrical double layer transistors (EDLTs) as a function of ionic liquid composition and temperature. We have successfully achieved effective carrier mobility up to 3.2 cm2V-1s-1 at charge densities larger than 1013 cm-2. At lower temperatures when a larger gate bias can be applied, the maximum attainable charge density reaches 6.5Ã-1013 cm-2 (0.34 holes per rubrene molecule), and more remarkably, two pronounced maxima in channel conductivity have been reproducibly and stably observed. This feature, which has not been reported for any EDLTs gated with electrolytes, is independent of ionic liquid compositions, current-voltage sweep rate, and crystallographic directions of rubrene crystals. We have identified that the first and second conductivity peaks occur at charge densities of 2.1Ã-1013 cm-2 and 5.2Ã-1013 cm-2, respectively. Capacitance-voltage (C-V) measurements have also shown two maxima at the same gate voltages as in current-voltage measurements. These observations imply that the double conductivity maxima are very likely related to the electronic structure on the surface of rubrene crystals.
6:00 AM - J6.9
Photo-patterned Polyimide Thin Films for Gate Dielectric Applications
Kwang-Suk Jang 1 Hye Jung Suk 1 Taek Ahn 2 Jae Won Ka 1 Mi Hye Yi 1
1Korea Institute of Chemical Technology Daejoen Republic of Korea2Kyungsung University Busan Republic of Korea
Show AbstractPolyimide is one of the most promising candidate materials for gate dielectric applications due to its high chemical resistance as well as good insulating properties. Rigid polyimides with high packing density are known to have high resistance to chemicals currently used in electronics industry such as echant and photoresist stripper. Because thermally cured polyimide films cannot be patterned by facile methods in most cases, photo-patternable polyimide gate dielectrics have been proposed recently. We have developed photo-patternable polyimide gate dielectric with a relatively high-dielectric constant and low leakage current density. The polyimide gate dielectric could be easily patterned by selective UV light exposure and immersion into gamma-butyrolactone to dissolve-out the uncrosslinked part. To investigate the potential of the polyimide as the gate dielectric, we fabricated pentacene TFT devices. Interestingly, hole mobility of pentacene TFT device was increased after the solvent immersion process for the patterning. Hole mobility of the device with the polyimide, UV-exposed, but not immersed in the patterning solvent, was in the range of 0.039 ~ 0.067 cm2/Vs. When using the patterned polyimide, UV-exposed and immersed in the patterning solvent, hole mobility was in the range of 0.10 ~ 0.21 cm2/Vs. There are two possible reasons of the above result. In most cases, gate dielectrics after the patterning process were damaged, but the crosslinked polyimide is robust against the patterning solvent. And the solvent-treated dielectric surface may affect the molecular ordering and morphology of pentacene film. After the solvent immersion process, the grain size of pentacene molecules was decreased. This might affect the device property. In summary, we have developed photo-patternable polyimide gate dielectric which is robust against the patterning process. The pentacene TFTs with the patterned polyimide were obtained with a carrier mobility of 0.10 ~ 0.21 cm2/Vs and I(on)/I(off) of ~ 10^6.
J2: Solution Process Organic Electronic Devices
Session Chairs
Tuesday AM, April 10, 2012
Moscone West, Level 2, Room 2008
9:00 AM - *J2.1
Solution-Processed Small-Molecule Solar Cells with 7% Efficiency
Alan J. Heeger 1 G. C Bazan 1 Y. Sun 1 G. C Welch 1 W. L Leong 1 L. G Kaake 1 D. Moses 1 C. J Takacs 1
1University of California, Santa Barbara Santa Barbara USA
Show Abstract
We have demonstrated efficient solution-processed small-molecule solar cells based on a novel molecular donor, DTS(PTTh2)2 A record power conversion efficiency (PCE) is achieved for small-molecule organic photovoltaics: PCE=7% under AM 1.5 G irradiation (l00 mW cm-2) from bulk heterojunction (BHJ) composites of DTS(PITh2)2:PC70BM (donor to acceptor ratio of 7:3) with short circuit current (Jsc) of 14.4 rnA cm-2, open circuit voltage (Voc) of 0.78 V and fill factor (FF) of 59%. These high values were obtained by using remarkably small percentages of solvent additive (0.25% v/v of diiodooctane, DIO) during the film forming process. Transmission electron microscopy was used to directly image crystalline DTS(PTTh2)2 domains in BHJ films and investigate changes with the varying concentrations of the solvent additive. These results provide innovative guidelines for the realization of high performance solution-processed organic solar cells fabricated using molecular materials. The final step in the preparation of p-DTS(PTTh2)2 involves end capping of the PT-DTS-PT core with hexyl bithiophene units via a microwave assisted Stille cross coupling reaction. Methyl transfer (instead of hexylbithiophene transfer) can occur leading to the formation of
(MePT)DTS(PTTh2)
. Trace impurities of
(MePT)DTS(PTTh2)
in BHJ solar cells fabricated from synthesis batches of p-DTS(PTTh2)2 significantly influence the photovoltaic properties, causing a ~50% reduction in efficiency and affecting all of the relevant device parameters Jsc, Voc and FF). From a broader perspective, despite molecular design, the suitability of a material for efficient devices is often only determined by trial and error in the device processing laboratory. As shown by the data, promising materials found to be unsuitable for device applications may suffer from highly dilute impurities that act to increase carrier recombination. The mechanism of charge transfer in blended films of organic molecules relevant to solar energy production is a central problem in the field. In particular, the structure property relationship regarding the process is of wide interest. We show that the timescale of ultrafast charge transfer between a small molecule donor and a fullerene acceptor is affected by the use of processing additives during film formation, decreasing to less than 50 fs. In light of these results, a model is put forth which describes ultrafast and high yield charge transfer as the result of excited state delocalization.
9:30 AM - J2.2
Vertical Organic Field Effect Transistors Realized by Fluorine Based Photo-lithography
Hans Kleemann 1 Alexander A Zakhidov 1 Bjoern Luessem 1 Karl Leo 1
1Technische Universitauml;t Dresden Dresden Germany
Show AbstractOrganic field effect transistors (OFETs) are promising candidates for flexible, transparent, and low cost electronics. Hence, they attract increasing scientific and economic interest and first products containing OFETs are ready for market entry. OFETs, however, are currently limited in their performance and accordingly in their usability for electronic applications. This is partially related to the rather low mobility of organic semiconductor in comparison to e.g. silicon. Moreover, there is a lack in appropriate structuring techniques for OFETs, which might allow to compensate for the low mobility of organic semiconductors. In this contribution, we present high performance vertical organic field effect transistors (VOFETs) with a vertical channel length down to 50nm. These devices are manufactured by a direct photo-lithography method, which is compatible to organic semiconductor materials. By this technique we can vertically stack source and drain electrodes and the organic semiconductor material in between. If a source-gate voltage is applied, a conductive channel is created in the semiconductor film and its resistance is controllable by the applied gate field. This vertical conductive channel of the VOFET, however, is a complex and 3-dimensional structure and thus OFET theory relying in the gradual channel approximation is not adoptable to such devices. Nevertheless, VOFETs presented in this study exhibit very high on/off ratios of 10^6 as well as high transconductance values. As a consequence of the extremely short vertical channel, which is likely governed by the film thickness, we clearly obtain short channel device characteristics as e.g. threshold voltage dependence on the source-drain field, and disappearance of the saturation region. The photo-lithography technique used for fabrication of these vertical transistors is based on fluorinated photo-resist compounds and thus it can be adopted for structuring of different organic materials under ambient conditions. Owing to this universality of the process, we present p-type VOFETs comprising Pentacene as well as n-type VOFETs comprising C60. Thus, the combination of high performance complementary VOFETs on the one hand and the advantages of the photo-lithographic patterning technique on the other hand, represents a promising way for high density integration in low-power electronic devices made of organic semiconductor materials.
9:45 AM - J2.3
Graphene Enabled Vertical Field Effect Transistors
Maxime G Lemaitre 1 Evan P Donoghue 2 Mitchell A McCarthy 2 Bo Liu 2 Bill R Appleton 1 3 Andrew G. Rinzler 2
1University of Florida Gainesville USA2University of Florida Gainesville USA3University of Florida Gainesville USA
Show Abstract
Recently, carbon nanotube enabled vertical field effect transistors (CN-VFETs) demonstrated high on-off ratios with record on-currents (for given device area) using an organic semiconductor for the channel material.1,2 Transconductance in these devices arises from a gate field modulation of the contact barrier at the organic semiconductor/nanotube interface. The porosity of the nanotube network allows unscreened access to the interface, while the low density of electronic states allows for gate field induced Fermi level shifts that boost the transconductance. The low density of states in the vicinity of the Dirac-point of graphene also makes it a good candidate source electrode for VFETs with some additional advantages. Here we demonstrate a novel transfer technique, which reduces disorder and improves areal yields, while eliminating the polymeric residue that typically plagues transferred films. The resulting planar graphene sheets do not suffer from the particulates and associated shorting pathways that necessitate thicker channel layers in nanotube-based devices. Thinner organic channel layers in these graphene based devices further boost the currents attained at low drain voltage. We also report a facile technique for fabricating perforated graphene films with tunable hole size and density, allowing for a direct assessment of the role of self-screening effects in VFETs. These initial graphene enabled VFETs achieve on/off ratios and on-current densities exceeding 106 and 250mA/cm2, respectively, with drive voltages below 4V. 1. M. A. McCarthy, B. Liu, A. G. Rinzler, Nano Lett. 10, 3467â?"3472 (2010). 2. M. A. McCarthy, B. Liu, R. Jayaraman, S. M. Gilbert, D. Y. Kim, F. So, A.G. Rinzler, ACS Nano, 5, pp 291â?"298 (2011).
10:00 AM - J2.4
Reversibly Tuning the Threshold Voltage in Organic Thin Film Transistors by Local Channel Doping Using Photoreactive Interfacial Layers and Acidic SAMs
Barbara Stadlober 1 Marco Marchl 2 Simon J. Ausserlechner 2 Manfred Gruber 3 Matthias Edler 5 Reinhold Hetzel 2 Heinz-Georg Flesch 2 Lukas Ladinig 2 Lucas Hauser 4 Michael Buchner 3 Thomas Rothlaender 1 Ferdinand Schuerrer 3 Gragor Trimmel 4 Thomas Griesser 5 Karin Zojer 3 Egbert Zojer 2
1Joanneum Research Weiz Austria2Graz University of Technology Graz Austria3Graz University of Technology Graz Austria4Graz University of Technology Graz Austria5University of Leoben Leoben Austria
Show AbstractWe present two easy methods to reversibly switch OTFTs from enhancement to depletion mode and back by either a photochemical reaction using photo-acid generators as interfacial layers, or by the usage of pH-sensitive acidic SAMs. The latter allow the fabrication of chemo-responsive OTFTs detecting the presence of bases like ammonia that compensate the acid doping. Based on these methods good quality depletion-load inverters with tuneable characteristics [1] were manufactured. In case of the photo-acid generator SAM a significant refinement of the concept of chemical channel doping is achieved by replacing the covalently bonded silane layers bearing sulfonic acid groups with photo-acid generator polymers. The goal hereby is to use them as an interface-modification layer, whose properties can be patterned photochemically, as the acid group is formed only upon illumination. Thus an accurate local control of VTh through the illumination dose is achievable by a method that is fully compatible with lithographic techniques omnipresent in conventional semiconductor industry. The inverters with acid/base-sensitive interfacial layers on the other hand are shown to be highly responsive switches to the presence of HCL and NH3 gases. [1] Marco Marchl , Matthias Edler , Anja Haase , Alexander Fian , Gregor Trimmel , Thomas Griesser , Barbara Stadlober , and Egbert Zojer., Adv. Mat. 22, 5361 (2010).
10:15 AM - J2.5
Highly Flexible Organic Thin Film Transistors Operating at Ultra-low Voltage
Piero Cosseddu 1 2 Stefano Lai 1 Massimo Barbaro 1 Annalisa Bonfiglio 1
1University of Cagliari Cagliari Italy2CNR-INFM Modena Italy
Show AbstractIn this work we propose a novel flexible structure for the fabrication of ultra-low voltage Organic Thin Film Transistors (OTFTs) working at 1V. All devices have been fabricated on flexible Polyethylenetherephtalate film (PET) and are based on a unique combination of different, ultra-thin, insulating films. The core of the structure is a Al2O3 thin film grown by UV-Ozone treatment at the top of an aluminium film that acts as the gate electrode. At the top of this structure an ultrathin Parylene C film has been deposited by chemical vapour deposition. Assisted by Atomic Force and Transmission Electron Microscopy we have investigated on the structural and morphological properties of the deposited thin insulating films. Very interestingly it was found that the UV-Ozone grown Al2O3 films have an average thickness of 6nm and are characterized by a very high surface corrugation which usually gives rise to high conductive paths through the oxide film. On the contrary, the deposition of a continuous Paylene C film (average thickness 20nm) led to a proper smoothing of the final surface roughness (reduced by almost one order of magnitude) and also contributed to an impressive lowering, by more than 5 orders of magnitude, of the leakage current. We have fabricated OTFTs using this approach employing TIPS-pentacene as p-type active organic semiconductor. Devices are characterized by a remarkable very small leakage current, around 20-50pA, charge carrier mobility up to 0.3 cm2/Vs, threshold voltages around -0.1V and very low substhreshold swings, close to 100-150mV/dec. Moreover, thanks to a proper shielding of electron trap sites, usually present on the Al2O3 layer, due to the presence of the continuous and smooth Parylene C film, we have succeeded in fabricating n-type OTFT using N1400 as semiconductor, obtaining a maximum mobility of 1x10-2 cm2/Vs. Interestingly enough, using this approach full swing complementary inverters operating at 1V have been fabricated reaching a maximum gain of 50. Finally, we have also demonstrated that, by simply reducing the thickness of the plastic substrate, it is possible to obtain highly mechanically stable transistors. In particular, we have fabricated ultra-low voltage transistors on very thin PET films (1.5 µm thick) that, thanks to the very small surface strain induced by the substrate can be bent down to very small bending radii (below 1mm) without getting damaged and most importantly without giving evidence of change of their electrical performances. These results represent a step forward for the fabrication of low-voltage flexible electronic devices to be employed for the fabrication of innovative applications in the wearable electronics field.
10:30 AM - J2.6
Solution-processed Dye-sensitized ZnO Phototransistors
Pichaya Pattanasattayavong 1 John G Labram 1 Stephan Rossbauer 1 Stuart Thomas 1 Henry J Snaith 2 Thomas D Anthopoulos 1
1Imperial College London London United Kingdom2University of Oxford Oxford United Kingdom
Show Abstract
Metal oxide semiconductors have recently emerged as an important class of materials for the development of next-generation opto/electronic devices. Although many of the proposed applications aim to supersede other transpiring technologies such as organic semiconductors, the two classes of materials can be combined to produce devices with enhanced properties and potentially new functionalities. One example is the dye-sensitized solar cell (DSSC) which is based on a combination of metal oxide nanoparticles and an organic or metal complex dye sensitizer. The same material combination could be extended to thin-film transistors (TFTs) by functionalizing the oxide semiconductor surface with the organic dye to produce bifunctional devices such as phototransistors. Here, we report the electrical characteristics and photoresponse of dye-sensitized ZnO TFTs which, despite their high transparency (> 80%), are found to respond to green light with extremely high responsivity and large optical-to-electrical gain. TFT fabrication and dye functionalization were all processed from solution using spray pyrolysis and dip coating techniques. A green inorganic light-emitting diode with
λmax = 522 nm was used to selectively excite the organic dye in the photoresponse study. With the increasing light intensity from dark condition to the maximum irradiance of â^¼1.53 mW/cm2, the threshold voltage (
VTH) of the device is strongly shifted in the negative direction, from -55 V to -116 V, while the drain current (
ID) increases by an amount that directly depends on the applied gate bias (
VG). Specifically,
ID increases by six orders of magnitude at
VG = -90 V and by only one order of magnitude at
VG = 50 V. The analysis shows that the responsivity (R), which is the ratio between photocurrent and incident optical power, can be as high as 10,000 A/W, and the sensitivity of the device is also controllable through the application of
VG. The present work can be viewed as a significant step towards the development of a new generation of high performance optical sensors that are highly transparent and can be processed onto large-area substrates using low-cost processing methods.
J3: Devices and Materials
Session Chairs
Tuesday AM, April 10, 2012
Moscone West, Level 2, Room 2008
11:15 AM - *J3.1
Nanterorsquo;s Microelectronic-grade Carbon Nanotube Formulations for Electronic Device Applications
Rahul Sen 1
1Nantero, Inc. Woburn USA
Show AbstractIn 2001 Nantero was incorporated with the goal of developing a universal memory, NRAMâ"¢, using carbon nanotubes (CNTs) as the switching element. NRAM incorporates all the positive attributes of currently available memory chips such as DRAM, SRAM and Flash and allows scaling to higher memory densities than any other technology. To enable development and production of the NRAM, Nantero had to envision and implement a process which used the existing CMOS fabrication tools and introduce a new nano-material such as CNTs into semiconductor fabs. Towards this goal, Nantero was successful in developing a CNT-based liquid formulation that meets CMOS process requirements such as high purity, stability and consistency. The formulation was qualified for use in several Fabs worldwide, and was integrated with other materials and tools to develop NRAM memory chips. This presentation will present our recent efforts to create high purity CNT formulations that are compatible with commercial, leading edge semiconductor manufacture technology nodes. Key characterization and application results related to specialized CMOS grade carbon nanotube formulations and their resulting CNT fabrics will be presented.
11:45 AM - J3.2
Charge Transport across Metal/Molecular (alkyl) Monolayer-si Junctions is Dominated by the LUMO Level
Omer Yaffe 1 Yabing Qi 2 Lior Segev 1 Luc Scheres 3 Sreenivasa Puniredd 4 Tal Ely 1 Hossam Haick 4 Han Zuilhof 3 Leeor Kronik 1 Ayelet Vilan 1 Antoine Kahn 2 David Cahen 1
1Weizmann Institute of Science Rehovot Israel2Princeton University Princeton USA3Wageningen University Wageningen Netherlands4Technion - Israel Institute of Technology Haifa Israel
Show AbstractMolecular electronics describes charge transport processes whereby molecules serve as active elements (e.g., rectifiers, switches, sensors) or passive ones (resistors or surface passivating agents) in electronic devices. Despite the diversity of investigated systems, many fundamental questions regarding the mechanisms for electrical current passing through molecules between two electrodes and the possibility of gaining predictive power and control over the electrical properties of molecular junctions remain unsolved. In short, even for one of the simplest systems, i.e., that of alkyl chains between Au electrodes, there is a large discrepancy between the average tunnel barrier height, extracted from current â?" voltage (I-V) measurements (~1.2 eV), and the barrier that is expected from the experimentally determined electrode work function and alkyl monolayer ionization potential, as found by Ultraviolet Photoelectron Spectroscopy, UPS (~5 eV). Thus, studying the alignment between the molecular orbitals and the Fermi level (EF) of the electrodes, is a crucial step towards understanding electrical transport through molecules. It would seem that having a semiconductor (SC) instead of a metal as one of the electrodes in a molecular junction increases the difficulty in understanding the mechanisms that govern charge transport through the junction. As we shall see, though, the intrinsic asymmetry of a junction with a SC, with respect to charge carrier type, provides unique information on the energy levels involved in transport, in ways that are impossible when using only metal electrodes. To find how charge is transported through a monolayer of saturated alkyl chains we measure and analyze electrical transport across heavy-doped semiconductor â?" molecular insulator / metal junctions. Our results provide unequivocal evidence that charge transport in these junctions is controlled by the energy difference between the metal Fermi level and the lowest unoccupied molecular orbital of the molecule, |EF â?" LUMO| (LUMO: lowest unoccupied molecular orbital), rather than by the energy difference with the highest occupied molecular orbital, |EF â?" HOMO| (highest occupied molecular orbital). This finding agrees with complementary spectroscopic measurements of the samples without Hg contact, but challenges the prevailing concept of HOMO-dominated transport.
12:00 PM - J3.3
Direct Measurement of Ion Mobility in a Conducting Polymer
Eleni Stavrinidou 1 Pierre Leleux 1 2 3 Harizo Rajaona 4 Dion Khodagholy 1 Sebastien Sanaur 1 George G Malliaras 1
1Ecole Nationale Supeacute;rieure des Mines St Etienne Gardanne France2Pocirc;le d'Activiteacute; Y. Morandat Gardanne France3Universiteacute; de la Meacute;diterraneacute;e Marseille France4Ecole Nationale Supeacute;rieure de l'Electronique et ses Applications Cergy France
Show AbstractConducting polymers are excellent candidates for a variety of applications that involve mixed electronic/ionic conductivity, including light-emitting electrochemical cells, dye-sensitized solar cells, batteries, electrochromic devices and biosensors. Yet, while the transport of electronic carriers is understood at a reasonable level of sophistication (Gaussian disorder, correlated disorder, etc.), ionic transport in these materials is not understood in any real detail. We developed a new experimental technique that allows us to directly probe ion injection and transport in conducting polymers. It consists of simultaneous electrical and optical measurements that can decouple ion mobility and concentration. We applied this technique to the state-of-the-art conducting polymer PEDOT:PSS poly(3,4-ethylenedioxythiophene) :poly(styrene sulfonate), and measured the mobility of several cations, including potassium and choline. Numerical simulations based on forward time iteration of drift-diffusion equations validated the analysis of the experimental results.
12:15 PM - J3.4
Moisture Assisted Decohesion of PEDOT:PSS Conducting Polymer Layers
Stephanie R Dupont 1 Eszter Voroshazi 3 2 Paul Heremans 3 2 Reinhold H Dauskardt 1
1Stanford University Stanford USA2K.U.Leuven Leuven Belgium3imec vzw Leuven Belgium
Show AbstractThe highly conductive polymer poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) is widely used as a hole transport layer or electrode in organic electronic devices because of its good film forming properties and high visible light transmissivity. In this presentation we demonstrate how the cohesion of the PEDOT:PSS layer is significantly influenced by moisture along with temperature and mechanical loads. Due to its hygroscopic nature the PEDOT:PSS absorbs water from the environment, weakening the layer. The water easily diffuses through the layer and breaks the hydrogen bonds formed between PEDOT:PSS grains resulting in an accelerated decohesion rate. We use a thin-film adhesion technique that enables us to precisely measure the energy required to separate layers. We determine this energy under systematically varied environmental conditions with changing temperature, relative humidity and pre and in-situ UV exposure. This extensive series of quantitative analysis provides the impact of the different environmental variables and most importantly their synergies, leading us to an in-depth understanding of the mechanisms of delamination. We complement this investigation by surface characterization with XPS and AFM of the debonded surfaces to investigate the physics and chemistry of this degradation process. Additionally, we will compare the effects of a range of adjacent layers such as oxides, metals, glass and ITO. This insight into the mechanism of decohesion yields guidelines for the design of more reliable organic devices containing PEDOT:PSS layers.
12:30 PM - J3.5
Stable Hole and Electron Doping of Graphene by Polymers and Applications
Sefaattin Tongay 1 2 Kara Berke 2 Max Lemaitre 1 Zahra Nasrollahi 2 David B Tanner 1 Arthur F Hebard 1 Bill R Appleton 2
1University of Florida Gainesville USA2University of Florida Gainesville USA
Show AbstractWe report on the n (electron) and p (hole) doping of graphene by modifying the graphene surface by various polymers. Modification of graphene with polymers decreases the graphene sheet resistance by 40-70%. Through such modification, we report sheet resistance values as low as 129 Ω, thus attaining values comparable to those of indiumâ?"tin oxide (ITO), while displaying superior environmental stability and preserving electrical properties over extended time scales. Electrical transport measurements reveal that, after doping, (1) the carrier density of holes increases, consistent with the acceptor nature of some of the polymers used, (2) after n-doping graphene, the carriers changes the sign. We find that the mobility of the graphene decreases due to enhanced short-range scattering and charged impurity scattering. The Drude formula predicts that competition between these two effects yields an overall increase in conductivity. We confirm changes in the carrier density and Fermi level of graphene through changes in the Raman G and 2D peak positions. Doped graphene samples display high transmittance in the visible and near-infrared spectrum, preserving graphene's optical properties without any significant reduction in transparency, and are therefore superior to ITO films in the near infrared. The presented results allow integration of doped graphene sheets into optoelectronics, solar cells, and thermoelectric solar cells as well as engineering of the electrical characteristics of various devices by tuning the Fermi level of graphene.
12:45 PM - J3.6
Prototype of a Light-emitting Organic Memory (LE-OMEM)
Peter Koerner 1 Richard C Shallcross 1 Eduard Maibach 1 Philipp Zacharias 1 Klaus Meerholz 1
1University of Cologne Cologne Germany
Show AbstractOrganic materials offer the opportunity to process device active layers from solution precursors, which is highly desirable over more cumbersome vacuum processing methods, as it permits the utilization of high throughput, low temperature and inexpensive printing techniques for device processing. Here, we report on a prototype of a nonvolatile light-emitting organic memory (LE-OMEM) device composed of multiple solution-processed layers, which is made possible by crosslinkable molecular materials. This presentation will focus on a LE-OMEM device with a 12x12 passive matrix crossbar geometry that demonstrates both optical and electrical multilevel recording, which is made possible by the incorporation of both electrically and optically susceptible dithienylethene (DTE) photochromes that may be toggled between two energetically distinct and thermally stable states. The DTE material is incorporated into an organic light-emitting diode (OLED) structure that enables readout capability on both optical (electroluminescence, EL) as well as electrical (voltage dependent current density) channels, where optimized unstructured devices have displayed ON/OFF ratios for both current and EL of greater than 1000. We discuss the design considerations when moving from larger unstructured devices (pixel size ca. 0.1 cm^2) to significantly smaller pixilated structures (ca. 0.003 cm^2). On the basis of this passive matrix device, we highlight the statistically viable number of available grey scale levels as a function of both external optical and electrical writing stimulus.
Symposium Organizers
Oliver Hayden, Siemens AG
Franky So, University of Florida
Paul Blom, TNO Holst Centre
Henning Richter, Nano-C, Inc.
Jongwook Park, Catholic University of Korea
Symposium Support
Nano-C, Inc.
National Science Foundation
Siemens AG
J8: Doping and Quantum Dots
Session Chairs
Wednesday PM, April 11, 2012
Moscone West, Level 2, Room 2008
2:30 AM - *J8.1
Dynamic Doping in Bright and Stable Light Emitting Electrochemical Cells
Daniel Tordera 1 Antonio Pertegas 1 Manuel Delgado 1 Sebastian Meier 2 David Hartmann 2 Klaus-Dieter Bauer 3 Juergen Bauer 3 Frank Vollkommer 3 Wiebke Sarfert 2 Henk J Bolink 1
1University of Valencia Paterna Spain2Siemens AG Erlangen Germany3OSRAM AG Augsburg Germany
Show AbstractLight-emitting electrochemical cells (LECs) are one of the simplest type of molecular electroluminescent devices. LECs have a simple architecture, are prepared from solution and operate with air-stable electrodes allowing for less rigorous encapsulation, which make them suitable for low cost/large area efficient lighting and signing applications.[1-4] In its simplest form they consist of a single active layer composed of an ionic transition-metal complex (iTMC) which supports all three processes of charge injection, charge transport and emissive recombination.[2-4] We showed that by using iridium(III) complexes capable of forming a supramolecularly-caged structure the lifetime of LECs can be increased to more than 3000 hours.[5,6] Additionally, using fast J-L-V scans during their operation the occurrence of dynamical doping was confirmed.[7] We will present our latest developments on iTMC based LECs, subsecond turn-on combined with a half lifetime in excess of 4000 hours at luminances above 600 cd/m2 and show additional proof of the occurrence of dynamical doping. Also we will present results on large area deposition on flexible substrates. (1) Pei, Q.; Yu, G.; Zhang, C.; Yang, Y.; Heeger, A. J. Science 1995, 269, 1086. (2) Handy, E. S.; Pal, A. J.; Rubner, M. F. J. Am. Chem. Soc. 1999, 121, 3525. (3) Gao, F. G.; Bard, A. J. J. Am. Chem. Soc. 2000, 122, 7426. (4) Slinker, J. D.; Rivnay, J.; Moskowitz, J. S.; Parker, J. B.; Bernhard, S.; Abruña, H. D.; Malliaras, G. G. J. Mat. Chem. 2007, 17, 2976. (5) Bolink, H. J.; Coronado, E.; Costa, R. D.; OrtÃ, E.; Sessolo, M.; Graber, S.; Doyle, K.; Neuburger, M.; Housecroft, C. E.; Constable, E. C. Adv. Mater. 2008, 20, 3910. (6) Costa, R. D.; Orti, E.; Bolink, H. J.; Graber, S.; Housecroft, C. E.; Constable, E. C. J. Am. Chem. Soc. 2010, 132, 5978. (7) Lenes, M.; Garcia-Belmonte, G.; Tordera, D.; Pertegas, A.; Bisquert, J.; Bolink, H. J. Adv. Funct. Mater. 2011, 21, 1581.
3:00 AM - *J8.2
Inorganic and Organometallic Low Cost Dopants for Transport Layers in Organic Electronic Devices
Guenter Schmid 1 Jan Hauke Wemken 1 Anna Maltenberger 1 Marina Petrukhina 3 Thomas Dobbertin 2 Arndt Jaeger 2
1Siemens AG Erlangen Germany2Osram Opto Semiconductor Regensburg Germany3State University of New York Albany USA
Show Abstract
Highly conductive transport layers are one of the technical prerequisites to manufacture organic light emitting diodes (OLEDs) with high luminous efficacies above 50 lm/W. The conductivity is usually enhanced by doping of the organic hole or electron conductor host material. In the class of inorganic and organometallic compounds, the electron withdrawing or donating properties can be varied in a very large range. In the presentation, we show a structure-property relationship within a series of p-dopants based on organometallic Lewis-acids. The conductivity enhancement of the hole conductor was balanced to other important OLED requirements, such as cost, transparency and off-state-appearance. The fundamental properties of the doped layers were characterized by current-voltage measurements, impedance, UV/Vis and photoluminescence spectroscopy. The maximum conductivity was found to be in the range of 10-4 to 10-3 Scm-1 without a significant additional absorption owing to a charge transfer band or the radical cation of the host material. The selected dopant was incorporated into a white OLED stack. Luminous efficacy and lifetime data will be presented. The synthetic depth of the preparation of the dopants determines its cost and thus, its contribution to an OLED product. The compounds were prepared in large quantities from readily available basic chemicals in only two steps. The compounds were characterized by standard chemical analytics and X-ray crystallography. Molecular modeling supported the course of Lewis acidity within the series of compounds.
3:30 AM - J8.3
An Air-stable n-Type Dopant for Vacuum Processed Organic Semiconductor Thin Films
Peng Wei 1 Torben Menke 2 Ben Naab 1 Karl Leo 2 Moritz Riede 2 Zhenan Bao 1
1Stanford University Stanford USA2Technische Universitauml;t Dresden Dresden Germany
Show AbstractControllable molecular doping of organic semiconductors has important advantages for organic light-emitting diodes (OLEDs), organic solar cells (OSCs) and organic thin-film transistors (OTFTs). Previous studies on doping of organic semiconductors showed that the addition of strong electron donors or acceptors as dopants can generate extra electrons or holes, respectively. This leads to a shift in Fermi level and significantly improves film conductivity. However, the design of n-type dopants is considerable challenging due to the requirement of a high HOMO level of the dopant, making these dopants unstable in ambient condition. We developed and synthesized a new benzoimidazole derivative as a strong n-type dopant for fullerene C60, a well-known n-channel semiconductor. The co-evaporated thin film showed a maximum conductivity of 5.6 S/cm at a 8.0wt% doping concentration, which is the highest reported value for molecular n-type conductors to our knowledge. o-MeO-DMBI-I can be stored and handled in air for extended periods without degradation, thus it is promising for various organic electronic devices.
3:45 AM - *J8.4
Polarized Phosphorescent Organic Light-emitting Devices Adopting Mesogenic Host-guest Systems
Chung-Chih Wu 1 Yu-Tang Tsai 1 Su-Hao Liu 1 Ming-Shiang Lin 1 Li-Yin Chen 1 Chih-Hung Tsai 1 You-Heng Lin 1 Anurach Poloek 2 Yun Chi 2 Chien-An Chen 3 Shaw H Chen 3 Hsiu-Fu Hsu 4
1National Taiwan University Taipei Taiwan2National Tsing-Hua University Hsin-Chu Taiwan3Rochester University Rochester USA4Tamkang University Taipei Taiwan
Show Abstract
The polarization control of light is of particular importance for optical information processing devices, such as displays, optical storage, and stereoscopic imaging systems etc. For instance, linearly polarized electroluminescence (EL) devices would be useful for backlights of liquid-crystal displays (LCDs). With polarized backlights, the displays can be made â?ogreenerâ? (i.e. more power-efficient) and simplified by removing polarizers needed for conventional non-polarized backlights and the optical losses it induced. As such, there had been substantial efforts in developing polarized organic light-emitting devices (OLEDs). Among them, linearly polarized OLEDs adopting mesogenic polyfluorenes/oligofluorenes together with appropriate molecular alignment techniques are some distinctive examples. S.H. Chen et al. also reported the use of liquid-crystalline oligofluorenes as hosts to guide molecular orientation of guest emitters, forming the polarized host-guest emitting systems permitting wide tuning of emission colors, including white emission. The previous efforts in polarized OLEDs were mainly on fluorescent OLEDs. Yet, the development of OLEDs in recent years has largely shifted toward phosphorescent OLEDs, since phosphorescent OLEDs could effectively utilize both singlet and triplet excitons and realize ideally 100% internal quantum efficiencies. As such, it is of both scientific and technical importance to explore the possibility of achieving polarized phosphorescent OLEDs. In this paper, we will present an initial attempt to realize workable and functional polarized phosphorescent OLEDs by using a recently developed mesogenic phosphorescent metal (Pt(II)) complex, which is combined with mesogenic oligomer hosts to form the corresponding mesogenic host-guest emitting systems. Both the host and guest molecules in the host-guest films were successfully aligned by using the rubbed conducting polymer as the alignment layer. With such aligned host-guest films, polarized red-emitting phosphorescent OLEDs were successfully implemented, showing a maximal brightness exceeding 2000 cd/m2.
4:15 AM - *J8.5
Near-unity Internal Quantum Efficiency Hybrid Organic and Quantum Dot Light Emitting Devices (QLEDs)
Peter Kazlas 1 Jonathan S Steckel 1 John Linton 1 Sridhar Sadasivan 1 John Ritter 1 Charles Hamilton 1 Robert Nick 1 Craig Breen 1 Benjamin S Mashford 1 Zhaoqun Zhou 1 Matthew Stevenson 1 James Perkins 1 Seth Coe-Sullivan 1
1QD Vision Lexington USA
Show AbstractQuantum dot (QD) semiconductor nanocrystals have been considered in a broad range of applications, from biological tagging to LEDs, lasers, displays, lighting and solar cells. In their photoluminescent mode of operation, QDs are currently in lighting products, and have the promise to be in liquid crystal display products in the near future. In electroluminescent mode, quantum dot light emitting devices (QLEDs) are an emerging class of thin-film hybrid organic-inorganic structures that can potentially achieve best-in-class performance amongst large-area emissive light sources. This paper will describe recent advancements in device design and materials performance that has enabled QLEDs to reach near-unity internal quantum efficiency. In addition, we will discuss printing methods that have enabled the creation of active matrix QLED displays that demonstrate the beneficial properties of QLED pixels.
J9: Displays I
Session Chairs
Wednesday PM, April 11, 2012
Moscone West, Level 2, Room 2008
4:45 AM - J9.1
3D amp; 2D Photopatterning of Highly Luminescent Polysilafluorene Conjugated Polymers: Lasing and Electroluminescent Devices
Jeffrey J McDowell 1 Moez Haque 2 Laden Abolghasemi 2 Florian Maier-Flaig 3 Carsten Eschenbaum 3 Tobias Bocksrocker 3 Jeffrey Castrucci 4 Peter R Herman 2 Uli Lemmer 3 Zheng-Hong Lu 4 Geoffrey Ozin 1
1University of Toronto Toronto Canada2University of Toronto Toronto Canada3Karlsruhe Institute of Technology Karlsruhe Germany4University of Toronto Toronto Canada
Show AbstractSuccessful commercialization of emerging display technologies is increasing dependant on reduced material costs and processing. Polymer based light emitting diodes are considered low cost alternative to small molecule counterparts. Polymer solutions can be made into thin films over relatively large areas without need for heating or high vacuum. Displays require creation of addressable subpixels consisting of at least one of each primary colour (red, green, blue) and the resolution of the ensemble must be high enough to render an image amicable to the human eye (< 50 µm). The work presented in this talk details our methods of making polychromic pixelated polymer displays with minimal steps. Our process began with the invention of a photopatternable polysilafluorene (PSF) electroluminescent polymers synthesized via a low cost route with narrow solid state emission in the deep blue. PSF is a suitable host for numerous higher wavelength emitters. Solutions of PSF with epoxidized vinyl side groups are loaded with photoacid and spun cast to make transparent, smooth films varying in thickness from 10 nm to several microns. Photopatterning occurs with single photon absorption of UV light through a shadow mask or the more versatile two photon absorption of 545 nm laser light via a direct laser writing (DLW) system. The latter method is used to produce three dimensional light emitting structures with submicron resolution. PSF shows stimulated emission in both solution and thin films. Observation of amplified spontaneous emission and lasing from photocrosslinked films on a diffraction grating demonstrate PSF can be used in combination with DLW to produce resonator cavities with sufficient gain for lasing. Our initial DLW efforts have focused on log pile and whispering gallery mode microtoroid resonators; the latter have extremely high Q factor (> 10^7) which could potentially reduce lasing thresholds to the limits of electrical excitation.
5:00 AM - *J9.2
Recent Technical Approaches for Flexible Displays Using New Flexible Substrate Material
Chang-Dong Kim 1 Soo-Young Yoon 1 Juhn-Suk Yoo 1 Weon-Seo Park 1 JongGeun Yoon 1 Myungchul Jun 1 Yong-Kee Hwang 1
1Ramp;D Center, LG Display Paju Republic of Korea
Show AbstractFlexible display is believed as an innovative device which can change our living mode in the future. Therefore many researchers focus many attentions on the key technology for realization of flexible displays. Recent technology advances show the possibility of building with real flexible display which has small package, low weight and design freedom and make active matrix organic light emitting diode (AMOLED) and electrophoretic display attractive in replacing existing glass display. To make real flexible displays, all display components should be replaced by the new flexible parts and processes to make real flexible display. Recently, soluble polyimide (PI) is introduced as a flexible substrate due to its several advantages such as simple processes for coating and curing, good chemical resistance for processing. Additionally, this new flexible substrate can be easily adopted on the current mass production factory. In this paper, we will discuss on flexible display as an emerging display, and review the recent technical approaches and highlight results of flexible display using soluble polyimide material as a new flexible substrate.
5:30 AM - *J9.3
Electroactive, Vibrantly Colored, Processable Conjugated Polymers in Display and Window Type Devices
John R. Reynolds 1
1Georgia Institute of Technology Atlanta USA
Show AbstractElectrochromic materials are potentially useful for color-changing displays and dynamic windows with the active materials varying from transition metal oxides, ionic metal complexes, conjugated organic molecules, and conjugated polymers. With their relatively fast switching rates, mechanical flexibility, low redox potentials, and highly transmissive oxidized states, conjugated polymers provide a suite of properties desirable for both application types. Our group has created a set of conjugated polymers with a complete color palette that spans the visible spectrum with neutral state colors that range from yellow, orange, red, magenta, green, cyan, blue, and black. Their high level of solubility allows for processing using a variety of methods including roll-to-roll coating, inkjet printing, and spray-coating. This presentation will outline our efforts to develop advanced characterization methods for measuring the electrochemical, spectroscopic, and colorimetric properties of these polymers, including monitoring color properties during dynamic switching, along with rapid spectral recording for definition of switching speeds. Using electrochromic devices (absorptive/transmissive and absorptive/reflective types) as a characterization platform, we demonstrate how these colored-to-transmissive switching polymers behave in regards to color, spectra, switching speed, lifetime, durability, and bistability.
J7: Lighting II
Session Chairs
Wednesday AM, April 11, 2012
Moscone West, Level 2, Room 2008
9:00 AM - *J7.1
Flexible Organic Light-Emitting Diodes for Lighting Applications
Ton van Mol 1 Joanne Wilson 1 Stephan Harkema 1 Paul Blom 1
1Holst Centre/TNO Eindhoven Netherlands
Show AbstractOrganic light-emitting diodes (OLEDs) have the potential to revolutionize the lighting market. For lighting applications OLEDs made on flexible foils have a number of beneficial properties, such as free form factor, tunable color, being difficult to break compared to glass, having a large area emitting surface, and being potentially very efficient and low cost. This presentation will give an overview of the key technological developments that are required in order to realize such devices. Highlights from our work will be presented, including devices on plastic and metal foil substrates with solution processed layers deposited using our R2R line, inclusion of metal grids and high conductivity PEDOT for ITO-free OLED devices, and state-of-the-art thin-film flexible barrier performance.
J10: Poster Session: Plastic Solar Cells
Session Chairs
Wednesday PM, April 11, 2012
Marriott, Yerba Buena, Salons 8-9
9:00 AM - J10.1
Correlating Phase Transitions with Thermal Annealing Temperatures for P3HT:PCBM Organic Photovoltaic Devices
Andrew Pearson 1 Paul Hopkinson 2 Tao Wang 1 Athene Donald 2 David Lidzey 1
1University of Sheffield Sheffield United Kingdom2University of Cambridge Cambridge United Kingdom
Show AbstractWe have studied a range of organic photovoltaic devices (OPVs) based on a thin-film blend of P3HT and PCBM, and show that by comparing device studies with a characterisation of the thermal transitions of the blend, we can provide a mechanistic description of the minimum temperature required to improve device efficiency. For as-cast P3HT:PCBM blend thin-films we evidence two glass transition temperatures corresponding to the existence of two compositionally different amorphous states. We demonstrate that an improvement in device efficiency only occurs once the film has been heated above the upper apparent glass transition temperature of the blend. If annealing is performed above the optimum temperature, excessive phase-separation and a partial reduction in film optical density leads to a general decrease in device efficiency. Both of these characteristic temperatures are dependent upon the composition of the blend. The temperature-dependent competition between such processes therefore opens a â?~windowâ?T within which device efficiency can be optimised and provides an opportunity to design effective annealing protocols for future polymer:fullerene blend OPVs.
9:00 AM - J10.11
Comparison of Nanoporous and Nanoparticulate ZnO Films of Similar Roughness for Dye Sensitized Solar Cell Applications
Constance Magne 1 2 Victoire-Marie Guerin 1 Thierry Pauporte 1 Jiri Rathousky 3 Tangui Le Bahers 1
1CNRS-Chimie ParisTech Paris France2Saint-Gobain Recherche Aubervilliers France3Academy of Sciences of the Czech Republic Prague Czech Republic
Show AbstractWe present a comparative study of two different ZnO porous film morphologies for DSC fabrications. Nanoparticulate ZnO were prepared by the classical doctor-blade technique starting from a paste containing ZnO nanoparticles. Nanoporous ZnO films were grown by a dye-assisted electrochemical growth technique. The film thicknesses were adjusted at similar roughness of about 300 in order to permit a worthy comparison. The effects on the cell performances of sensitization by dyes belonging to three different families, namely, xanthene (eosin Y) and indoline (D102, D131, D149 and D205) organic dyes as well as a ruthenium polypyridine complex (N719), have been investigated. It is shown that the mesoporous electrodeposited matrix exhibits significant morphological changes upon the photoanode preparation that yield to a dramatic increase in the layer internal specific surface area. In the case of indoline dyes, better efficiencies were found with the electrodeposited ZnO porous matrices compared to the nanoparticulate ones in spite of significantly shorter electron lifetimes measured by impedance spectroscopy. The observation is interpreted in terms of much shorter transfer time in the oxide in the case of the electrodeposited ZnO films. Among the tested dyes, the D149 and D205 indoline organic dyes were found the most interesting.
9:00 AM - J10.13
Competition between Charge-transfer Exciton Dissociation and Direct Photocarrier Generation in Molecular-scale Heterojunctions
Jun'ya Tsutsumi 1 Toshikazu Yamada 1 Hiroyuki Matsui 1 Tatsuo Hasegawa 1
1National Institute of Advanced Industrial Science and Technology Tsukuba Japan
Show AbstractDonor-acceptor interface plays a crucial role in the electron-hole charge separation process of organic photovoltaic cells (OPCs). It has been demonstrated in the last decade that efficient conversion of photo-generated excitons to the geminate electron-hole pairs becomes possible with donor-acceptor bulk heterojunction which is a percolation structure of the donor and acceptor phases. On the other hand, it is known that the ultimately-ordered donor-acceptor interface is realized in a series of molecular donor-acceptor compounds, or the charge-transfer (CT) complexes. For example, an alternating mixed stack of donor and acceptor molecules is formed to afford a rich variety of narrow-gap organic molecular semiconductors whose gap energy can be tuned between 0.5 and 2 eV, depending on the combination of ionization energy of donors and electron affinity of acceptors. The materials should provide a model system for investigating the microscopic photovoltaic processes in OPCs which have not been disclosed yet. Here we report fabrication and photovoltaic (PV) characteristics of prototypical single crystal PV cells based on a typical mixed-stack CT complex of dibenzotetrathiafulvalene (DBTTF) with tetracyanoquinodimethane (TCNQ). We used functional organic metal electrodes for selective hole- and electron-ejections to fabricate metal-insulator-metal (MIM)-type single-crystal PV diode. It was found that the cell presents clear rectification and photovoltaic characteristics above hν > 0.8 eV [1]. High-spatial-resolution laser beam-induced current (LBIC) technique was used to investigate the interfacial charge-separation and diffusion characteristics of the CT excitons in the cell. The measurement allowed selective detection of hole and electron photocurrents through the metalâ?"semiconductor interfaces. We observed an exceptionally long diffusion length in the LBIC photocurrent profiles; this length was as much as 20 μm at excitation photon energy of above 1.3 eV, which can be ascribed to direct generation of photocarriers. Note that the exciton diffusion length of usual organic semiconductors is in a range of a few tens of nm. We discuss that the readiness to undergo CT exciton dissociation and the long diffusion characteristics of photocarriers, which should illuminate fundamental aspects of photoelectric conversion in the OPCs. [1] J. Tsutsumi et al., Phys. Rev. Lett. 105, 226601 (2010).
9:00 AM - J10.14
Hybrid ZnO/Low-band Gap Polymer Photovoltaic Devices
Jason J Amsden 1 2 Insun Park 2 Jun-Mo Park 2 Ki-Young Yoon 2 Jun Young Kim 1 Raja Shahid Ashraf 4 Iain McCulloch 4 TaeLim Choi 2 Changhee Lee 1 Klaus Mullen 3 Do Yeung Yoon 2
1Seoul National University Seoul Republic of Korea2Seoul National University Seoul Republic of Korea3Max Plank Institute for Polymer Research Mainz Germany4Imperial College London United Kingdom
Show AbstractHybrid photovoltaic devices composed of semiconducting nanocrystal acceptors and organic polymer donors are an intriguing alternative to traditional organic photovoltaic devices based on polymers and C61-butyric acid methyl ester (PCBM). Semiconducting nanocrystals such as CdSe, TiOx, and ZnO have several potential advantages including relatively high electron mobility, tunable band gap, and various morphologies. In particular, ZnO is an attractive possibility due to its environmentally friendly solution processing and non-toxicity. However, to date, hybrid photovoltaic device performance lags far behind that of devices using PCBM as the electron acceptor due to morphology. Furthermore, devices using ZnO nanoparticles as an acceptor and poly[2-methoxy-5-(3�,7�-dimethyloctyloxy)-p-phenylene vinylene](MDMO-PPV), a relatively large band-gap polymer as a donor have significantly better performance than those using regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) as an acceptor despite P3HT having a lower band gap and better overlap with the solar spectrum.(1-3) In this presentation, I will discuss the reason for this performance discrepancy and include recent results using additional low band gap polymers such as poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b�]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT),(4) poly[N-9�-heptadecanyl-2,7-carbazole-alt-5,5-(4�,7�-di-2-thienyl-2�,1�,3�-benzothiadiazole)] (PCDTBT),(5) Poly[2,7-(9,9-di-octyl-fluorene)-alt-4,7-bis(thiophen-2-yl)benzo-2,1,3-thiadiazole] (PFO-DBT),(6) Poly[2,7-(9,9-bis(2-ethylhexyl)-dibenzosilole)-alt-4,7-bis(thiophen-2-yl)benzo-2,1,3-thiadiazol] (PSiF-DBT),(7) Silaindacenodithiophenes,(8) and A,B-Alternating Poly(arylenevinylene) Copolymers.(9) (1) Park, I.; Lim, Y.; Noh, S.; Lee, D.; Meister, M.; Amsden, J. J.; Laquai, F.; Lee, C.; Yoon, D. Y. Org. Electron. 2011, 12, 424. (2) Beek, W. J. E.; Wienk, M. M.; Janssen, R. A. J. Adv. Funct. Mater. 2006, 16, 1112. (3) Beek, W. J. E.; Wienk, M. M.; Janssen, R. A. J. Adv. Mater. 2004, 16, 1009. (4) Mühlbacher, D.; Scharber, M.; Morana, M.; Zhu, Z.; Waller, D.; Gaudiana, R.; Brabec, C. Adv. Mater. 2006, 18, 2884. (5) Blouin, N.; Michaud, A.; Leclerc, M. Adv. Mater. 2007, 19, 2295. (6) Zhou, Q. Appl. Phys. Lett. 2004, 84, 1653. (7) Wang, E. Appl. Phys. Lett. 2008, 92, 033307. (8) Ashraf, R. S.; Chen, Z.; Leem, D. S.; Bronstein, H.; Zhang, W.; Schroeder, B.; Geerts, Y.; Smith, J.; Watkins, S.; Anthopoulos, T. D.; Sirringhaus, H.; de Mello, J. C.; Heeney, M.; McCulloch, I. Chemistry of Materials 2010, 23, 768. (9) Choi, T.-L.; Han, K.-M.; Park, J.-I.; Kim, D. H.; Park, J.-M.; Lee, S. Macromolecules 2010, 43, 6045.
9:00 AM - J10.15
Toward High Efficient TiO2 Nanodendrite Array - P3HT Heterojunction Solar Cells Based on Interfacial Modification Using a Metal-free Organic Dye
Wen-Pin Liao 1 Shu-Chien Hsu 1 Jih-Jen Wu 1
1National Cheng Kung University Tainan Taiwan
Show AbstractConjugated polymer-based photovoltaic (PV) devices have attracted increasing attention in the past decade due to solution processability and easy fabrication. The fabrication and photovoltaic performance of the TiO2 nanodendrite (ND) array â?" poly(3-hexylthiophene) (P3HT) heterojunction solar cells will be reported in the presentation. The aligned rutile TiO2 nanorod (NR) array was first synthesized on the FTO substrate by hydrothermal method and the branches of the TiO2 nanodendrites (ND) were then formed using an aqueous chemical bath deposition (CBD). TEM characterization reveals that the successful growth of TiO2 ND is attributed to the presence of rutile {101} twinning structure. Significant enhancement of the cell efficiency is achieved by interfacial modification using metal-free organic dye, D149. With the TiO2 film thickness of 1.5 μm, the D149-modified TiO2 ND array/P3HT solar cell possesses an efficiency of 3.12%. The charge separation and recombination in the cells are investigated by time-resolved photoluminescence (TRPL) and impedance spectroscopy. The mechanism for photocurrent generation in the D149-midified TiO2 ND/P3HT solar cells will be discussed in the presentation.
9:00 AM - J10.16
Enhanced Charge Extraction in Inverted Hybrid Photovoltaic Cells Assisted by Graphene Nano-flakes
Yun-Ming Sung 2 Fang-Chi Hsu 1 Di-Yan Wang 3 I-Sheng Wang 3 Chia-Chun Chen 3 Hsueh-Chung Liao 4 Wei-Fang Su 4 Yang-Fang Chen 2
1National United University Miaoli Taiwan2National Taiwan University Taipei Taiwan3National Taiwan Normal University Taipei Taiwan4National Taiwan University Taipei Taiwan
Show AbstractWe use graphene nanoflakes (GNFs) to greatly enhance the charge extraction out of photoactive blend in inverted hybrid poly(3-hexythiophene):(6,6)-phenyl C61 butyric acid methyl ester (P3HT:PCBM)/ZnO-nanorod photovoltaic cells. Instead of a continuous film, solution processed GNFs with dimensions less than 200 nm Ã- 200 nm are homogeneously scattered on top of the well-aligned ZnO-nanorods. Those GNFs play key roles, including to serve as an electron drain to collect electron flow out to ZnO-nanorods, enhance the carrier mobility of the device and promote holes to drift toward the surface in contact with the anode. As a result, there is a large enhancement in photocurrent and photovoltage of 35% and 27%, respectively, leading to an improved cell efficiency by up to about 100%.
9:00 AM - J10.17
The Optical and Electrical Properties of Organic Solar Cells with Gold Nanoparticles Incorporated in the Active Layer
Charlie Wang 1 Wallace C.H. Choy 1 Chunhui Duan 2 Dixon Fung 1 Wei Sha 1 Fengxian Xie 1 Fei Huang 2 Yong Cao 2
1the University of Hong Kong Hong Kong Hong Kong2South China University of Technology Guangzhou China
Show AbstractBulk heterojunction polymer solar cells are one of the very promising candidates for photovoltaic devices due to the efficient charge transfer from conjugated polymers to fullerene derivatives. Compared with inorganic solar cells, however, one important hindrance for the efficiency improvement of PSCs is the limited light absorption due to the thin active layer limited by the short exciton diffusion length and low carrier mobility. In this work, we will demonstrate the impact of the incorporation of monofunctional poly(ethylene glycol) (PEG)-capped Au NPs into the active layer of polymer blend. A newly synthesized polymer poly[2,7-(9,9-dioctylfluorene)-alt-2-((4-(diphenylamino) phenyl)thiophen-2-yl)malononitrile] (PFSDCN) is used as the donor of the active layer. The improvement of open-circuit voltage (VOC), short-circuit current (JSC), fill factor (FF) are all obtained with an appropriate amount of Au NPs incorporated into the active layer. In order to understand the improvement, we experimentally and theoretically investigate the effects of LSPR introduced by Au NPs on the optical properties of PSCs, particularly the PSC active layer. We also experimentally and theoretically study the effects of Au NPs on PSC electrical properties. Our results show that due to the interesting feature of the strong lateral distribution of LSPR near field along the active layer, light absorption is enhanced by incorporating Au NPs into the active layer. The understanding can be applied to other metallic NPs incorporated organic solar cells. Meanwhile, our results show that electrical properties can counter-diminish the optical enhancement from LSPR which reduces the overall performance improvement. It is important that both optical and electrical properties need to be studied and optimized simultaneously. Our results show that after optimization, power conversion efficiency can be improved by ~32%. The study therefore contributes to better understanding the uses of Au NPs for enhancing PSC performances.
9:00 AM - J10.18
SXPS Characterization of a Merocyanine / Fullerene Hetero Junction for Organic Solar Cells: Growth Behavior and Occurrence of a High Photovoltage
Eric Mankel 1 Philip Reckers 1 Corinna Hein 1 Thomas Mayer 1 Wolfram Jaegermann 1
1Technische Universitaet Darmstadt Darmstadt Germany
Show AbstractNovel absorber molecules are investigated for high efficient organic solar cells. Especially merocyanines provide high photovoltages as donor materials in photoactive organic hetero junction cells. The electronic alignment and the growing behavior of a merocyanine (donor) / fullerene (acceptor) hetero junction is presented. Fullerene was stepwise evaporated onto a 30nm thick merocyanine layer. After each deposition step synchrotron induced photoelectron spectra were taken in situ using the the TGM7 dipole beamline and the SoLiAS (Solid Liquid Analysis System) end station at the electron storage ring Bessy II in Berlin. During the first deposition steps the fullerene molecules diffuse into the merocyanine layer. After some steps intercalation pauses and the growth of a pristine fullerene layer starts. The band diagram of the heterojunction was determined considering the two different growth regimes. The merocyanine / fullerene HOMO offset is determined to be 850meV leading to a high LUMO offset, the driving force for exciton splitting. In situ bias illumination of the interface with 150W/m2 of a halogen lamp leads to a reversible shift of the C60 emission features, indicating a high photovoltage. The height of the photovoltage depends on the fullerene layer thickness. It starts at 200mV for submonolayer coverage and increases up to approximately 1V for coverages of some ten nanometers. Reversing the experiment by stepwise deposition of merocyanine on a 30nm thick fullerene layer leads to no intercalation and no photovoltage showing that the sequence of the layer deposition has a high impact on the functionality of the solar cell device.
9:00 AM - J10.2
Stabilization and Photovoltaic Performance of 3D Surface Engineered Si-ncs upon DC Microplasma Treatment in Liquid
Vladimir Svrcek 1 Davide Mariotti 2 Toshihiro Yamanari 1 Koiji Matsubara 1 Michio Kondo 1
1National Institute of Advanced Industrial Science and Technology (AIST) Tsukuba, Japan2University of Ulster belfast United Kingdom
Show AbstractRecent progress in silicon nanocrystals (Si-ncs) research and in the understanding of carrier multiplication phenomena has highlighted significant potential implications for the development of next generation solar cells. In this context, the control of surface characteristics and the capability of tuning Si-ncs surface properties are crucial research aspects. The combination of quantum effects due to the small size with the enhancement of the surface area offers the possibility of tuning Si-ncs properties not only through the core of the material but also through accurate engineering of the surface. Furthermore, an accurate three dimensional (3D) control of Si-ncs surface characteristics is fundamental to achieve high-performance devices. Indeed, Si-ncs that were stabilized by organic ligands have shown promising results, however â?obulkyâ? and long organic molecules cannot provide adequate surface coverage (~ 60 %) of the small Si-ncs and as a result the carrier transport within the device is compromised. In this contribution we discuss how an alternative approach that uses an atmospheric-pressure DC microplasma in liquid medium can be efficiently used for 3D-enginering and stabilization of Si-ncs optoelectronic properties. Induced non-equilibrium liquid/Si-ncs surface chemistry and efficient replacement of hydrogen terminations with oxygen-/organic-based terminations is the key factor. In particular, the microplasma treatment drastically enhances (x 10) the silicon nanocrystals photoluminescence (PL) properties. Enhanced PL properties are stable after several days of storage in either ethanol or water. Furthermore, we show that 3D surface engineering of Si-ncs directly in liquid media significantly impacts in the overall solar cell performance. Our investigation shows a very promising approach to achieve enhanced hybrid solar cells performance without using any surfactants. In particular, we demonstrate that 3D surface engineering of Si-ncs by microplasma processing in liquid medium can be used to enhance the electronic interactions with conjugated polymers such as with polythieno[3,4-b]thiophenebenzodithiophene (PTB7). Our results suggest that surface engineering and the PTB7/Si-ncs coupling is electronically favorable for exciton dissociation and carrier generation/transport. We illustrate that bulk-heterojunction cells consisting of Si-ncs embedded in PTB7 show clear improvement after Si-ncs surface engineering. Hybrid bulk heterojunction solar cells show relatively good rectification behavior, improved short circuit current (ISC) and a very high open circuit voltage (VOC) exceeding 1 V. The results confirm that the PTB7 and 3D surface engineered Si-ncs with quantum confinement effects via atmospheric-pressure DC microplasma in liquid medium could be a promising low-cost and environmental friendly technology for preparation of new class of energy materials and for their integration in high-performance solar cell devices.
9:00 AM - J10.20
Efficient Organic Solar Cells with a Spray-coated Active Layer
Wanyi Nie 1 Robert Coffin 1 Jiwen Liu 1 Yuan Li 1 David Carroll 1
1Wake Forest University Winston Salem USA
Show AbstractWe investigated spray deposition of the absorbing layer of bulk heterojunction solar cells comprised of poly[4,8-bis(1-pentylhexyloxy)benzo[1,2-b:4,5-bâ?~]dithiophene-2,6-diyl-alt-2,1,3-benzoxadiazole-4,7-diyl and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM). Specifically we examined, host solvents and substrate temperatures. Interestingly, we find that spray deposition leads to higher efficiency solar cells than is observed for spin-coating using the same host solvent. Spraying the donor and acceptor blend from 1,2-dichlorobenzene in on to a 80 °C substrate results in an average power conversion efficiency of 5.4% (max.=5.8%) compared with 3.5 % achieved spin-coating from the same solvent. This high efficiency is observed despite AFM topography images revealing a very rough surface indicative of spray coating. We observe a low fill factor consistent with previous reports of spray deposition, however, high short circuit current and open circuit voltage compensate for this.
9:00 AM - J10.22
Variation of the Photocurrent Spectra Due to Energy Dependent Hole Mobility in Organic Bulk Hetero-junction Solar Cells
Buddika Abeyweera 1 Bruce W Alphenaar 1
1University of Louisville Louisville USA
Show AbstractA detailed study of the photocurrent spectra of organic bulk heterojunction solar cells is presented, providing insight on the energy dependent factors limiting photo-charge capture. Increasing the thickness of the active layer in an MDMO-PPV / PCBM solar cell reduces the magnitude of the photocurrent due to the low mobility of the photogenerated holes. Measurements show that the photocurrent reduction is predominately due to loss in carriers generated at the polymer absorption maximum, while the low energy response is relatively unaffected. In a thick enough sample, the low energy response (1.5-2 eV) dominates, and a photocurrent peak is no longer observed at the main absorption maximum (2.6 eV). The normal photocurrent spectral response can be made to return by applying bias to the sample. The main photocurrent peak increases by a factor of 5 under a bias of -1V compared to the low energy response. A similar effect is observed for samples with thinner active layers, although the change is less pronounced. The results imply that hole transport is blocked for carriers generated in the polymer compared to those generated in the PCBM. The difference can be attributed to the relatively low energy of the holes generated in the polymer, caused by the PCBM / polymer band offset. This work was funded by Dept. of Energy (Grant No. DEFG02-07ER46375) and National Science Foundation (GrantNo.NSF-DMR-0906961).
9:00 AM - J10.23
Quantitative Nanoorganized Structural Evolution for High Efficient Bulk Heterojunction Polymer Solar Cell
Hsueh Chung Liao 1 Yu-Tsun Shao 2 Meng-Huan Jao 1 Tsung-Han Lin 1 Sheng-Yong Chang 2 Cheng-Si Tsao 3 U-Ser Jeng 4 Yang-Fang Chen 2 Wei-Fang Su 1
1National Taiwan University Taipei Taiwan2National Taiwan University Taipei Taiwan3Institute of Nuclear Energy Research Taoyuan Taiwan4National Synchrotron Radiation Research Center Hsinchu Taiwan
Show AbstractWe have developed an improved small-angle X-ray scattering (SAXS) modeling and analysis methodology to quantitatively evaluate the nanostructures of a blending bulk heterojunction (BHJ). This method was first applied to resolve the various structures of self-organized poly(3-hexylthiophene) /C61-butyric acid methylester (P3HT/PCBM) thin active layer in solar cell from the studies of both grazing-incidence small-angle X-ray scattering (GISAXS) and grazing-incidence X-ray diffraction (GIXRD). Tuning the various length scales of PCBM-related structures by different annealing process is a typical processing which help for better understanding the enhanced power conversion of P3HT/PCBM solar cell (from 1.2% to 3.8%). The quantitative structural characterized by this method includes (1) the mean size, volume fraction and size distribution of aggregated PCBM clusters, (2) the specific interface area between PCBM and P3HT, (3) the local cluster agglomeration,(4) the correlation length of PCBM moleculesâ?T network within P3HT phase and (5) the specific surface area of PCBM cluster which can all be well-correlated to the device performance with different thermal history. The in-situ GISAXS measurements were also conducted to provide informative details of the temporal evolution of PCBM-related structures during phase separation. This work established a useful SAXS approach to present an insight into the modeling of the morphology of P3HT/PCBM film. Based on the developed model, simultaneous GISAXS and GIWAXD technique was performed to characterize a distinctive nanostructure induced by the newly developed strategy: embedding optimized amount (2.8x10-3wt%) of inorganic nanoparticles into the P3HT:PCBM BHJ film. Beside the typical process of thermal annealing, it is demonstrated for the first time that the P3HT:PCBM BHJ nanostructure can be manipulated by including inorganic nanoparticle copper disulfide (Cu2S) or Cadmium selenide (CdSe), leading to the promising improved power conversion efficiency from 3.5% to 4.3% (23% improvement). To sum up, the results of this investigation significantly extend the current knowledge of relationship of BHJ morphology to device performance. Benefited from the established characterization technique, the effect of nanoparticle-tuned self-organization of BHJ can be tailored which successfully provides an alternative strategy to optimize the nanostructure that is beneficial for roll-to-roll large area processing.
9:00 AM - J10.24
High Efficiency and High Photo-stability Zinc-phthalocyanine Based Planar Heterojunction Solar Cells with a Double Interfacial Layer
Tae-Min Kim 1 Ji Whan Kim 1 Hyun Sub Shim 1 Jang-Joo Kim 1
1Seoul National University Seoul Republic of Korea
Show AbstractMetal-phthalocyanines (MPcs) and fullerene (C60) are widely used as donor and acceptor molecules, respectively, for small molecular planar heterojunction organic photovoltaic cells (OPVs). However, MPc/C60 based small molecular planar heterojunction photovoltaic cells show low power conversion efficiencies (ηp) of 1~2% and short lifetimes. Moreover, MPc/C60 planar heterojunction solar cells showed the characteristics of photo-degradation under the illumination of light even for a short time range in a nitrogen environment. Here, we present that the use of MoO3 and CuI as a double interfacial layer improves the ηp value and the photo-stability in ZnPc based solar cells at the same time. The device without the interfacial layer resulted in low efficiency and low photo-stability. We used CuI as a template layer to enhance the absorption of the ZnPc film. The absorption coefficient of the ZnPc film increased 1.6 times when thermally deposited on a CuI layer. Resultingly, the OPV with a CuI interfacial layer increased the efficiency significantly to 3.3%. However, the photo-stability was lowered even further. In order to improve the photo-stability, a MoO3/CuI double interfacial layer was used. The double interfacial layer retains the role as a template layer and acts a buffer layer between ITO and organic layer. The device with the double interfacial layer shows high efficiency and very much improved photo-stability at the same time. The TOF-SIMS analysis revealed that the decomposition of CuI and the diffusion of Cu is the origin of the photo-degradation of the device. Insertion of the MoO3 layer between the ITO and CuI prevents the decomposition of CuI to Cu and the diffusion of Cu under UV illumination. Our results indicate that the selection of materials for anode engineering and the combination of interfacial layers are important to improve the efficiency and stability of the planar heterojunction solar cells.
9:00 AM - J10.25
Textured Film for Efficient Light Trapping in Thin-film Organic Photovoltaic Cells
Changsoon Cho 1 Hoyeon Kim 2 Ji-Won Seo 1 Seunghyup Yoo 2 Jung-Yong Lee 1
1Korea Advanced Institute of Science and Technology Daejeon Republic of Korea2Korea Advanced Institute of Science and Technology Daejon Republic of Korea
Show AbstractProper light trapping schemes are crucial in increasing the power conversion efficiency (PCE) of organic photovoltaic (OPV) cells, which typically have low absorption efficiency due to their thin active layers. Although many studies on nanostructured photoactive layers have been done for efficient light trapping, the enhancements have not been sufficient. Furthermore, structured active layers are often subject to electrical defects and thus can complicate the manufacturing process. On the other hand, structuring performed on the external surface of the substrates do not affect the electrical properties of the devices, yet can significantly enhance the PCE of the devices by improving the chance of photon absorption. Our study indicates that ideal Lambertian scattering on the external surface of a glass substrate can result in 28.2% absorption enhancement in the archetypal P3HT:PCBM60-based OPV cells. In this work, we investigate the influence of various micron-scale substrate-texturing methods and geometries on the performance of OPV cells. In particular, simulation based on both geometrical ray tracing and wave-optical transfer-matrix formalism is employed in combination for quantitative analysis of OPV cells with the textured substrates. Among the various structures we studied, a one-dimensional periodic array of V-groove with a vertex angle of 90 degree is predicted to yield approximately 14% absorption enhancement in P3HT:PCBM60-based OPV cells. This agrees relatively well with our experimental results showing ca. 8% improvement in short-circuit current density with both open circuit voltage and fill factor unaffected. Experimental details and the strategies for further optimization will also be provided.
9:00 AM - J10.26
Direct Formation of Inorganic Nanocrystals in P3HT for Photovoltaic Applications
Kiran Jain 1 V. Agrawal 1 S. Chand 1
1National Physical Laboratory Delhi India
Show AbstractInorganic-organic nanocomposite or hybrid solar cells have attracted researchers due to the unique possibilities of tuning inorganic semiconductor nanoparticles in size and shape. One requisite for high photovoltaic efficiencies is the purity of the inorganic semiconductor. That means that in the active layer no stabilizer or capping layer should be present. To solve this problem, research was carried out on the development of in-situ preparation methods of inorganic metal sulfides directly in the organic semiconductor matrix. In this contribution we present novel copper and indium xanthates as precursors which are soluble in apolar organic solvents and are thus compatible with common conjugated polymers used in organic photovoltaics. However, the major advantage of this procedure is that the conversion of the precursor into CdS /PbS takes place below 140°C resulting in a dense network of the inorganic particles directly in the conjugated polymer matrix. By using P3HT as polymer and CdS:polymer and CdPbS/polymer composite was prepared in situ. The synthesis temperatures and compositions were optimized for high efficieny. The formation process as well as the morphology of the nanocomposite layer was analyzed by complementary spectroscopic and microscopic methods.
9:00 AM - J10.27
Side-chain Modified Quarterthiophene Derivative Enhances Device Performance of Organic Solar Cells
Hyojung Cha 1 Tae Kyu An 1 Chan Eon Park 2 Soon-Ki Kwon 2 Yun-Hi Kim 1
1Pohang University of Science and Technology Pohang Republic of Korea2Gyeongsang National University Jinju Republic of Korea
Show AbstractA conjugated polymer as a donor material, poly(3,4â?Tâ?Tâ?T-di(decylthiophenyl) quaterthiophene) (PDTQT), containing of decylthiophenyl side chains on the polymer backbone, was synthesized for use in organic photovoltaic cells. The crystallinity of poly(quaterthiophenes) (PQTs) was remarkably reduced by substituted-decylthiophenyl side chain of quaterthiophene backbone units. A well-interpenetrated nano-scale morphology of PDTQT:PC71BM blended active layer arose from this modification, which increased the power conversion efficiency relative to devices based on highly crystalline PQTs. Bulk heterojunction solar cells based on PDTQT:PC71BM blends with a 1:4 weight ratio annealed at 120°C presented the best photovoltaic performances, with a high short-circuit current density (Jsc) of 9.8 mAcm2, an high open-circuit voltage (Voc) of 0.91 V, a fill factor (FF) of 0.36 and a high power conversion efficiency (PCE) of 3.2 % under illumination of AM 1.5 with light intensity of 100 mWcm2. PDTQT:PC71BM are characterized by UV-visible absorption and Photoluminescence spectra, AFM and TEM images, and GIXD patterns. The charge transport characteristics and morphology studies indicated that an optimized interpenetration network composed of PDTQT: PC71BM could be achieved in a blend ratio of 1:4 annealed at 120°C. Decylthiophenyl side chain substitution appears to be an effective strategy for high device performance of organic photovoltaic cells.
9:00 AM - J10.28
Interfacial Engineering of Metal Oxide Semiconductor for the Improved Efficiency and Stability of Organic Bulk-heterojunction Solar Cell
Gyoung Seok Hwang 1 Dai Gun Yoon 1 Byung Doo Chin 1
1Dankook University Youngin Republic of Korea
Show AbstractThe interface between photo-active layer and charge extraction layer is critically important for a better design of the polymer-fullerene solar cell with improved power conversion efficiency and device stability. Significant progress has been made in the performance of organic bulk heterojunction solar cells through the development of active layer materials, while detailed experimental studies on how to minimize interfacial charge collection losses are not systematically performed yet. Comparing the devices with thin layer of PEDOT:PSS and semiconducting p-type tungsten oxide (WO3) inserted between the active organic layer (poly(3-hexyl-thiophene):1-(3-methoxy-carbonyl)propyl-1-phenyl-(6,6)C61 - P3HT:PCBM) and tin-doped indium oxide (ITO) anode, 3.2% and 2.8% of power conversion efficiency was obtained, respectively. Such a limited performance of charge collection at WO3 interface might be explained by the relatively low conductivity behavior and interfacial composition mismatch. However, in case of the inverted devices (ITO/titanium oxide (TiOx)/P3HT:PCBM/PEDOT:PSS or WO3/Ag), efficiencies were almost comparable each other (~3.0%). Although PEDOT:PSS was slightly modified with small amount of additive hydrophobic solvents, inferior interfacial adhesion of PEDOT:PSS on the active layer limits the efficient hole extraction. Upon using of the CW3 as hole collection layer, operational stability under AM 1.5 illumination was significantly improved both for normal and inverted devices. For a further improvement of efficiency and stability, we have designed various condition of bulk heterojunction by the surface-mediated self assembly and contact transfer process. Vertical phase separation of P3HT:PCBM blends were adjusted by the templating method on top of silicon oxide with methacryloxypropyltrimethoxysilane (MPS) and octadecyltrichlorosilane (OTS) modified surface, followed by the transfer process. Relationship between the surface functionality of metal oxides (TiOx, WO3) and the interfacial composition of bulk heterojunction transfer layer was investigated in detail.
9:00 AM - J10.3
Post-Deposition Dipping Method for Improving the Electronic Properties of a Narrow Bandgap Conjugated Polymer
Yeong Don Park 1 Jin Kuen Park 2 Kilwon Cho 3 Guillermo Bazan 2
1University of Incheon Incheon Republic of Korea2University of California, Santa Barbara Santa Barbara USA3POSTECH Pohang Republic of Korea
Show AbstractWe have synthesized poly[(4,4-didodecyldithieno[3,2-b:2',3'-d]silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl] (P1), a class of narrow bandgap conjugated polymers currently under investigation for use in polymer solar cells. Herein, we present a systematic study of the simple physical method to facilely eliminate low MW part, and simultaneously to improve the molecular structure and electrical properties in a spin-coated P1 thin-film by immersing it into a poor solvent for a few minutes. The molecular structures of conjugated polymer films thus prepared can be translated to different levels of order and the low MW component from pristine P1 thin-film can be removed facilely by dipping method. FETs with improved performance therefore can be easily fabricated. Correlation of nanoscalar structural features and electrical property allowed us to determine the appropriate dipping time and how small MW influences electronic properties. This method is also straightforward and obviously useful in practical polymeric device fabrication.
9:00 AM - J10.30
Shell Thickness Dependent Photoinduced Hole Transfer in Hybrid Conjugated Polymer/Quantum Dot Nanocomposites: From Ensemble to Single Hybrid Level
Zhihua Xu 1 Mircea Cotlet 1 Corey R Hine 2 Mathew M Maye 2
1Brookhaven National Lab Upton USA2Syracuse University Syracuse USA
Show AbstractSemiconducting organic/inorganic nanocomposites are appealing optoelectronic materials due to the possibility of combining desired properties from both components in one, thus creating new properties that may not be present in each individual material. In particular, blends of conjugated polymers (CPs) and semiconductor quantum dots (QDs) have attracted interests for applications such as light-emitting diodes (LEDs), photovoltaics (PV), and biosensing. The interaction between the excited states of a CP and a QD, which determines the optoelectronic properties of the resulting hybrid material, needs to be fully understood. Photoinduced hole transfer (HT) from QD to CP which is an essential process in achieving high power conversion efficiency in PV cells, but a detrimental process for applications such as LEDs and biosensing, remains little understood. Here, we investigate photoinduced HT from a CdSe/ZnS core/shell QD with varying shell thickness to a water-soluble conjugated polymer poly(9,9â?²-bis(6-N,N,N-trimethylammoniumhexyl)fluorene-alt-phenylene (FHQ). For this donor/acceptor hybrid, the emission peaks of FHQ (450nm) and QD (634-638nm) are well separated, enabling to selectively excite only the QD in the hybrid, so that HT from photoexcited QD to CP can be probed exclusively by monitoring the steady-state and transient photoluminescence (PL) decay from QD. In this work, HT in FHQ/QD composite is investigated both in solution and solid thin films by ensemble-based PL measurement. Furthermore, single particle spectroscopy is used to resolve the dynamics and heterogeneity of HT at single CP/QD interface and to probe the effect of this process on the PL blinking dynamics of QDs.
9:00 AM - J10.31
Spray-coating Technique for the Realization of Polymer Solar-cells
Gianpaolo Susanna 1 2 Luigi Salamandra 1 2 Thomas Brown 1 2 Andrea Reale 1 2 Francesca Brunetti 1 2 Aldo Di Carlo 1 2
1University of Rome Tor Vergata Rome Italy2CHOSE-Center for Hybrid and Organic Solar Energy Rome Italy
Show AbstractOrganic solar cells have gained in the last years a growing interest within the research community. Among all organic solar cells, small molecule and polymer based technology are very promising. In particular, polymer bulk-heterojunction based technology has shown wide margins of improvement in terms of energy conversion efficiency, reaching values above 8% (certified by NREL), and uncertified 9%, actually doubling the performance in the last two years [1-4]. Despite the low efficiency, when compared with the conventional inorganic solar cells, the potential of a Roll-to-Roll (R2R) process and the low-cost large-area definition on flexible substrates, make the BHJ-SCs technology an interesting and economic solution for the production of energy. The solar cell is built up in a multilayer structure in which the deposition of the layers can be performed with different techniques like for example casting, spin-coating, screen and ink-jet printing [5]. Recently conventional spray-coating was introduced as a large area deposition method for the layers of organic solar cells (OSCs) [6]. In this context, we present polymer solar cells that use as active layer P3HT:PCBM, in which we replaced the standard spin-coating deposition of the active layer and of the hole transporting layer (PEDOT:PSS) with the spray-coating technique, reaching power conversion efficiency above 4% [6]. In these cells a fine tuning of the realization parameters such as the solvent used, the substrate temperature, the film thickness, time of spray, distance between sample and airbrush, substrate temperature has been performed. Furthermore different cells architectures are presented as function of the required characteristics of the substrate and of the electrode deposition. These results point out the spray-coating as a powerful printing technique to be embedded in a R2R process for the realization of flexible organic photovoltaic devices. References [1] C.J. Brabec et al. Organic Photovoltaics: Materials, Device Physics, and Manufacturing Technologies, Wiley-VCH, Weinheim (2008). [2] Konarka Power Plastic (www.konarka.com) [3] Solarmer (www.solarmer.com) [4] J.Nelson, Materials Today, 14 (2011), pp. 462-470 [5] F.C. Krebs et al. Sol. Energy Mater. Sol. Cells 93 (2009), pp. 1968â?"1977. [6] G.Susanna et al. Sol. Energy Mater.Sol Cells, 93 (2011), pp. 1775-1778.
9:00 AM - J10.34
Revealing Bulk Heterojunction Blend Morphology by Spectroscopic Ellipsometry
Sebastian Engmann 1 Vida Turkovic 1 Gerhard Gobsch 1 Harald Hoppe 1
1University of Technology Ilmenau Ilmenau Germany
Show AbstractThe nanoscale morphology of bulk heterojunction solar cells is very crucial for their performance. Hence, a lot of effort has been invested to study and control it. In order to elucidate the nanoscale morphology, many different complex techniques have been applied. Recently, TEM-tomography has been used to portray the structure of the polymer-fullerene active layer in all three dimensions. In this work we used another non-destructive method, in order to gain information about the predominant morphology and the donor-acceptor distribution within P3HT:PCBM blend films. We demonstrate that spectroscopic ellipsometry can be used to investigate the spatial composition over the film thickness and provides detailed information about the shape and extension of phase separation. The shape of nano-inclusions of the fullerene phase within the polymer matrix, as well as the concentration profile of the both components was determined. Morphological aging was investigated to trace the change in the concentration profile initiated by surface-directed spinodal decomposition of the film.
9:00 AM - J10.36
Systematic Variation of Poly(thienylene vinylene) Molecular Weight and Its Effect on the Phase Behavior, Morphology, and Performance of Bulk Heterojunction Photovoltaic Cells
Bryan D. Paulsen 1 Josh C Speros 2 Elizabeth A Jackson 2 Scott P White 3 Marc A Hillmyer 2 C. Daniel Frisbie 1
1University of Minnesota Minneapolis USA2University of Minnesota Minneapolis USA3University of Iowa Iowa City USA
Show Abstract
Polymer/fullerene bulk heterojunction (BHJ) photovoltaic cells are continually proving themselves to be an attractive route to potentially inexpensive solar energy conversion. Molecular weight is among the most important properties governing polymer behavior, and in the rapidly developing field of polymer based solar cells, understanding molecular weight effects is necessary to maximizing device efficiency and realizing commercialization. The trend of increased performance with increasing molecular weight is generally accepted, yet not rigorously understood. Here, semicrystalline poly(3-hexadecyl-2,5-thienylene vinylene) (P3HDTV), with a low band gap of 1.65 eV, was synthesized by acyclic diene metathesis polymerization with tuned chain length (Mn = 6 kg/mol to Mn = 33 kg/mol). Neat P3HDTV samples with systematically varied molecular weight were characterized by size-exclusion chromatography, UV-Vis spectroscopy, differential scanning calorimetry, and wide-angle X-ray scattering. This characterization revealed dramatic increases in polymer ordering and aggregation leading to the appearance of liquid crystal behavior at high molecular weight. The molecular weight dependent hole mobility of neat P3HDTV was investigated in field effect structures. Hole mobility was observed to increase over two orders of magnitude, from the 3.1 ± 0.5 Ã- 10-5 to 8.0 ± 0.3 Ã-10-3 cm2/V s, with increasing molecular weight. P3HDTV was blended with the electron acceptor methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM) to form the BHJ photovoltaic cell active layer. Completed cells were characterized with diode measurements in the dark and under simulated solar illumination, and with monochromatic external quantum efficiency (EQE) measurements. BHJ cells were optimized for active layer thickness and P3HDTV/PCBM composition. Cells incorporating high molecular weight P3HDTV readily showed PCE of ~1%. In a series of 100 nm thick, constant composition devices, increasing molecular weight produced large increases in cell short circuit current (Jsc) and power conversion efficiency (PCE), with both increasing by a factor of two. The improved hole transport and solar cell performance observed with increasing molecular weight was attributed to improved level of polymer ordering, which using DSC and UV-Vis was shown to be persistent in the nano-segregated P3HDTV/PCBM BHJ blends even at extreme PCBM loading. Finally, combining the aforementioned characterization techniques with atomic force microscopy imaging, the thorough temperature-composition phase diagram was mapped in order to relate solar cell performance with composition, phase behavior, and morphology.
9:00 AM - J10.37
Solution-processed Organic Solar Cells from Dye Molecules: An Investigation of Diketopyrrolopyrrole:Vinazene Heterojunctions
Bright Walker 1 2 Xu Han 3 Chunki Kim 2 Alan Sellinger 3 Thuc-Quyen Nguyen 2
1Ulsan National Institute of Science and Technology Ulsan Metropolitan City Republic of Korea2University of California at Santa Barbara Santa Barbara USA3Stanford University Palo Alto USA
Show AbstractAlthough one of the most attractive aspects of organic solar cells is their low cost and ease of fabrication, the active materials incorporated into the majority of reported bulk heterojunction (BHJ) solar cells include a semiconducting polymer and a fullerene derivative, classes of materials which are both typically difficult and expensive to prepare. In this study, we demonstrate that effective BHJs can be fabricated from two easily synthesized dye molecules. We find that solar cells incorporating a diketopyrrolopyrrole (DPP) as a donor and a dicyanoimidazole (VinazeneTM) acceptor function as an active layer in BHJ solar cells, producing a relatively high open circuit voltage of up to 1.23 V and power conversion efficiencies (PCEs) of 1%.
9:00 AM - J10.38
Hybrid Bulk-heterojunction Solar Cells with Ultra-thin Titania Nanosheet as an Electron Buffer Layer
Eiji Itoh 1 Yasutake Maruyama 1 Katsutoshi Fukuda 2
1Shinshu Univ. Nagano Japan2Kyoto University Uji Japan
Show AbstractThe n-type wide-gap semiconductors, such as titanium oxide and zinc oxide are used preferably because they are deposited by solution based technique, and their electronic and optical features suitable for the electron-extracting/injection buffer layer due to their excellent electron withdrawing and transport properties and the exciton and hole blocking natures. Recently, Sasaki et al. developed a set of transition metal oxides with 2 dimensional structure (called â?onanosheetâ?) including titania. The nanosheet has several important features: (i) ultimate two-dimensionality with a thickness of ~1nm and a lateral size ranging from sub-micrometers to 10 micrometers, (ii) high crystallinity and well-defined composition, and (iii) novel and enhanced physical properties due to the quantum size effect. For example, the band gap of titania nanosheet (TN) (~3.8eV) is much larger than that of conventional anatase titania (~3.2eV) which is favorable for solar cell application. Moreover, the exfoliated TN can be deposited by low temperature process, such as electrophoresis, Langmuir-Blodgett technique, and the layer-by-layer (LBL) deposition of the diluted dispersion of aniodic nanosheet in pure water and polymer based polycations(PDDA) which is suitable for fabricating uniform, nano-controlled TN ultra-thin film. In this study, we have fabricated the hybrid bulk-heterojunction solar cells with ultra-thin TN as an electron buffer layers. We have investigated both "normal" and "inverted" cell consisting of ITO/NiO (or MoO3)/P3HT:PCBM/TN/Al and ITO/TN/P3HT:PCBM/MoO3/Ag structures. Only one or two nanosheet (1.5~3nm thick) layer markedly suppress the leakage current and improved both VOC and FF in inverted cells. That is we could obtain the pin-hole free ultra-thin nanosheet film by the insertion of ultra-thin TN film deposited by layer-by-layer technique. The reduction in FF and the current density above VOC for the device with thicker TN layers are ascribed to the interfacial potential barrier at ITO/TN interface and the series resistance across the multilayers of TN and PDDA. The FF is markedly improved by the insertion of anatase TiO2 layer between ITO and TN resulting in the improved efficiency of 2.7%. In normal cell, on the other hand, the contact barrier is essentially low compared with inverted cell. The hydrophobic organic active layer was covered by the repeated TN deposition and the improved efficiency was successfully obtained by the insertion of TN layer as an electron buffer layers. The detail will be discussed in the symposium.
9:00 AM - J10.39
Hybrid Organic-Inorganic Photovoltaic Devices with Composite Nanomesh Transparent Electrode on Clear Plastic Substrates
Nima Mohseni Kiasari 1 Bobak Gholamkhass 1 Saeid Soltanian 1 Frank Ko 2 Peyman Servati 1
1The University of British Columbia Vancouver Canada2University of British Columbia Vancouver Canada
Show AbstractOrganic photovoltaic (OPV) devices are amenable to low-cost roll-to-roll production on plastic substrates using solution processing at low temperature. The photoactive layer is in form of a blend of acceptor polymers and donor fullerene with nanometer scale domains that promote efficient exciton dissociation and charge transport to respective electrodes. However, the power conversion efficiency (PCE) and stability of the device are a direct function of segregation of acceptor and donor polymer domains during operation of OPV. In addition, most OPV devices are based on costly and brittle indium tin oxide (ITO) coated glass, which leads to a high cost and incompatibility to flexible plastic substrates. This work presents our latest results on OPV devices based on blends of poly (3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) fabricated on clear plastic substrates, with composite nanomesh transparent electrodes, having high-performance nanostructured ZnO hole blocking layers. In comparison to conventional ITO, the nanomesh electrode (deposited by electrospinning process) provides the required mechanical flexibility for plastic substrate, low sheet resistance (<50 Ohm/sq) and high transparency (>90%). A nanowire ZnO hole-blocking layer is deposited by a combination of electrochemical solution deposition and transfer of chemical vapour deposition grown nanowires, which provides the selectivity for electrons in an inverted OPV structure. The nanostructure ZnO layer also provides a vertically penetrating contact layer in contact to the photoactive layer, acting as electron selective branches for improved electron transport and quantum efficiency. In addition, we believe the nanostructured morphology of the ZnO layer serve as an inorganic mechanical grid for stabilizing the nanoscale morphology of the photoactive layer, hence delaying the phase segregation and improving stability. The device is finished by hole collecting electrode based on higher work function metals (e.g., silver), providing improved stability in comparison to Al based layers. Preliminary results display a PCE in excess of 2% for this ITO-free hybrid organic-inorganic PV device on plastic substrate. We have systematically investigated the effect of thickness, roughness, doping and crystal quality of ZnO layer on the performance of OPV devices. Thicker ZnO films initially improve the performance due to increased surface area, but when the thickness increases beyond 50 nm, it suppresses the performance due to the low conductivity of the contact layer, increased loss of high energy photons, and reduced open circuit voltage possibly due to the parasitic role of ZnO:P3HT heterojunctions.
9:00 AM - J10.4
Performance Enhancement in Organic Solar Cells Using a Grating Electrode
Hoi Lam Tam 1 Qingyi Yang 1 Gui Xin Li 1 Furong Zhu 1
1Hong Kong Baptist University Kowloon Tong Hong Kong
Show AbstractLight trapping is a promising approach to improve light absorption in organic solar cells by scattering the incident light to oblique angle and increasing the dwell time of the light inside the devices. In this work, the absorption enhancement in organic solar cells is realized using a poly(methyl methacrylate) (PMMA)/indium-tin-oxide (ITO) grating electrode. Photovoltaic properties such as external quantum efficiency and power conversion efficiency of a set of structural identical cells made with a planar ITO anode (control device) and a PMMA/ITO grating electrode were analyzed. The grating incorporated organic solar cells resulted in a 13% increase in short circuit current density (Jsc) as compared to the planar device. The results reveal that over 60% of enhancement in Jsc is due to an enhanced spectral responsivity via the light scattering effect. Our experiment results agree with FDTD simulation in showing that the light coupled into the waveguide mode in the active layer is mainly responsible for the absorption enhancement in the devices. This was contributed by the PMMA/ITO grating electrode.
9:00 AM - J10.41
Study the Effect of Surface Velocity Recombination Ratio and Mobility of the IV Curve of Bilayer Organic Devices
Seyyed Sadegh Mottaghian 1 Mahdi Farrokh Baroughi 1
1South Dakota State University Brookings USA
Show AbstractThis paper studied the effect of different Surface Velocity Recombination rates (SVR) at both back and front contacts and electron, hole mobilities in the bulk of material on the IV curve of bilayer organic solar cells. A comprehensive charge transport model was developed to quantitatively study the role of each parameters in the organic and hybric organic-inorganic devices. SVRs and mobilities are used as sweep parameters for simulation to investigate their effect on the shape of illuminated IV in bilayer organic solar cell. These simulation results along with the experimental results from the relevant literatures will be utilized to understand the physics behind the illuminated-IV shape in the bilayer organic solar cells.
9:00 AM - J10.43
X-shaped pi;-extended Conjugated Molecules for Organic Field-effect Transistors and Organic Photovoltaics
Jicheol Shin 1 Hyun Ah Um 1 Kyung Hwan Kim 1 Tae Wan Lee 1 Dong Hoon Choi 1
1Korea University Seoul Republic of Korea
Show AbstractOrganic semiconducting materials based on Ï?-extended linear-conjugated systems have been very intriguing and significant development has been achieved in these materials over the one decade. Accordingly, a number of researchers have attempted to synthesize Ï?-conjugated small molecules, dendrimers, oligomers, and polymers because of their strong potential applications to electronics and optoelectronics. Organic semiconductors have recently attracted much attention for applications in organic electronics such as organic field-effect transistors (OFETs), organic photovoltaic (OPV) cells, and nano-/meso-/microscopic object devices. In this work, we synthesized a new anthracene based Ï?-conjugated X-shaped molecules for use as soluble p-type organic semiconductors. The UV-visible absorption spectroscopy, PL spectroscopy, cyclic voltammetry, and thermal analyses were performed for understanding their fundamental physical properties. Eventually, the molecules were employed to fabricate thin film transistors, single-crystal FETs, and OPV. The device properties were investigated in detail.
9:00 AM - J10.5
Zwitterion-modified ITO as the Cathode for Highly-efficient Inverted Polymer Solar Cells
Kuan Sun 1 Jianyong Ouyang 1
1National University of Singapore Singapore Singapore
Show AbstractPolymer solar cells (PSCs) are regarded as the next-generation solar cells, but their poor stability has been severely impeded their application. The stability can be significantly improved by adopting an inverted architecture. The inverted structure can give rise to the improvement in the photovoltaic efficiency as well. The major challenge in developing inverted PSCs is to effectively lower the work function of ITO. The methods to lower the work function of ITO strongly affect the photovoltaic performance. Here, we report a novel method to lower the work function of ITO by coating a thin layer of zwitterions. Inverted PSCs with zwitterion-modified ITO as cathode exhibit photovoltaic efficiency comparable to that with normal device architecture.
9:00 AM - J10.7
Nanoimprinted Polymer Solar Cells: Effects of Nanostructure Geometries
Yi Yang 1 Kamil Mielczarek 2 Anvar Zakhidov 2 1 Walter Hu 3 1
1University of Texas at Dallas Richardson USA2University of Texas at Dallas Richardson USA3University of Texas at Dallas Richardson USA
Show AbstractIn recent years, research progress in organic photovoltaic (OPV) devices has been advanced tremendously, driven by the potential for low cost, large area and flexible devices. As an important member in the OPV family, the polymer:fullerene solar cells draw the most research interest due to the relatively high power conversion efficiency (PCE) achieved when compared to others. The nanoscale morphology in polymer:fullerene based photovoltaic devices is a key factor determining the overall device performance. The short exciton diffusion length in combination with the discrete and randomly distributed phases have caused significant charge recombination and thus lowered the efficiency. In our work, we demonstrate a nanoimprint process which allows the formation of ordered and continuously interdigitized morphology in active layers for both efficient charge separation and collection. Through the fabrication of imprinted poly(3-hexylthiophene) (P3HT):fullerene solar cells with different feature sizes, we comprehensively study the geometric effect of nanostructure on device performance which demonstrates that the PCE is highly dependent on the density and aspect ratio of nanostructures. Besides P3HT, similar work is extended to a low band gap polymer poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-bâ?²]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT):fullerene solar cells to study the process window of nanoimprint for polymer solar cells.
9:00 AM - J10.8
Effects of Nanostructure Geometries on Nanoimprint Induced P3HT Chain Orientation
Yi Yang 1 Anvar Zakhidov 2 1 Walter Hu 3 1
1University of Texas at Dallas Richardson USA2University of Texas at Dallas Richardson USA3University of Texas at Dallas Richardson USA
Show AbstractConjugated polymers have drawn lots of research interest as cost effective functional materials for organic electronic devices such as solar cells, light emitting diode and field effect transistors. In organic electronics, not only the overall crystallinity of the conjugated polymers is important, but also the conductivity for each direction is equally critical in determining the charge transport. It has been found that crystallization and chain alignment with controlled orientations can be realized in imprinted poly(3-hexylthiophene) (P3HT) nanostructures which lead to higher hole mobilities. Although this nanoiprint lithography induced chain alignment in P3HT has been widely observed, there are contradictory results reported in literature for the exact alignment direction within the imprinted P3HT nanostructures, which make it difficult for device structure design. One possible answer to these conflicting results might come from the different nanostructure geometries used in these reported works. Therefore to solve this issue, we have characterized imprinted P3HT nanostructures with different feature sizes using grazing incidence X-ray diffraction (GIXRD) and found that chain orientation is determined by the aspect ratio. The effect of residual layer thickness on chain alignment direction is also discussed.
9:00 AM - J10.9
Solution-processable Metal Oxide Materials for Optimized Tunnel Junction Layers in Multijunction Architecture Organic Photovoltaics
Sarah R Cowan 1 Andres Garcia 1 Dana Olson 1
1National Renewable Energy Laboratory Golden USA
Show AbstractThe ability of metal oxides to selectively collect and block charge makes them ideal candidates for recombination layers in organic tandem solar cells. The tunnel junction layer, which electrically connects cells in the multijunction stack, fulfills multiple design parameters: (1) optical transparency, (2) â?ometallicâ? electrical contact of the quasi-Fermi levels of one cell to the next, and (3) efficient charge recombination of cell current to prevent voltage and power loss. Leakage between the mechanically connected front and back cells (i.e. inefficient recombination at the tunnel junction) results in a reduction in potential, and hence, power conversion efficiency. This study utilizes doped metal oxides (ZnO, NiO, MoO3) to tune the work function of tunnel junction layers for charge selectivity and efficient recombination at the interlayer, toward optimized multijunction organic photovoltaics.
9:00 AM - J9.10
SXPS Characterization of an In situ Solution Processed Bulk Heterojunction Model System
Julia Maibach 1 Eric Mankel 1 Thomas Mayer 1 Wolfram Jaegermann 1
1Technische Universitaet Darmstadt Darmstadt Germany
Show AbstractThe electronic properties of wet processed absorber molecules for organic solar cells are investigated in situ by synchrotron induced photoemission spectroscopy (SXPS). As a model system for a typical bulk heterojunction we used α,Ï?-Dihexylsexithiophene (α,Ï?-DH6T) as soluble and evaporable donor and Phenyl-C61-butyric acid methyl ester (PCBM) as typical acceptor system. The aim of the work is the comparison of the solution processed and the evaporated materials. The experiments were conducted at the SoLIAS (Solid Liquid Interface Analysis System) end station at the electron storage ring BESSY II in Berlin. This system offers the possibility to deposit thin films from solution under inert atmosphere and directly transfer the sample into the UHV analysis system. The films investigated were deposited from chlorobenzene solution on sputter-cleaned ITO substrates via drop-casting under clean Argon atmosphere. Firstly, both materials were characterized as pure films from solutions with a concentration of 2 mol/l. The influence of the solvent on the electronic properties was explored in more detail by comparison of α,Ï?-DH6T films prepared by both thermal evaporation and drop casting. In a second step a blend of PCBM and α,Ï?-DH6T was prepared. Therefore the solutions were mixed in a 1:1 ratio by volume prior to the drop casting. However, the obtained spectra of the composite film show a dominance of α,Ï?-DH6T. This could be caused by a segregation of the two materials during the drying process and thus leading to a α,Ï?-DH6T-rich surface layer similar to the thiophene-rich surface layer observed in P3HT-PCBM bulk heterojunctions.
J7: Lighting II
Session Chairs
Wednesday AM, April 11, 2012
Moscone West, Level 2, Room 2008
9:30 AM - *J7.2
High Performance Multiphoton-emission OLEDs
Junji Kido 1
1Yamagata University Yonezawa Japan
Show AbstractThe recent development of high performance OLEDs will be discussed. High quantum efficiencies can be obtained by using phosphorescent emitters such as iridium complexes. We synthesized a variety of wide gap, or high T1 level, materials for host and charge-transporting layers and succeeded to fabricate extremely high efficiency OLEDs. In particular, electron-transporting materials with pyridine side groups exhibited high electron mobility due to hydrogen bonding that controls the orientation of the molecules. High external quantum efficiency (QE) of 25â?"30 percent was achieved for blue, green and red OLEDs, which correspond to the internal QE of nearly 100 percent. By using multiphoton-emission (MPE) structures, a green phosphorescent OLED exhibited an internal QE of 220 percent. Recentry, we developed the MPE devices with two emitting units by spin-coating. The charge generation layers were composed of MoO3 which were vacuum-evaporated. The MPE device exhibited doubly higher EQE than that of conventional unstacked solution-processed single OLED device.
10:00 AM - *J7.3
Highly Efficient OLED Panel for a General Lighting Application
Sehwan Son 1 Yeon K Lee 1 Minchoon Park 1 Yong S Ahn 1 Jungdoo Kim 1 Munkyu Joo 1 Yun H Ham 1 Jaemin Moon 1 Minsoo Kang 1
1LG Chem Daejeon Republic of Korea
Show AbstractWe present current status of OLED lighting devices developed at LG Chem. All of the devices were fabricated using combination of blue fluorescent and yellow phosphorescent materials to emit white color at high brightness level, 3,000 nit. We applied an internal light extraction structure, having a thin form factors, to the devices. High power efficiency up to 60 lm/W, T70 device lifetime over 10,000 hours at 3,000 nit were achieved using 10x10 cm2 one-pixel panels. Only commercially viable processed and cell structures were applied to these panels.
10:30 AM - J7.4
Fully Solution-processable Multilayer Green Organic Light-emitting Diodes Incorporating Crosslinkable Bipolar Hosts and Iridium-(III)-Complexes
Georgios Liaptsis 1 Klaus Meerholz 1
1University of Cologne Cologne Germany
Show AbstractReducing manufacturing costs of organic light-emitting diodes (OLEDs) is a challenging issue. A key to cost reduction is solution processing instead of vacuum techniques. To realize highly efficient multi-layered OLEDs from solution we introduce oxetane-moieties to our small molecule precursors. This allows for crosslinking e.g. via photo-initiated cationic ring-opening polymerization (CROP) yielding an insoluble network. Consequently, we are able to deposit complex multi-layer stacks to produce highly efficient OLEDs. Here, we introduce a new series of crosslinkable bipolar host materials for triplet emission based on well-known spirobifluorene derivatives. The spiro-configured carbon atom decouples the two pi-systems responsible for hole- or electron conduction both, spatially and electronically. This decreases intramolecular donor-acceptor interactions, yielding high triplet energy levels and high conductivity. Further, we establish novel crosslinkable Iridium-(III)-complexes for highly efficient green emitting OLEDs. Incorporating this host-guest system into a well balanced injection and charge-transporting layer stack consisting of hole- and electron-transporting layers, we are able to fabricate state-of-the-art OLEDs entirely from solution.
10:45 AM - J7.5
Efficient Solution-processed Tandem-OLEDs with Fully Recovered Currents
Anne Koehnen 1 Richard C Shallcross 1 Klaus Meerholz 1
1University of Cologne Cologne Germany
Show AbstractOrganic electronics is receiving much attention as an alternative cost-effective technology compared to the traditional inorganic ones. Especially research activities concerning organic light-emitting diodes (OLEDs) are enforced in university groups as well as industry since they are promising candidates for next generation displays and solid-state lighting. To achieve high brightness and efficiency at low current density so-called Tandem-OLEDs (vertically stacked devices with two or more electroluminescent units connected in series via a charge generation layer (CGL)) have been introduced about ten years ago. Mostly these Tandem-OLEDs have been realised by evaporation of organic materials. Only few publications report on solution-processed electroluminescent units since depositing further materials can lead to the washing-up of already deposited ones. However, though various device structures have been investigated, only current efficiency enhancements could be shown since the operating voltages of the Tandem OLEDs are still high than their respective single unit references. Here, we present solution-processed polymeric Tandem-OLEDs (Tandem-PLEDs) realized by thermal "crosslinking" of the OLED materials. By optimizing the CGL, the current of the Tandem-PLEDs could be fully recovered with respect to the single-unit device. Therefore, the two-unit devices show high current and power efficiencies at the same time. Compared to the single-unit device an enhancement of almost 90% in current efficiency and 70% in power efficiency was obtained for singlet-emitting Tandem-PLEDs with yellow emission (max. efficiencies: 17 Cd/A and 10 lm/W). Currently, we are extending this approach to white-emitting PLEDs.
11:30 AM - J7.6
Extremely Efficient Flexible Organic Light-emitting Diodes Using Graphene Anode
T. H Han 1 Y. Lee 2 3 M. R Choi 1 S. H Woo 1 S. H Bae 2 3 B. H Hong 4 J. H Ahn 2 3 T. W. Lee 1
1Pohang University of Science and Technology (POSTECH) Pohang, Gyungbuk Republic of Korea2SKKU Advanced Institute of Nanotechnology (SAINT) and Center for Human Interface Nano Technology (HINT) Seoul Republic of Korea3Sungkyunkwan University Suwon Republic of Korea4Seoul National University Seoul Republic of Korea
Show AbstractThe application of graphene sheets as transparent anode is limited due to its relatively low work function (WF) (~ 4.4eV) and conductivity compared with that of conventionally used ITO anode (~4.7 â?¤ WF â?¤ 4.9 eV and 10 ohm/sq) in optoelectronic devices. The relatively low work function of graphene makes the hole injection unfavorable because of high injection barrier at interface between anode and organic layers in organic light-emitting diodes (OLEDs). We modified graphene anode via p-doping with HNO3 and AuCl3 to improve conductivity of graphene sheets and our polymeric hole injection layer (GraHIL) which has gradient work function to makes holes be injected easily to the organic layer despite high injection barrier for holes at interface between graphene anode and organic layer. In this paper, we fabricated flexible fluorescent and phosphorescent OLEDs by modifying the graphene anode to have a high work function and high conductivity, and thus achieved extremely high luminous efficiency (37.2 lm/W in fluorescent OLEDs, 102 lm/W in phodphorescent OLEDs), which is significantly higher than that of the conventionally optimized device using an ITO anode (24.1 lm/W in fluorescent OLEDs, 85.6 lm/W in phosphorescent OLEDs). We also demonstrated that when our GraHIL is introduced on top of graphene anode, the nearly ohmic contact is formed between graphene anode and organic layers by using Dark Injection Space Charge Limited Current (DI SCLC) transient measurement. Futhermore, DI SCLC measurement exhibhits that the hole injection efficiency of GraHIL with graphene anode is higher than that with ITO anode by avoidance of metal diffusion from ITO into the organic layers. Finally, we fabricated flexible white OLEDs lighting devices using the modified graphene anode.
11:45 AM - J7.7
High Efficient Green Organic Light Emitting Diodes with Distribution Bragg Reflector Microcavity
Chaoyu Xiang 1 Wonhoe Koo 1 Franky So 1
1University of Florida Gainesville USA
Show AbstractIntroducing DBR microcavity into OLEDs is an effective way to enhance light out-coupling and color purity. OLEDs show better efficiency with double emitting layers. By combining the DBR and double emitting structure, we achieved very high efficient green organic light emitting diodes (OLEDs). The noncavity devices containing double emitting layers with a green phosphorescent emitter, fac-tris(2-phenylpyridine) iridium [Ir(ppy)3], doped in 4,4â?T,4â?-tris(N-carbazolyl)-triphenyl triphenyl amine (TCTA) and 4,4â?T-(N-carbazolyl)-biphenyl (CBP), respectively, show a high efficiency as 93cd/A. An enhancement by a factor of 2.4 is achieved with DBR microcavity, resulting in 225 cd/A in current efficiency. We found that such a high enhancement of the current efficiency in the microcavity device comes from two parts. While the number of photon outcoupled into air was partially increased by 30% with the microcavity effect, the major enhancement was caused by the better match of the emission wavelength to the highest eyeâ?Ts sensitivity in the photopic vision function corresponding to 550 nm. Finally, with a hemisphere lens, additional improvement by a factor of 2.4 in the normal direction was achieved due to extraction of substrate mode. The total enhancement of our devices in the normal direction is calculated by a factor of 5.7.
12:00 PM - J7.8
Carborane Based Optoelectronic Materials for High Performance Blue PHOLEDs with High Thermal and Electrochemical Stabilities
Soonnam Kwon 1 Kyung R Wee 1 Sang O Kang 1
1Korea University Chochiwon Republic of Korea
Show AbstractThe development of optoelectronic materials satisfying high electrochemical and thermal stability is urgent for commercial application of blue phosphorescent organic light emitting devices (PHOLEDs). We have developed a series of new types inorganic/organic hybrid materials which meet the specification for the realization of blue PHOLEDs. As part of our ongoing research effort, we have turned our attention to three-dimensional aromatic molecules, namely, cluster carboranes and found that depending on geometry of carboranes highly desirable optophysical properties for blue PHOLEDs were identified. Thus, we present our recent results in which carborane clusters not only function as organic linker but also facilitate otherwise unattainable only with organic entities, thermal and electrochemical stabilities. The first rationale development of 3-D network molecules interweaving carboranes with functional organic units has been demonstrated, and their application to blue PHOLEDs has also been achieved. In addition, we found that carborane clusters effectively localized triplet wave-function in each functional organic unit attached and delocalized charges across entire molecular network, resulting in high triplet energy and high charge mobility. Most of all, when new carboranes were engaged as HTL and EML materials, they reached to high T1 of 2.9, 3.0 eV with enhanced hole mobility of around 10-3 cm2/Vs bearing high Tg of 120 °C, 140 °C, respectively. Optimized devices showed EQE of 19% with deep blue color coordinates of (0.15, 0.23) and enhanced device lifetime. Detailed analysis on the photophysical property of each 3-D network structure produced by carboranes (o-, m-, and p-carboranes) will be elaborated by DFT calculations.
12:15 PM - J7.9
Green Tandem Organic Light-emitting Diodes with Reduced Efficiency Roll-off
Dong Woo Song 1 Cephas Small 1 Do Young Kim 1 Franky So 1
1University of Florida Gainesville USA
Show AbstractIn recent years, researches for tandem organic light-emitting diodes (OLEDs) have been widely performed for providing high luminance with enhanced current efficiency or for controlling emission spectra with units emitting different colors [1,2]. Despite the importance of charge generation units (CGUs) for operating tandem OLEDs, there are only a few in-depth studies on the CGUs in tandem OLEDs [3-5]. In this presentation, we report a series of studies on charge generation in CGUs and demonstrate efficient green phosphorescent tandem OLEDs with reduced efficiency roll-off. For the charge generation study, we have fabricated inverted diodes with CGUs and a control inverted diode without any CGU, and analyzed their diode I-V characteristics. The inverted diodes had the following structure: ITO/tris[3-(3-pyridyl)-mesityl]borane (3TPYMB)/CGU/1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC)/Al. The CGU consisted of Cs2CO3 doped bathophenanthroline (BPhen)/Al/MoO3 or 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN). Drastic increases in current densities of around 10 mA/cm2 at a forward bias of 10 V for the devices with CGUs were observed, while that for the control device remained less than 10-4 mA/cm2 at the same voltage. The device with HAT-CN electron accepting layer (EAL) showed a sharp increase in current density than that with MoO3. The mechanism of current in the devices with CGUs is proposed by the charge generation at the interface between an EAL and a hole transporting layer (HTL), followed by the tunneling of electrons through the depletion region in the n-doped BPhen layer. Based on the charge generation study, we have fabricated Ir(ppy)3-based green phosphorescent tandem OLEDs with the CGUs. We employed TAPC as the main HTL for obtaining high efficiency and N,Nâ?T-bis(naphthalen-1-yl)-N,Nâ?T-bis(phenyl)-benzidine (NPB) as the charge generating HTL for making well operating charge generating interface. We achieved a highly efficient green tandem OLED with the current density of 128 cd/A at 0.1 mA/cm2 (or at 128 cd/m2) and 120 cd/A at 1 mA/cm2 (or at 1200 cd/m2). References [1] T. Matsumoto, T. Nakada, J. Endo, K. Mori, N. Kavamura, A. Yokoi and J. Kido, SID Symp. Digest Tech. Papers 34 (2003) 979. [2] L. S. Liao, K. P. Klubek and C. W. Tang, Appl. Phys. Lett. 84 (2004) 167. [3] M. K. Fung, K. M. Lau, S. L. Lai, C. W. Law, M. Y. Chan, C. S. Lee and S. T. Lee, J. Appl. Phys. 104 (2008) 034509. [4] X. D. Gao, J. Zhou, Z. T. Xie, B. F. Ding, Y. C. Qian, X. M. Ding and X. Y. Hou, Appl. Phys. Lett. 93 (2008) 083304. [5] X. Qi, N. Li and S. R. Forrest, J. Appl. Phys. 107 (2010) 014514.
12:30 PM - J7.10
Spatial Control of p-n Junction in a Light Emitting Electrochemical Cell
Jiang Liu 1 Isak Engquist 1 Xavier Crispin 1 Magnus Berggren 1
1Linkouml;ping University Norrkouml;ping Sweden
Show AbstractPolymer light emitting electrochemical cells (LECs) have attracted much attention due to their low voltage operation and insensitivity to electrode materials. Based on the planar LEC structure, we have developed a new device configuration, the light emitting electrochemical transistor (LECT), in which the location of the light emitting p-n junction and the current level can be controlled via a gate terminal. Specifically, an electrochemically active conducting polymer is employed as gate material to modulate the doping profile within the light emitting polymer, and thereby control the position of the light emitting p-n junction. In this configuration, we are able to move the light emitting p-n junction back and forth within an interelectrode gap of 500 µm under a voltage bias lower than 5 volt. Additionally, the LECT exhibits current modulation capabilities, as evidenced by measured transfer curves. The channel current on/off ratio, estimated for gate voltages of 0 V and 4 V, ranges from 10 to 100 and the transconductance value beyond the gate threshold voltage (-2.3 V) is found to be in the range from 1.5 to 3 μS. This device provides a new method to study the fundamental physics of the LECs and opens a new avenue towards tuneable light sources and addressable circuits.
12:45 PM - J7.11
Phosphorescent Emitters with Narrow Emission Spectra for Display Applications
Alicia Wolfe 1 Eric Turner 1 Xiao-Chun Hang 1 Jian Li 1
1Arizona State University Tempe USA
Show AbstractThe development of emissive materials for use in organic light emitting diode displays has been the focus of significant research in recent years. As the market for portable, battery driven devices and green products has matured, increasing emphasis has been placed on energy efficiency. The development of phosphorescent emitters with narrow emission spectra is vital to this goal. Narrowly emitting phosphors have the ability to achieve 100% internal quantum efficiency while delivering maximum light output in the desired range. While efficient phosphors based on platinum and iridium have been developed, their emission spectra are often broad, which results in reduced power conversion efficiency as only a portion of the emitted light is useful for display purposes. In this presentation, we will discuss our recent work developing a new class of efficient platinum based emitters with full width half max values at room temperature in the range of 30 nm. The photophysical, electrochemical, and electroluminescent properties of this novel series of compounds will be discussed.
Symposium Organizers
Oliver Hayden, Siemens AG
Franky So, University of Florida
Paul Blom, TNO Holst Centre
Henning Richter, Nano-C, Inc.
Jongwook Park, Catholic University of Korea
Symposium Support
Nano-C, Inc.
National Science Foundation
Siemens AG
J13: Organic Photovoltaics I
Session Chairs
Thursday PM, April 12, 2012
Moscone West, Level 2, Room 2008
2:30 AM - *J13.1
Small Molecular Solar Cells toward Improved Efficiency and Stability
Jang-Joo Kim 1 Ji Whan Kim 1 Hyo Jung Kim 1 Tae-Min Kim 1 Won-Ik Jeong 1 Yang-Eun Lee 1
1Seoul National University Seoul Republic of Korea
Show AbstractWe will present a few methods to improve the efficiency and stability in small molecule based organic solar cells, including the formation of bulk heterojunctions (BHJs) through alternative thermal deposition (ATD), the use of a micro-cavity structure and interface modifications. By ATD which is a simple modification of conventional thermal evaporation, the thicknesses of alternative donor and acceptor layers were precisely controlled down to 0.1 nm, which is critical to form BHJs. The formation of a BHJ in copper(II) phthalocyanine (CuPc) and fullerene (C60) systems was confirmed by AFM, GISAXS and absorption measurements. From analysis of the data, we found that the CuPc|C60 films fabricated by ATD were composed of the nanometer sized disk shaped CuPc nano grains and aggregated C60, which explains the phase separation of CuPc and C60. On the other hand, the co-deposited CuPc:C60 films did not show the existence of separated CuPc nano grains in the CuPc:C60 matrix. The OPV cells fabricated using the ATD method showed significantly enhanced power conversion efficiency compared to the co-deposited OPV cells under a same composition. We will also present by numerical simulation that adoption of microcavity structure in the planar heterojunction can improve the short circuit current in single and tandem OSCs. Interface modifications also allowed us to achieve high efficiency and high stability OSCs.
3:00 AM - J13.2
Tuning the Color of Highly Efficient, Semi-transparent Organic Photovoltaic Devices
Jens Czolk 1 Andreas Puetz 1 Jan Mescher 1 Henry Vogeler 1 Dimitar Kutsarov 1 Uli Lemmer 1 Alexander Colsmann 1
1Karlsruhe Institute of Technology (KIT) Karlsruhe Germany
Show AbstractWindow integrated photovoltaics for automotive and building applications are a promising market segment for organic solar modules. Besides semi-transparency, window integrated applications require a reasonable transparency color perception. These boundary conditions are well matched by a blend of PSBTBT:[70]PCBM with an extended absorption to the infrared. We therefore fabricated and investigated highly efficient (η â?^ 3%), semi-transparent PSBTBT:[70]PCBM solar cells under different illumination scenarios such as bright sunlight or cloudy skies. However, a reasonable transparency perception does not necessarily imply good color rendering. The color rendering of items under illumination of transmitted light is of utmost importance when integrating semi-transparent photovoltaic devices in windows or overhead glazing. For example, (transmitted) white light that mainly contains blue and yellow parts of the visible spectrum will render white surfaces acceptably but is incapable of rendering reddish objects. The transmitted light through PSBTBT:[70]PCBM solar cells exhibits remarkable color rendering properties making this polymer/fullerene blend and the respective solar cells suitable for real-life window applications. Unfortunately, most solar cells with efficient polymer/fullerene blends such as the reddish PCDTBT:[70]PCBM, neither appear color neutral nor exhibit good color rendering properties. In this work we successfully balanced the transparency color perception and color rendering properties by incorporating complementary absorbing dyes into the device. In light of roll-to-roll mass production processes for solution deposited solar cells we investigated two concepts: integration of dye doped PMMA layers into the device architecture and blending of the dyes into a highly conductive PEDOT:PSS anode. As a result, the transparency color perception of the modified devices equaled daylight and the color rendering index (CRI) approached unity while we observed only very little effect on the overall device power conversion efficiency. Another way of affecting the solar cellâ?Ts color is the systematic fine tuning of thin film interferences within the device. We deliberately simulated the influence of the cathode TCO and other layers thicknesses on the light interference pattern and hence the light absorption and reflection utilizing optical parameters that were obtained in-house. By carefully selecting the electrode thickness, outstanding color perception and rendering properties can be realized for a variety of absorber materials. These techniques allow for choosing the absorber blend independently of its natural color perception so that the entire range of highly efficient organic absorber materials is available for the fabrication of color neutral, semi-transparent photovoltaic devices. [Ref.: A. Colsmann et al., Adv. Energy Mater, 1 (2011) 599]
3:15 AM - J13.3
Direct Observation of Photoinduced Bound Charge-pair States at Organic-inorganic Semiconductor Interface and Their Detrimental Effect on the Performance of Hybrid Photovoltaic Devices
Yana Vaynzof 1 Artem A Bakulin 1 Simon Gelinas 1 Henning Sirringhaus 1 Richard H Friend 1
1Cambridge University Cambridge United Kingdom
Show AbstractIt is generally considered that photoinduced charge transfer in hybrid photovoltaic devices immediately results in a pair of free charge carriers due to the increased screening of electron-hole interaction by the higher dielectric constant of inorganic materials. This idea inspired extensive research and development of hybrid organic-inorganic devices where, upon illumination, excitons are created in the highly-absorbing organic material and efficiently dissociated at the interface with the inorganic counterpart of the device. However, these promising hybrid photovoltaic devices generally show very moderate performance. One possible reason for this could be the existence of bound charge pairs (BCP) at the organic-inorganic interfaces. A BCP state comprises of a positive charge localized on an organic molecule (polaron) Coulombically-attracted to an electron in the inorganic material. We employed a novel interface-selective ultrafast optoelectronic technique and observe the existence of BCP states at the organic-inorganic interface formed between electron-accepting zinc oxide and conjugated polymers such as poly(3-hexylthiophene) (P3HT) or poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2â?T,2â?Tâ?T-diyl) (F8TBT). We have developed a state model and utilized a genetic algorithm to quantify the yield of BCP states. In the case of the P3HT/ZnO and F8TBT/ZnO devices, we find that 52% and 54% of photo-induced charges remain in BCP states, respectively, which substantially hinders the device efficiency.[1] We have previously reported that organic interface modification can result in a three-fold increase the external quantum efficiency of hybrid photovoltaic devices.[2] Using the newly developed ultrafast technique and state model allows us to elucidate the physical processes responsible for this remarkable improvement. We find that the interfacial modification improves the light harvesting and decreases the number of bound charges to 28% and 24% in P3HT and F8TBT devices respectively, accounting for the observed 300% enhancement of the device efficiency. To summarize, the efficiency of exciton harvesting and disassociation to free charge carriers strongly depends on the properties of the interface, and can be dramatically enhanced by an organic monolayer modification of the inorganic surface. Our results are of fundamental importance for understanding of the photo-physical processes taking place in organic-inorganic hybrid photovoltaic cells. References: [1] Y. Vaynzof, A. A. Bakulin, S. Gelinas, H. Sirringhaus and R. H. Friend, â?oDirect Observation of Photoinduced Bound Charge-Pair States at Organic-Inorganic Semiconductor Interfaceâ?o, submitted to Phys. Rev. Lett. [2] Y. Vaynzof, D. Kabra, L. Zhao, P. K. H. Ho, A. T-S. Wee and R. H. Friend, â?oImproved Photoinduced Charge Carriers Separation in Organic-Inorganic Hybrid Photovoltaic Devicesâ?, Appl. Phys. Lett. 97, 033309 (2010).
4:00 AM - J13.4
Understanding the Roles of Geminate and Nongeminate Recombinatiion in Determing the Bias Dependence of the Photocurrent and Fill Factor in Organic Solar Cells
Ian Howard 1 Ralf Mauer 1 Frederic Laquai 1
1Max Planck Institute for Polymer Research Mainz Germany
Show AbstractDespite significant study by many research groups, the efficiency-limiting processes that govern the performance of bulk heterojunction photovoltaic devices still remain ambiguous. In particular the role of interfacial charge-transfer (CT) states in determining the bias dependence of the photocurrent is debated. Understanding the bias dependence of the photocurrent is necessary to optimize device fill factors, and therefore resolution of this debate is important. In this contribution we directly determine the field dependence of bimolecular recombination at the charge densities relevant to photovoltaic devices using a novel optical pump electrical probe experiment that enables the ultra fast time resolution of optical spectroscopy to be combined with high signal to noise achievable in current measurements.[1] We relate these results to temperature dependent transient absorption (pump-probe) spectroscopy over the entire relevant timescale from femtoseconds to milliseconds and in a broad spectral range covering 500 to 2000 nm. [2,3] In general the results demonstrate the importance of i.) ultrafast free carrier generation, ii.) suppression of interfacial CT state formation, and especially iii.) high and balanced charge carrier mobilities to achieve good fill factors and high power conversion efficiencies. [1] R. Mauer, I.A. Howard, F. Laquai, J. Phys. Chem. Lett. 2011, 2 (14), 1736-1741. [2] I.A. Howard, R. Mauer, M. Meister, F. Laquai, J. Am. Chem. Soc. 2010, 132 (42), 14866-14876. [3] R. Mauer, I.A. Howard, F. Laquai, J. Phys. Chem. Lett. 2010, 1 (24), 3500-3505.
4:15 AM - J13.5
Matching the External to the Internal Quantum Efficiencies to Enhance Light Harvesting in Organic Photovoltaic Cells
Rafael Betancur 1 Alberto Martinez-Otero 1 Pablo Romero-Gomez 1 Xavier Elias 1 Silvia Colodrero 3 Hernan Miguez 3 Jordi Martorell 1 2
1The Institute of Photonic Sciences - ICFO Castelldefels (Barcelona) Spain2Departament de Fiacute;sica i Enginyeria Nuclear, Universitat Politegrave;cnica de Catalunya Terrassa Spain3Instituto de Ciencia de Materiales de Sevilla, Consejo Superior de Investigaciones Cientiacute;ficas (CSIC) Sevilla Spain
Show AbstractTo enhance photoconversion efficiency (PCE) of organic photovoltaic (OPV) cells it is important to optimize the photon harvesting provided the absorption band of the materials used is rather narrow. In addition, OPV devices suffer from a short exciton diffusion length and a low mobility of charged carriers which prevent the use of thick materials for a more effective photon absorption [1]. From an optical perspective, such organic solar cells can be seen as a stack of several layers with different refractive index. At each interface between any of these layers there is a significant reflection that prevents the most effective photon harvesting. If such reflection could be reduced minimized, the external quantum efficiency (EQE), defined as the ratio between current and incoming photons, would approach the internal one, which is the quotient between current and absorbed photons. In the current work we show experimentally that for an inverted P3HT:PCBM cell it is possible to find a combination of layer thicknesses such that optical interference leads to an EQE that amounts to 91% of the internal quantum efficiency (IQE). We observe that the total reflectivity is close to its minimum possible value in a wavelength range of more than 100 nm. In that range the EQE closely matches the IQE. In a model developed to determine the role played by optical interference in the light harvesting efficiency we confirmed that the EQE can closely match the IQE for that wavelength range. Additionally we observed that a similar cell with an active material 1.7 times thicker exhibited a lower PCE. The poor photon harvesting in the later cell configuration is confirmed by an EQE that amounts only to 72% of the IQE. Such optical interference control of the EQE may enable fabrication of devices with active layers thinner than the absorption length, which has been considered as the optimal solution to prevent problems in the charge transport pathways or cul-de-sacs in OPV devices [1]. [1]. K.S. Nalwa, J.-M. Park, K.-M. Ho, S. Chaudhary. Adv. Mater. 2011, 23, 112â?"116. [2]. B.V. Andersson, D.M. Huang, A.J. Moulé, O. Inganäs. Appl. Phys. Lett. 2009, 94, 043302.
4:30 AM - J13.6
Transparent Conductive Oxide Less Flexible Dye-sensitized Solar Cells with Flat and Cylinder Shapes
Shuzi Hayase 1 Kengo Sadamasu 1 Jun Usagawa 1 Yuhei Ogomi 1 Shyam Pandey 1
1Kyushu Institute of Technology Wakamatsu-ku Kitakyushu Japan
Show AbstractDye-sensitized solar cells consist of transparent conductive oxide glasses (TCO-glass). TCO-glasses are expensive now. We have focused on structures and fabrication process of transparent conductive oxide less DSC (TCO-less DSC). We prepared nano-composite films consisting of a protected metal mesh/porous titania/dye. By employing the nano-composite films, TCO-less flexible DSCs with 6.1% efficiency (F-1), TCO-less cylinder DSC with 5.6% efficiency (F-2), TCO-less fiber DSC with 2.4 % efficiency (F-3) were fabricated. A new flexible TCO-less cylinder DSC with 4.1 % efficiency (F-4) is also reported. In addition, TCO-less iodine free DSCs consisting of Co2+/Co3+ redox species with 3.5 % efficiency are discussed from the view point of high open circuit voltage (Voc) and low damage toward metal electrodes. Hybrid and tandem cells are candidates for high efficiency solar cells since they can expect to harvest light in near infrared (NIR) and infrared (IR) regions. Hybrid cells (F-5) were fabricated by pressing two nano-composite films (top and bottom electrodes). Tandem cells (F-6) consisting of porous titania/dye top electrode and porous SnO2/dye bottom electrode are expected to harvest light in infrared (IR) region, where developments of high efficiency bottom SnO2/dye electrodes are essential. A problem is low ff values for DSCs consisting of SnO2 nanoporous electrodes. The reason of the low efficiency of SnO2 based solar cell was discussed by using hybrid cell structures. It was concluded that serious charge recombination occurs through dyes, not SnO2 itself. F-5 and F-6 were proved to work properly as tandem and hybrid cells by using model dyes. Near IR and IR dyes for titania and SnO2 electrodes are needed for completing the development of these tandem and hybrid solar cells.
4:45 AM - J13.7
Polymer Solar Cells with Enhanced Lifetime by Improved Electrode Stability
Roland Roesch 1 Kai-Rudi Eberhardt 1 Gerhard Gobsch 1 Harald Hoppe 1
1Ilmenau University of Technology Ilmenau Germany
Show AbstractWe present polymer solar cells with a photoactive layer based on PCDTBT:PCBM exhibiting lifetimes of more than 1000 hours under continuous illumination and periodic characterization. The accelerated aging was performed under approximately AM1.5 conditions using a metal halide lamp exhibiting considerable amount of UV-radiation. The samples were kept at about 50°C and were characterized by IV-measurements every 30 minutes. Furthermore luminescence imaging and lock-in thermography were applied to yield conclusive information about the degradation mechanisms. For improved understanding of the optimal design of the layer architecture, the top contact electrode material was varied, and some solar cells were kept unsealed.
5:00 AM - J13.8
Enhanced Exciton Diffusion and Power Conversion Efficiency in Organic Photovoltaic Cells Containing a Dilute, Electron Donor Layer
S. Matthew Menke 1 Wade A Luhman 1 Russell J Holmes 1
1University of Minnesota Minneapolis USA
Show AbstractWe use photoluminescence quenching measurements to examine the exciton diffusion length (LD) of the electron donor boron suphthalocyanine chloride (SubPc) and its dependence on dilution in a wide-energy gap host matrix. By diluting SubPc, both the exciton lifetime and photoluminescence efficiency are increased, leading to an increase in the self-Forster radius for energy transfer. This increase in the self-Forster radius leads to a doubling in the exciton diffusion length of SubPc from a value of LD=7.7 nm in neat film to a value of LD=15.6 nm in dilute films containing 25 wt.% SubPc. The increased diffusion length leads to a significant improvement in the short-circuit current density and power conversion efficiency (ηP) of planar heterojunction organic photovoltaic cells (OPVs) based on the donor-acceptor pairing of SubPc and C60. Since LD is often much shorter than the optical absorption length, the performance of planar heterojunction OPVs can be limited by inefficient exciton diffusion. Here, we overcome this limitation and demonstrate a high power conversion efficiency of ηP=(4.2±0.1)% for planar heterojunction OPVs containing a dilute SubPc donor layer, compared to ηP=(3.0±0.2)% for devices with a neat donor layer. Interestingly, the performance of the dilute-donor planar OPV rivals that of optimized mixed and graded heterojunction OPVs. This suggests that with improved control over exciton diffusion in the donor, high ηP can be realized in architectures that are often considered exciton-diffusion limited.
5:15 AM - J13.9
Recombination Mechanisms in High Voltage Polymer-fullerene Bulk Heterojunction Solar Cells
Eric T Hoke 1 Tim A Miller 2 George F Burkhard 1 Aaron Lindenberg 2 Michael D McGehee 2
1Stanford University Stanford USA2Stanford University Stanford USA
Show AbstractThe best polymer-fullerene solar cells have open circuit voltages which are ~0.9V less than the optical bandgap of the polymer. In comparison, inorganic solar cells can produce voltages within 0.4V of their bandgap. Much of the voltage loss in organic solar cells can be attributed to the offset in band energy levels between the donor and acceptor materials required for efficient exciton splitting into free charges. However, it is not clear what minimum offset in energy levels is required for efficient charge generation. It has been proposed that recombination to triplet states can compete with charge generation when the energy of the triplet states lies below that of the charge separated state, which occurs when the offset in band energy levels is made too small. Triplet recombination has been shown to be significant in several polymer-polymer blends and blends of large bandgap polyfluorene copolymers with PCBM. We perform a thorough electronic and spectroscopic study of several high efficiency copolymer blends with bis-indene C60 (ICBA) which produce voltages ~0.7V less than the polymer bandgap. We find from light intensity dependent internal quantum efficiency measurements that geminate recombination limits the performance of these devices. From photoinduced absorption measurements, we conclude that the yield of triplets is not sufficient to explain the loss in photocurrent and that there is a kinetic competition between charge formation and other recombination channels.
5:30 AM - J13.10
Patterned Graphene Films as Transparent and Flexible Conducting Electrodes for Organic Transistor and Solar Cell Devices Applications
Sangchul Lee 1 Gunho Jo 3 Seok-Ju Kang 1 Jun-Seok Yeo 1 Yongsung Ji 1 Chun Hum Cho 1 Dong-Yu Kim 1 Yung Ho Kahng 2 Byoung Hun Lee 1 Takhee Lee 3
1Gwangju Institute of Science and Technology Gwangju Republic of Korea2Gwangju Institute of Science and Technology Gwangju Republic of Korea3Seoul National University Seoul Republic of Korea
Show AbstractOrganic electronic devices have been attracting much attention due to several advantages such as cost-efficient large-area device fabrication, limitless materials variety, flexibility, and wide range of device applications.[1] For such device applications, choosing appropriate contact electrode material is significantly important.[2,3] In particular, finding a suitable electrode material for efficient charge injection to organic active layer and also tuning the interface in-between them for better injection have been considered as key issues for improved device performance. Recently, graphene has received great attention as a promising transparent and flexible conducting electrode for organic and inorganic devices. In this presentation, we report on patterned graphene electrode for electrical and optical devices applications.[4-6] First, an efficient charge injection in pentacene organic transistors induced by the use of graphene electrodes was investigated. Through channel length-dependent, temperature-variable electrical characterizations, and Kelvin probe measurements, we found that favorable orientation of interface dipole layer occurred between graphene electrodes and pentacene organic semiconductor for consequential efficient injection of charges.[4] Secondly, the compatibility of graphene electrodes with the well-known substrate treatment techniques for improving the performance of pentacene transistors were systemically investigated. Reduction of channel resistance and charge injection barrier height were observed in terms of various surface treatments such as self-assembled monolayer coating and thermal annealing.[5] Lastly, doping graphene electrodes for high performance organic solar cells was researched. By doping graphene films which possess mechanical flexibility, we fabricated efficient flexible organic solar cells with graphene electrodes.[6] The results of these studies suggest that the use of flexible graphene electrodes will provide significant enhancement for various types of organic devices. References [1] S. R. Forrest., Nature 2004, 428, 911. [2] C.-a. Di et al., Adv. Mater. 2008, 20, 3289. [3] S. Pang et al., Adv. Mater. 2009, 21, 3488. [4] Lee et al., Adv. Mater. 2011, 23, 100. [5] Lee et al., Appl. Phys. Lett. 2011, 99, 083306. [6] Lee et al., submitted. Acknowledgements: The National Research Laboratory Program, the National Core Research Center grant, and the World Class University program by the Korean Ministry of Education, Science and Technology.
J14: Poster Session: Organic Light Emitting Devices, Displays, and Sensors
Session Chairs
Thursday PM, April 12, 2012
Marriott, Yerba Buena, Salons 8-9
9:00 AM - J14.1
Short Triplet Exciton Diffusion in Co-host Phosphorescent Blue OLEDs
James S Swensen 1 Birgit Schwenzer 1 Asanga B Padmaperuma 1 Lelia Cosimbescu 1 Phillip K Koech 1 Evgueni Polikarpov 1 Daniel J Gaspar 1
1PNNL Richland USA
Show AbstractCo-host phosphorescent OLEDs are made when the hole transport layer (HTL) and electron transport layer (ETL) are blended together with the phosphorescent emitter to form the emissive layer (EML). The advantage of the co-host EML are that there are no real interface at the start of the EML because it is made from the same materials as the HTL and ETL and the charge balance in the EML can be tuned by varying the co-host EML content. Exciton formation in a co-host EML should form via charge trapping on the phosphorescent emitter. In this work, we present a study that explores the diffusion of triplet excitons in co-host EMLs. We show that the degree to which a triplet exciton is quenched by a low triplet energy transport material is dependent upon the location of the emission zone in the channel. We compare co-host EML results to results for a hole dominated, high triplet energy host material. Finally we show that the distance required to minimize quenching by a low triplet energy transport layer is between 50 and 100 Ã..
9:00 AM - J14.10
Organic LED Encapsulation with a Hybrid of SiO2 and Silicone Polymer
Bhadri Visweswaran 1 2 Prashant Mandlik 3 Lin Han 1 2 Jeff Silvernail 3 Ray Ma 3 James C Sturm 1 2 Sigurd Wagner 1 2
1Princeton University Princeton USA2Princeton University Princeton USA3Universal Display Corporation Ewing USA
Show AbstractWith the surge of Organic LEDs in displays and lighting, there has been a need for an ultrathin encapsulating chemical and electrical barrier layer for their employment in flexible substrates. The encapsulating layer needs to be free of pinhole defects and must be able to encapsulate particles. Current OLED encapsulation techniques are either rigid or too thick for flexible substrates, or are made of costly multilayers. A hybrid of SiO2 and silicone polymer that is made by plasma-enhanced chemical vapor deposition in the parallel-plate geometry overcomes these difficulties. Films are deposited on substrates held at room temperature from the source gases hexamethyl disiloxane (HMDSO) and oxygen. The resulting hybrid is an excellent barrier against the permeation of atmospheric gases and is an equally excellent electrical insulator. We have studied how the deposition conditions affect the growth of the hybrid layer on substrates with trenches, or on substrates mounted at an angle to or an offset from the substrate holder. Depositions were conducted on silicon wafers with microtrenches or with exposed undersides. First, microtrenches were fabricated with an inductively coupled plasma deep etcher; hybrid films were deposited on this structure. The thickness of the film on the side wall of the trenches in function of distance from the top edge was measured for different trench widths and depths. Second, a silicon wafer was mounted perpendicularly to the substrate electrode. The thickness of the film deposited on the face of the silicon wafer was measured in function of the distance from the edge of the wafer. Third, two silicon wafers were sandwiched back to back and mounted at an offset to the substrate electrode to expose the underside. The thickness of the film deposited on the bottom face, which is in the shadow of the plasma, was measured as a function of the distance from the edge. These three thickness measurements enable us to characterize film growth as a function of the plasma parameters. We will report these dependences and our interpretation of the film growth mechanism.
9:00 AM - J14.11
Printable Hybrid Electrode with Metal Grid, Oxide, and Organic Materials for the Inverted Organic Light Emitting Diodes
Sung Min Jo 1 Dai Gun Yoon 1 Junhan Cho 1 Byung Doo Chin 1
1Dankook University Youngin Republic of Korea
Show AbstractA simple printing method was developed to replace indium-tin oxide (ITO) for the fabrication of inverted organic light emitting diodes (OLED). Using metal grid structure, which was formed by the surface-mediated fine line inkjet printing process, hybrid transparent electrodes with tunable work function and transparency were fabricated. On the glass substrate with hydrophobic surface treatment, inkjet printing of silver metal grid with a scale of 8-20um width was performed using Dimatix piezoelectric printhead or electrohydrodynamic (EHD) meniscus. Jetting profiles and surface energy condition of substrates were precisely controlled for the optimum smooth-line patterning and desired pitch-to-pitch separation for optimum transparency and conductivity. By the combination of silver grid geometry, thin hole-transporting polymer (Clevios PH500), and semiconducting tungsten oxide (WO3), transparency in the range of 78-83% at 550nm was achieved (with 93% glass transmittance), resulting in the comparable i-V-L characteristics of OLED (22-23 cd/A) to those with ITO-based one (25 cd/A for green device). Silver grid/solution-processible zinc oxide (ZnO) hybrid electrode also yielded the efficient inverted bottom emission device, while the inverted top emitting structure with this hybrid electrode (Ag/ZnO and n-doped layer/phosphorescent emitters/p-doped and WO3/Ag grid/ capping layer) provided a feasible way for the fabrication of flexible and large area OLED compatible with n-type oxide transistor array. Unlike the imprinting, transfer patterning, or selective surface modification, inkjet printing on the substrate with controlled surface energy is advantageous since the additional processes such as mask or mold patterning are not required. The value of parameters for the interfacial properties of hole- and electron transport layers (energy level, roughness, and conductivity) were clearly demonstrated and analyzed in comparison with the device performance of inverted bottom and top-emitting OLEDs.
9:00 AM - J14.13
The Dependence of Electrical Current in Organic Light Emitting Diodes on Optical Microcavity Effects and Its Use as a Novel Sensing Modality
Kanika L Agrawal 1 Matthew E Sykes 1 Kwang H An 2 3 Max Shtein 1
1University of Michigan Ann Arbor USA2University of Michigan Ann Arbor USA3Currently at GE Global Research Niskayuna USA
Show Abstract
The recombination of electrically generated excitons with trapped interfacial charges is a well known phenomenon that occurs in organic light emitting devices (OLEDs).1-3 Here we demonstrate that this effect can be used to externally influence the current density by varying exciton lifetime in an archetypal bi-layer OLED. Specifically, by using an external capping layer to modify the optical microcavity modes present in the OLED, we show that the exciton lifetime can be modulated, causing the emitter decay rate and current density to vary proportionately and measurably. We ascribe this effect to interfacial exciton-polaron quenching that generates a back-diffusing hole current through the hole transport layer. These findings have potential applications in a new class of electro-optical sensors for, e.g., monitoring nanoscale refractive index variation or molecular detection.
1M. Pope and C. Swenberg, Electronic Processes in Organic Crystals (Oxford University Press, Oxford, 1982).
2M.A. Baldo et al., Physical Review B.66, 035321 (2002).
3Ichikawa et al., Japanese Journal of Applied Physics40, L1068 (2001).
9:00 AM - J14.14
MoO3 as Combined Hole Injection Layer and Tapered Spacer in Combinatorial Multicolor Microcavity Organic Light Emitting Diodes
Rui Liu 1 Chun Xu 1 Rana Biswas 1 Joseph Shinar 1 Ruth Shinar 2
1Iowa State University Ames USA2Iowa State University Ames USA
Show AbstractMulticolor microcavity (microC) organic light-emitting diode (OLED) arrays were fabricated simply by controlling the hole injection and spacer MoO3 layer thickness. The normal emission was tunable from ~490 to 640 nm and can be further expanded. A compact, integrated spectrometer with two-dimensional combinatorial arrays of microC OLEDs was realized. Introducing MoO3 yields more efficient and stable devices, and reduces device breakdown. The pixel current density reaches ~4 A/cm2, resulting in a maximal normal brightness Lnorm,max ~ 140,000 Cd/m^2, which is important for photoluminescence-based sensing and absorption measurements. The results also illustrate a new breakdown mechanism in OLEDs.
9:00 AM - J14.15
High Efficiency Solution-processed Small Molecule Electrophosphorescent Organic Light-emitting Diodes
Min Cai 1 Teng Xiao 1 Emily Hellerich 1 Ying Chen 1 Ruth Shinar 2 Joseph Shinar 1
1Iowa State University Ames USA2Iowa State University Ames USA
Show AbstractWe demonstrate very high efficiency (up to 60 lm/W and 69 Cd/A without any outcoupling enhancement, which can double with a high-quality microlens array) spin-coated small molecule OLEDs based on green-emitting tris[2-(p-tolyl)pyridine] iridium(III) (Ir(mppy)3)-doped 4,4'-bis(9-carbazolyl)-biphenyl (CBP) blended with electron- and hole-transporting molecules. An additional electron-transporting/hole-blocking 4,7-diphenyl-1,10-phenanthroline (BPhen) layer is thermally evaporated on the active layer, followed by the LiF/Al cathode.
9:00 AM - J14.16
Indium-tin-oxide-free Tris(8-hydroxyquinoline) Al Organic Light-emitting Diodes with 80% Enhanced Power Efficiency
Min Cai 1 Teng Xiao 1 Rui Liu 1 Ying Chen 1 Ruth Shinar 2 Joseph Shinar 1
1Iowa State University Ames USA2Iowa State University Ames USA
Show AbstractEfficient ITO-free small molecule organic light-emitting diodes (SMOLEDs) with multilayered highly conductive poly(3,4-ethylenedioxy thiophene): poly(styrenesulfonate) (PEDOT:PSS) as the anode are demonstrated. SMOLEDs with the structure PEDOT:PSS/MoO3/N,Nâ?T-diphenyl- N,Nâ?T-bis(1-naphthylphenyl)-1,1â?T-biphenyl-4,4â?T-diamine (NPD)/tris(8- hydroxyquinoline) Al (Alq3)/ 4,7-diphenyl-1,10-phenanthroline (BPhen)/LiF/Al exhibited a peak power efficiency of 3.82 lm/W, which is 81% higher than that of similar ITO-based SMOLEDs (2.11 lm/W). The increase in the device performance is believed to be due to the advantageous higher workfunction, lower refractive index and decreased surface roughness of PEDOT:PSS as compared to ITO, as well as to Ohmic hole injection from the PEDOT:PSS to the NPD layer via the MoO3 interlayer. The results demonstrate that a polymeric anode has the potential to substitute ITO in OLEDs with strongly improved device performance.
9:00 AM - J14.17
Tunable Transparent Electrodes Based on Dielectric/Metal/Dielectric
Sungjun Kim 1 Kihyon Hong 1 Jong-Lam Lee 1
1POSTECH Pohang Republic of Korea
Show AbstractOrganic optoelectronics such as organic light emitting diodes (OLEDs), organic sensors, and organic photovoltaics (OPVs) cells have advantages such as low cost, flexible structures, and potential for easier processability. Many of these devices incorporate several components; this is especially the case in OLEDs and OPVs in which a transparent electrode is used as a bottom or a top electrode. Thus, a highly-transparent and conducting electrode materials are required to reduce operation voltage and to increase internal/external quantum efficiencies. Expecially, in order to maximize the performance of devices, the optical properties of transparent electrode should be adjusted and optimized for each system. In this work, we demonstrate a novel way of tuning the optical transmittance of transparent electrode using dielectric/metal/dielectric (DMD) structure. DMD multilayer has an advantage in that it can fulfill the optimum â?~zero-reflectionâ?T condition with metal film, realizing improved transparency characteristic. We found that the refractive index of outer dielectric can tune the zero-reflection condition for specific wavelength, and the wavelength for maximum transmittance of DMD becomes longer as the refractive index of outer dielectric is increased. We measured and simulated how optical transmittance of DMD films is affected by the refractive indices of several kinds of metal oxide materials varies from 1.9 to 2.5. (WO3, ZnS and TiO2). Ag is used as metal layer because it has good electrical conductivity and the lowest refractive index among metals resulting low absorption. WO3 is employed for inner dielectric due to large work function (~ 6.2 eV) enhancing hole injection properties. It is calculated that the wavelength for maximum transmittance of D/Ag/WO3 can be tuned from 480 nm to 620nm by increasing refractive index of D from 1.9 to 2.5. It is well matched with measured transmittance of DMD using WO3, ZnS and TiO2.We fabricated the Ag films sandwiched with outer dielectric layer with different refractive index and WO3, then fabricated and characterized three kinds of OLEDs emitting red(Ir(piq)3 doped CBP), green(C545T doped Alq3) and blue (Firpic doped TCTA) on these materials.
9:00 AM - J14.18
ITO-free Organic Light Emitting Diodes with Semitransparent Au Electrode and MoO3 Hole Injection Layer
ChaeFwi Lim 1 Chiyoung Lee 1 KiYeul Yang 1 Jaegab Lee 1
1Kookmin University Seoul Republic of Korea
Show Abstract
Organic light emitting diodes (OLEDs) are promising for flexible displays because they offer several advantages such as the ease processing, light weight, bright self-emission, and wide viewing angle over liquid crystal displays (LCDs). Indium tin oxide (ITO) is the most extensively used metallization in OLEDs because it offers transparency in the visible range of the electromagnetic spectrum as well as electrical conductivity. But ITO is hard to adapting the flexible display because ITO has a rigid and brittle property. Therefore, we developed Au single layer for the anode in OLEDs to replace ITO electrodes, and used MoO3 thin layer to replace hole injection layer (HIL, 2-TNATA). This resulted in Au/MoO3/NPB/Alq3:C545T/Alq3/LIF/Al structures, which yielded the lower turn-on voltage (3V) and higher luminescence intensity (14600 cd/m2 with 150 mA/cm2 at 7V) than those of ITO-based OLEDs (ITO/2-TNATA/NPB/Alq3:C545T/Alq3/LIF/Al) showing turn-on voltage (3.5V) and luminance intensity (3413 cd/m2 with 23 mA/cm2 at 7V). Transparency and sheet resistance of gold thin films varied with the film thickness; for example, â^¼60% transparency and 8â^¼10 â"¦/sq of sheet resistance for 15nm-thick Au film. The high work-function (5.1â^¼5.3 eV) and low sheet resistance (8â^¼10 â"¦/sq) of gold film allowed for low turn-on voltage and low driving voltage in OLEDs, leading the higher current at the same voltage compared with that of ITO electrode OLEDs. However Au anodes have difficulties with hole-transferring from HIL to emitting layer (EML) possibly due to the metal-polymer interface effects. The replacement of HIL with MoO3 significantly improved the electrical and optical performances due to the band alignment at the interface. *4,4',4"-Tris(N-(2-naphthyl)-N-phenyl-amino)-triphenylamine(2-TNATA) *N,N'-bis(naphthalen-1-yl)-N,N'-bis(phenyl)-benzidine(NPB) *Tris(8-hydroxy-quinolinato)aluminium(
Alq3) *2,3,6,7-tetrahydro-1,1,7,7,-tetramethyl-1H,5H,11H-10-(2-benzothiazolyl)quinolizino [9,9a,1gh]coumarin(C545T)
9:00 AM - J14.19
Effect of Exposure to Nitroaromatic Compound on Organic Field Effect Transistors Based on Polythiophene and Electron Donor Blend Film
Hoyoul Kong 1 Byung Jun Jung 1 Jasmine Sinha 1 Howard E Katz 1
1Johns Hopkins University Baltimore USA
Show AbstractVery recently many research groups have reported electrical performance changes of organic field effect transistors (OFETs) exposed to various chemical compounds such as humidity, polar solvents, and explosives. Most OFET devices showed decreased current output and mobility after exposure the chemical compounds. The degradation of the electrical performance by exposure to those compounds was affected by reduced charge transport which was caused by charge trapping at grain boundaries. Here we fabricated polythiophene OFETs with or without electron rich material. When the pure polythiophene devices were exposed to nitroaromatic solutions, the current of them was significantly decreased. In contrast, the OFET devices with blend polythiophene film with small amount of electron rich material showed clearly increased current after exposure to the nitroaromatic solutions, which was caused by changed energy level of the complex of electron donor and acceptor. Especially, as the devices were exposed to more dilute nitroaromatic solutions, the relative current increase was much bigger.
9:00 AM - J14.2
Fused Indole Derivatives as High Triplet Energy Hole Transport Materials for Deep Blue Phosphorescent Organic Light-emitting Diodes
Min Su Park 1 Chil Won Lee 1 Jun Yeob Lee 1
1Dankook Univ. Gyeonggi-do Republic of Korea
Show AbstractWe have synthesized high triplet energy and high glass transition temperature hole transport materials derived from fused indole core structure and investigated the device performances of deep blue phosphorescent organic light-emitting diodes with the hole transport materials. The fused indole based hole transport materials showed a high triplet energy over 2.9 eV and high glass transition temperature over 120 oC. The high quantum efficiency over 20% and color coordinate of (0.137, 0.182) could be achieved in deep blue phosphorescent organic light-emitting diodes.
9:00 AM - J14.20
Fluorene-type Polymer Photodetectors Doped with Iridium and Platinum Complexes as Opto-electrical Conversion Devices
Hirotake Kajii 1 Hiroki Ohmori 1 Yusuke Sato 1 Akihiro Katsura 1 Yutaka Ohmori 1
1Osaka University Suita Japan
Show AbstractOrganic photodetectors (OPDs) have high potential especially for signal processing and optical sensing systems in flexible devices. Phosphorescent materials have heavy atom effects and induce intersystem crossing (ISC), which transforms excitons from singlet into triplet. In general, triplet excitons have a longer lifetime than singlet excitons. This long lifetime can be expected to reduce exciton recombination and improve conversion efficiency. Solution processed organic devices utilizing conjugated polymers have attracted considerable interest because of their advantages in large-area device fabrication. Fluorene-type polymers have emerged as an important class of conjugated polymers due to their efficient emission, high stability and relatively high mobility. To enhance the efficiency of the fluorene-type polymers based OPDs, we investigated the properties of OPDs with Ir or Pt complexes doped in poly(9,9-dioctylfluorene) [F8] or poly(9,9-dioctylfluorene-co-benzothiadiazole) [F8BT], of which the device structures consisted of (+)ITO/F8 or F8BT :dopant (1 wt%) (100 nm)/Al/Ag (-). In order to compare the difference between singlet and triplet excitons in OPDs, the green emissive tris(8-hydroxyquinoline)aluminum [Alq3] and tris(2-phenylpyridine)iridium(III)[Ir(ppy)3] are used as the fluorescent and phosphorescent materials, respectively. Under illumination, the devices show good photoresponses. Then, we find that Ir(ppy)3-doped F8 and F8BT devices show more efficient conversion than both Alq3-doped and non-doped devices. This result suggested that longer exciton lifetime helps the exciton diffusion to organic/metal interface, where excitons are dissociated into carrier pairs. We also investigate the characteristics of the devices utilizing phosphorescent dyes of Ir and Pt complexes with different triplet levels. The photoresponse properties of bilayer devices utilizing solution-processable starburst molecular, methoxy-substituted 1,3,5-tris[4-(diphenylamino)phenyl]benzene (TDAPB) as a hole transporting layer were investigated because F8BT with an electron-withdrawing group was used as an electron-transporting material. The solution-processed bilayer device structures consisted of (+)ITO/F8BT:Ir(ppy)3 (1wt%) (50nm)/TDAPB (30nm)/MoOx(5nm)/Au(10nm)/Ag. Owing to the PL quenching and long exciton lifetime related with the ISC between F8BT and Ir(ppy)3, Ir(ppy)3 doped devices showed larger photocurrent than non-doped device under high applied voltage and Ir(ppy)3 doped bilayer devices exhibited the IPCE of approximately 30 % at 7 V. A blue LD was used to generate optical pulses as an example of opto-electrical processing. The cut-off frequency was estimated as more than 10 MHz under a bias voltage. We demonstrated that these OPDs can be used as opto-electrical conversion devices for transmitting moving picture signals.
9:00 AM - J14.21
Electrical Sensing Properties of Gold Nanoparticle/Organic Hybrid Films
Edith Chow 1 Karl H Muller 1 Burkhard Raguse 1 Lech Wieczorek 1 James S Cooper 1 Lee J Hubble 1 Melissa Webster 1
1CSIRO Lindfield Australia
Show AbstractChemiresistors consisting of gold nanoparticle/organic hybrid films are highly sensitive and inexpensive devices for monitoring organic analytes. Previous studies using chemiresistors have been limited to gas-phase operation, although more recently, operation in the liquid phase has been realized through control of the electrode geometry and spacing [1]. The chemiresistors consist of films of gold nanoparticles individually coated with an organothiol self-assembled monolayer and rely on a change in electrical resistance of the film when an analyte partitions into the organothiol layer. The response of a chemiresistor is governed by the partition coefficient of the analyte between the organothiol layer and the solution, the thickness of the gold nanoparticle/organic hybrid film, the analyte concentration and the analyte-water flow velocity. As our model system, we have studied the partitioning of the analytes hexane, heptane and octane into the hexanethiol layer of gold nanoparticle films [2, 3]. By varying the organothiol coating of these nanoparticles and using these to form a chemiresistor array, it is anticipated that we can broaden the range of potential applications. References: [1] B. Raguse, E. Chow, C. S. Barton and L. Wieczorek, Gold nanoparticle chemiresistor sensor: Direct sensing of organics in aqueous electrolyte solution, Anal. Chem., 2007, 79, 7333. [2] E. Chow, K.-H. Müller, E. Davies, B. Raguse, L. Wieczorek, J. S. Cooper and L. J. Hubble, Characterization of the sensor response of gold nanoparticle chemiresistors, J. Phys. Chem. C, 2010, 114, 17529. [3] K.-H. Müller, E. Chow, L. Wieczorek, B. Raguse, J. S. Cooper and L. J. Hubble, Dynamic response of gold nanoparticle chemiresistors to organic analytes in aqueous solution, Phys. Chem. Chem. Phys., 2011, 13, 18208 -18216.
9:00 AM - J14.22
Hybrid Assemblies for Detection Applications
Iain A Wright 1 Edwin C Constable 1 Catherine E Housecroft 1
1University of Basel Basel Switzerland
Show Abstract
Harnessing the complementary properties of both inexpensive functional organic molecules and inorganic nanocrystalline components presents a unique opportunity for the generation of new classes of hybrid materials.
New sensing technologies could be an ideal way to exploit the benefits of such hybrid species, with the potential to provide rapid, efficient and reliable detection of trace quantities of a number of analytes in aqueous solution. The presence of the metal containing nanoparticles can allow this to be achieved via a variety of optical and electrochemical detection methods including luminescence and photoconductivity changes amongst others.
With this potential in mind, we have recently begun to develop new molecules featuring an analyte receptor unit at one end, and anchoring thiol moieties at the other. The molecules have been designed to allow for both easy adsorption onto nanocrystalline surfaces and for strong and specific binding to a variety of anions and cations including Na+, Ca2+, PO43-, and F-. The molecules have been designed along robust synthetic principles which allow for straightforward derivatisation or further functionalisation, serving to demonstrate the flexibility that this approach towards new technologies could allow.
The current synthetic progress towards the receptor molecules under examination will be discussed, alongside relevant physical data and planned future work.
We acknowledge the support of the European Commission of HYSENS (FP7-NMP-2010-SMALL-4, Project 263091)
9:00 AM - J14.23
Organic Thermal Sensor with Nonvolatile Memory Component
Xiaochen Ren 1 Paddy, Kwok Leung Chan 1
1The University of Hong Kong Hong Kong Hong Kong
Show AbstractOrganic thin film transistor (TFT) has been demonstrated for various sensing applications including thermal information. Combining organic temperature sensor with nonvolatile memory device for data storage would be ideal for electronic artificial skin or food chain health monitor applications. In this work, we fabricated a two-transistor (2T) thermal memory device that can sense the temperature from 293K to 363K and store the data for later readout. The first transistor is working as the sensing component, in which the drain-source current at subthreshold region shows temperature dependent property and its value in pentacene TFT is 600 pA at 293K and increases 50 times from 293K to 363K. The second transistor is working as the memory component, by inserting a thin layer of silver nanoparticles into pentacene TFT, the transistor shows a memory window of 90V in transfer I-V curve. The sensing component is connected to the gate electrode of the second transistor for the programming. The output current of memory component is read at room temperature, and its value is only proportional to the temperature while the device is being programmed. The gate connection structure enable it to achieve low output current at room temperature (off state), hence we achieved a dynamic range around 8 bit with an off current of 1 nA. The retention property of memory sensor at various temperature and dielectric materials will also be discussed.
9:00 AM - J14.24
Highly Sensitivity P3HT/PCBM Volatile Organic Compounds Senor
Che-Pu Hsu 1 Ming-Chung Wu 1 Wei-Fang Su 1
1National Taiwan University Taipei City Taiwan
Show AbstractVolatile organic compounds (VOCs) are compounds that have high vapor pressure and low water solubility, and it is also the common pollutant found in the indoor air that impact human health. Some of these VOCs are carcinogenic and have been associated with sick building syndrome, fatigue and headache. To improve the sensing performance of VOCs sensor is an important issue in recently years. In this study, we adopt a polymer blend of P3HT/PCBM to as the sensor materials. In the fabrication process of our VOCs sensor, the P3HT/PCBM blend solution was prepared from 10 mg of P3HT (Mw~60000, PDI~1.6, RR>95%) and 8mg of PCBM (Nano-C Inc.) dissolved in 1 ml chlorobenzene. Then, the solution was deposited on glass slide by spin coating at 700 rpm for 1 min, yielding the film thickness of ~ 80 nm. After drying the P3HT/PCBM thin film by vacuum pump for 2 hrs, we can obtain our P3HT/PCBM VOCs sensor successfully. The absorbance of P3HT/PCBM VOCs sensor was measured by UV-Vis absorption spectroscopy after the sensor was exposed to various VOCs for different time. From the data of absorbance spectra, we can observe the intensity at 560 nm and at 600 nm showing an obvious variation. Table 1 is data analysis for P3HT/PCBM VOCs sensor exposed to various volatile organic compounds for 24 hrs. The results of our study display that P3HT/PCBM thin film is suitable for highly sensitivity volatile organic compounds sensor application.
9:00 AM - J14.25
Microfluidic Label-free DNA Sensor Based on Pentacene Thin Film Transistors
Jun-Ho Jeun 1 Jung-Min Kim 1 Dong-Hoon Lee 1 Rohit Chand 1 Yong-Sang Kim 1 2
1Myongji University Yongin-si Republic of Korea2Myongji University Yongin-si Republic of Korea
Show AbstractDiagnosis of disease, personalized medicine and drug discovery requires a low-cost and fast detection technique. Most of the DNA sensors involve labeling step which is unnecessary and requires large optical systems and specialized analysis procedure. One of the best candidates for disposable tool is pentacene thin film transistor due to their low-cost fabrication process as well as fast detection. In this study, we integrated pentacene TFTs and PDMS micro-channel for the use as biosensor for flow-detection of single-strand DNA and hybridization on pentacene layer. At first, we flowed double-distilled deionized water through PDMS microchannel on pentacene channel. Sequentially, we flowed ss-DNA (25 mer ployA) dissolved in DDI water on pentacene channel. When ss-DNA flows through PDMS micro-channel, it gets immobilized on pentacene surface. After that, when we flow complementary ss-DNA, the two ss-DNA hybridizes to form a ds-DNA. Immobilization of ss-DNA and ds-DNA dramatically differentiates the electrical response of OTFT. The process produced a dramatic change in the channel current (IDS) and field-effect mobility (μFET) of the devices. We demonstrated the feasibility of OTFT as a disposable sensor for DNA immobilization and hybridization.
9:00 AM - J14.26
Electrochromic Parameters of the Electrical Resistivity vs. the Thickness of Layers Applied in Flexibles Electrochromic Devices
Andre Felipe da Silva Guedes 1 Eleani M Costa 2 Vilmar P Guedes 1 Idaulo J Cunha 3 Simone Tartari 1
1INTELLECTOS Torres Brazil2PUCRS Porto Alegre Brazil3UFSC Florianoacute;polis Brazil
Show AbstractThe synthesis and application of new organic materials, nanostructured, for the development of technology based in organic devices have received much interest in the scientific community. In recent years entered in the market the first polymeric electronics products (organic semiconductor materials), among them there are the electrochromic devices, which are called smart windows, by controlling the passage of light or heat through a closed environment, like an ordinary window. The main functional aspects of electrochromic devices for use in architectural and automotive industry are to control the passage of light and temperature for thermal and visual comfort. These devices can be flexible and very thin, and do not contain heavy metals. The devices are formed by layers of organic material deposited in various architectures. In this work, the electrodeposition from organic materials in the case Polyaniline, which provides stability in the optical and electrical parameters, were used to develop prototypes of organic electrochromic devices. These materials were characterizated by: absorption spectroscopy ultraviolet-visible (UV-Vis), scanning electron microscopy (SEM), measurement of thickness and electrical measurements. The work objective was to the establishment of the relationship between thickness of the active layer and the value of the electrical resistivity of the layer deposited by electrodeposition technique. The experimental results enabled the equationing of the electrical resistivity in function to the thickness of the deposited layer. The linear fit of these results expresses the thickness of the conducting layer, α, and the lowest value of the electrical resistivity, B, associated with the gap between the valence band and conduction band. The results demonstrate that, when the layer of organic material is completely conductive, we obtain the thickness of the organic material deposited on the substrate.
9:00 AM - J14.27
Electrode-independent Electron Injection Using an Organic p-n Junction for Flexible Organic Light Emitting Diodes
Jeong-Hwan Lee 1 Ji Whan Kim 1 Sei-Yong Kim 1 Seung-Jun Yoo 1 Jae-Hyun Lee 1 Jang-Joo Kim 1
1Seoul National Univ. Seoul Republic of Korea
Show AbstractRecently organic light emitting diodes (OLEDs) have been successfully launched in small sized mobile displays, and a large effort has been made to develop larger sized and flexible OLEDs. Flexible OLEDs on plastic substrates require flexible driving circuits fabricated at low temperature. Oxide thin film transistors (TFTs) are considered as a candidate for this purpose because the materials satisfy the requirements along with high electron mobilities. Moreover, the transistors can be fabricated using a solution process. Since the transition metal oxides are n-type semiconductors, inverted OLEDs are preferred to conventional OLEDs to utilize the oxide TFTs in active matrix OLEDs. The salient feature of inverted OLEDs compared with conventional OLEDs is the electron injection from the bottom electrode to an electron transporting layer (ETL). Most commoly used transparent electrodes such as indium tin oxide (ITO) and indium zinc oxide (IZO) or thin aluminium (Al) and silver (Ag) have work functions larger than 4.3 eV, while the lowest unoccupied molecular orbital (LUMO) level of organic materials used for the ETL is in the region of 2.8-3.3 eV, resulting in a large electron injection barrier from the electrode to the ETL. Moreover, solution processible electrodes such as graphene and PEDOT:PSS have been proposed as transparent electrodes for displays and solar cells. It would be very valuable to develop an electron injection layer operating efficiently independent of the work function of the cathode. In this study, we report an organic p-n junction as an efficient and cathode independent electron injection layer (EIL) for inverted bottom emission OLEDs (IBOLEDs). Forward bias in the OLEDs to inject electrons and holes into the device corresponds to the reverse bias in the p-n junction to generate electrons and holes at the junction. The charge carriers created by the external field at the junction encounter a lower barrier height than the injecting carriers from the electrode to the organic layer when they are extracted from the organic layer to the cathode by an external bias voltage, resulting in an efficient injection of electrons. An overpotential to generate electrons and holes was minimized by using a p-doped copper phthalocyanine (CuPc)/n-doped 4,7-diphenyl-1,10-phenanthroline (Bphen) layer as the charge generation layer, and this was the first successful application of an organic p-n junction as an EIL for organic electronics to the best of our knowledge. The organic p-n junction composed of a p-CuPc/n-Bphen layer shows almost the same electron injection characterisitics for the cathodes with different work functions whereas the injection characterisitics of the n-Bphen EIL significantly depends on the work function of the cathode. These facts indicate that the new pn-i-p structure with the organic p-n junction can be efficiently applied for high performance flexible organic electronics, regardless of the electrodes.
9:00 AM - J14.28
White Light Electroluminescence from Graphene-enhanced Single Polymer Comprising Two Color Emitters of Equal Molar Ratios
Raymond Chien-Chao Tsiang 1 Reagen Y Su 1
1National Chung Heng University Ming-Hsiung, Chiayi Taiwan
Show AbstractPoly(3-hexylthiophene-alt-9,9-dioctylfluorene) (PTAF), has been synthesized and used to fabricate a White polymeric light-emitting diode (WPLED). This conjugated alternating copolymer, PTAF, was synthesized by Suzuki cross-coupling reaction and comprises 50 mol% of 3-hexylthiophene which is an orange-red color chromophore and 50 mol% 9,9-dioctylfluorene which is a bluish-green color chromophore. Nanocomposite consisting PTAF and graphene nanosheets enhances the optoelectronic properties and the device ITO/PEDOT:PSS/(PTAF + 1% graphene)/Ca/Al shows two-color white electroluminescence. The white luminescence from a single polymer affords the WPLED device a simple structure and low fabrication cost.
9:00 AM - J14.29
New Blue Emitter System and Solution Processible Aluminum Electrode for OLEDs
Beomjin Kim 1 Hye Moon Lee 2 Jongwook Park 1
1Catholic University of Korea Bucheon-si Republic of Korea2Korea Institute of Materials Science (KIMS) Changwon Republic of Korea
Show AbstractThe molecular design and synthesis as well as the device performance of novel organic molecules for highly efficient blue emission in OLEDs will be discussed. One of the synthesized blue emitting materials exhibited excellent color coordinates of (0.156, 0.088) and external quantum efficiency of 7 % with three times longer life time than MADN, commercial compound, for deep-blue OLED emitters without doping. Several other compounds were designed in order to control the emitting efficiency based on the model compounds. According to the size of the side group, optimization of chemical structures in organic emitter will be proposed. Furthermore, low work function metal such as aluminum had not been available for solution process in OLED device, but novel solution process was recently achieved to get OLED emission. The related process method and data will be discussed.
9:00 AM - J14.3
Improvement of Efficiency of Polymer Light-emitting Diodes Using Solution-processed Small Molecular Electron Transport Layer
Yongwon Kwon 1 2 Myeongjin Park 1 2 Hyunkoo Lee 1 2 Changhee Lee 1 2
1Seoul Nat'l University Seoul Republic of Korea2Inter-University Semiconductor Research Center Seoul Republic of Korea
Show AbstractMultilayer polymer light emitting diodes (PLEDs) have been fabricated with SuperYellow (PDY-132) purchased from Merck as an emitting layer (EML) and solution-processed small molecular electron transport layers (ETLs). Both EML and ETL are formed by using spin-coating method. Three kinds of ETLs such as 2,2',2''-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi), 4,7-diphenyl-1,10-phenanthroline (BPhen), and 3-(4-biphenyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (TAZ) are utilized and dissolved in methanol which does not dissolve the underlying EML. By employing the ETL, the efficiency of the devices are improved compared to the device without ETL. The maximum external quantum efficiency (EQE) of the devices with TPBi, BPhen, and TAZ are 2.9 %, 1.7 %, and 1.6 %, respectively, while the device without the ETL shows EQE of 1.5 %. The ETL prevents quenching of radiative excitons at the cathode interface by acting as an efficient exciton blocking layer. The better EQE of the device with TPBi is partly attributed to the better film morphology compared to BPhen and TAZ, as evidenced from atomic force microscopy images.
9:00 AM - J14.30
Charge Transport and Recombination in Organic Light-emitting Diodes
Paul Blom 1 2 Martijn Kuik 1 Gert-Jan Wetzelaer 1 Herman Nicolai 1
1University of Groningen Groningen Netherlands2Holst Centre Eindhoven Netherlands
Show AbstractCharge transport and charge recombination are recognized as key ingredients in the performance of polymer light emitting diodes (PLEDs). In the last two decades a large effort has been put on the characterization of the transport of the dominant charge carrier, holes. It has been demonstrated that the hole transport is governed by hopping between localized states, characterized by a mobility that depends on density, electric field and temperature. The strongly reduced electron currents are generally attributed to the immobile trapping of electrons. We observe that in conjugated polymers the electron transport is limited by traps that are Gaussianly distributed in energy within the band gap. Remarkably, we show that the electron trap distribution is identical for a large variety of polymers, hinting at a common origin for electron traps. Photogenerated current measurements on PLED device structures reveal that next to the known Langevin recombination also trap-assisted recombination is an important recombination channel in PLEDs. The dependence of the open-circuit voltage on light intensity enables us to determine the strength of this process. Numerical modeling of the current-voltage characteristics incorporating both Langevin and trap-assisted recombination yields a correct and consistent description of the PLED, without the traditional correction of the Langevin pre-factor. At low bias voltage the trap-assisted recombination rate is found to be dominant over the free carrier recombination rate. As a result, we show that the ideality factor in the diffusion regime of a bipolar diode is governed by the recombination of trapped electrons with free holes.
9:00 AM - J14.33
Microporous Phase-separated Films of Polymer Blends for Enhanced Outcoupling of Light from OLEDs
Rui Liu 1 Zhuo Ye 1 Joong-Mok Park 1 Min Cai 1 Ying Chen 1 Kai-Ming Ho 1 Joseph Shinar 1 Ruth Shinar 2
1Iowa State University Ames USA2Iowa State University Ames USA
Show AbstractThin microporous films were formed by dropcasting a toluene solution containing various ratios of polystyrene: polyethylene glycol blends on a glass substrate, with OLEDs on the ITO that coated the opposite side of that substrate. We demonstrate for the first time that such easily-fabricated films with surface and bulk micropores in the index-matching polystyrene can serve as random microlens-like arrays to improve forward OLED light extraction by up to ~60%. A theoretical interpretation of the angular emission profile of the device, considering the geometrical change at the substrate/air interface and the scattering by the pores within the films, was established in excellent agreement with the experiments. The use of such blended thin films provides an economical method, independent of the OLED fabrication technique, for improving the outcoupling efficiency.
9:00 AM - J14.34
Synthesis and Device Performances of Thermally Stable Fused Aromatic Amine Based Green Phosphorescent Host Material
Mounggon Kim 1 Chil Won Lee 1 Jun Yeob Lee 1
1Dankook University Yongin-si Republic of Korea
Show AbstractWe have designed and synthesized a fused aromatic amine based host material, 10-phenyl-9,9-bis(4-(pyridin-3-yl)phenyl)-9,10-dihydroacridine(TPDAN2). A fused amine was a core structure and pyridine was introduced in the core structure to have bipolar charge transport properties. The TPDAN2 showed a highest occupied molecular orbital 5.97 eV and lowest unoccupied molecular orbital of 2.37 eV with a triplet energy of 2.76eV. It also showed high glass transition temperature of 125 degree Celcius due to the rigid core structure. Green phosphorescent organic light-emitting diodes were fabricated using the TPDAN2 as the host material and a quantum efficiency of 13.5% was achieved at luminance of 1000 cd/m2.
9:00 AM - J14.4
Corrugated Structure through Spin-Coating Process for Enhanced Light Extraction from Organic Light-emitting Diodes
Woo Jin Hyun 1 Sang Hyuk Im 2 O Ok Park 1 3 Byung Doo Chin 4
1Korea Advanced Institute of Science and Technology (KAIST) Daejeon Republic of Korea2Advanced Materials Division, Korea Research Institute of Chemical Technology (KRICT) Daejoen Republic of Korea3Daegu Gyeongbuk Institute of Science and Technology (DGIST) Daegu Republic of Korea4Dankook University Yongin, Gyeonggi Republic of Korea
Show Abstract
Light extraction from organic light-emitting diodes (OLEDs) is one of the most important issues for highly efficient OLED because, in conventionally structured OLED, less than 20% of the light generated from the emissive layer can escape from the device as useful radiation. In order to extract the light confined inside the devices for increasing external quantum efficiency, defined as the ratio of the number of emitted photons into air to the number of injected electrons, rapid progress have been made in the research about out-coupling techniques for the light extraction from OLEDs, however, those methods have required complex and high cost equipment needing vacuum systems or additional steps for preparing a template. Considering practical application, where it is essential that fabrication techniques are applicable to large area and cost effective, it is very desirable to develop a simple fabrication technique for out-coupling structure. In this study, we developed a very simple fabrication technique for out-coupling structure without use of a mold and a vacuum process to enhance the light extraction efficiency of OLEDs. Corrugated structure can be prepared by only spin-coating process with a small amount of a mixture of SiO2 and TiOx sol solution in a short time. The width and the height of the corrugation were characterized by varying the spin-coating speed and the mixing ratio of the two sol solutions. OLEDs were fabricated with and without the corrugated structure used as an out-coupling structure and then their device performance were compared. The OLEDs with the corrugated structure introduced at the backside glass substrate shows enhanced light extraction efficiency by about 10% without a change of electroluminescence spectrum. The enhancement attributes to decreased incidence angle of the light at the interface between the glass substrate and air, which reduces the trapped light by total internal reflection. This mold and vacuum process free approach could provide a simple and cost effective way to fabricate out-coupling structure over large area for practical applications.
9:00 AM - J14.5
Flexible Al-doped ZnO Films Grown on PET Substrate Using Linear Facing Target Sputtering for Flexible OLEDs
Hyun-Soo Shin 1 Han-Ki Kim 1
1Kyung Hee University Yongin Republic of Korea
Show AbstractWe report the characteristics of flexible Al-doped zinc oxide (AZO) films prepared by a plasma damage free linear facing target sputtering (LFTS) system on PET substrate for use as a flexible transparent conducting electrode in flexible organic light emitting diodes (OLEDs). The electrical, optical, and structural properties of LFTS grown flexible AZO electrodes were investigated as a function of DC power. We obtained a flexible AZO films with a sheet resistance of 39 Ohm/sqaure and an average transmittance of 84.86 % in the visible range although it was sputtered at room temperature without activation of the Al dopant. Due to effective confinement of the high-density plasma between the facing AZO targets, the AZO film was deposited on the PET substrate without plasma damage and substrate heating caused by bombardment of energy particles. Moreover, the flexible OLED fabricated on the AZO/PET substrate showed performance similar to the OLED fabricated on ITO/PET substrate despite of a lower work function. This indicates that LFTS is a promising plasma damage-free and low temperature sputtering technique for deposition of flexible and indium free AZO electrode for use in cost-efficient flexible OLEDs.
9:00 AM - J14.6
Simple Structure and High Efficiency Phosphorescent OLEDs Using Narrow Band-gap Bipolar Host Material
YoungHoon Son 1
1Kyung Hee University Seoul Republic of Korea
Show AbstractOrganic light-emitting diodes (OLEDs) have been accepted as a technology for mobile and television (TV) displays and for potential future use in the lighting industry. In particular, phosphorescent OLEDs (PHOLEDs) have been of considerable interest in recent years as singlet and triplet excitons contribute to the emission of photons. Highly efficient OLEDs with over 20% external quantum efficiency (EQE), an almost ideal value, have been reported using this electrophosphorescent technology. During the last couple of years, blue PHOLEDs with over 20% EQE have also been reported by several groups. However, the driving voltage is relatively high compared with those of red and green PHOLEDs. Therefore, high driving voltage and short lifetime issues have to be addressed for meaningful application of blue PHOLEDs. In order to make highly efficient PHOLEDs, the triplet energy of the host should be higher than that of the phosphorescent guest to facilitate the exothermic energy transfer from the host to the guest. Thereby, reverse energy transfer from the guest back to the host is prohibited effectively and triplet excitons are confined on guest molecules. However, the host materials with wide band-gaps often cause an increase in driving voltages of the PHOLEDs. Therefore, the selection of suitable host materials for PHOLEDs is very imperative to achieve the high efficiency. In this paper, we report high efficiency and low driving voltage blue PHOLEDs with a simple organic triple stacked structure. Narrow band-gap characteristics of our bipolar transporting host materials are a key parameter to give such results. These new host materials were synthesized based on very well-known hole transporting material, 4,4�,4�-tri(N-carbazolyl)triphenylamine (TCTA), which is consisted of one triphenylamine and three carbazoles. We substitute α-carbolin for three carbazoles in TCTA one by one. As the results, three new materials, KHU-BH-1, KHU-BH-2 and KHU-BH-3 were synthesized. Using a high-triplet-energy-hole transporting material of TAPC and a high-triplet-energy-electron transporting material of TmPyPB, the organic three stacked structure has been realized with three new narrow band-gap blue host materials. These host materials have bipolar characteristics and high triplet energy of >2.8 eV. Very low onset voltages of 2.8~3.0 V and driving voltages of 4.2~4.6 V to obtain a brightness of 1000 cd/m2 are achieved in this three stacked device configuration. Maximum external quantum efficiency above 20% is reported.
9:00 AM - J14.7
Charge Injection/Extraction at the Interface between Molybdenum Oxide and Organic Polymers in OLED and OPV Devices
Jonathan Griffin 1 Alastair Buckley 1 Darren C Watters 1
1University of Sheffield Sheffield United Kingdom
Show AbstractMolybdenum oxide has recently been used in multiple papers for both OLED and OPV devices with promising results, however relatively little work has been done on the study of the energy level structure of molybdenum oxide and how this relates to device performance. OLED (F8:F8BT) and OPV (PCDTBT:PCBM) devices have been fabricated using reactively sputtered molybdenum oxide as a hole injection/extraction layer for these devices. Molybdenum oxide layers were deposited from a molybdenum target using an oxygen argon blend for the plasma, the oxygen concentration during deposition was varied from 0-70% in order to vary the stoichiometry and the energy level structure of the layer. Results show that for OLED devices the performance increases until around 20-25% when a plateau is reached and the performance remains constant, this is attributed to the film itself becoming almost fully oxidised and mainly consisting of Mo 6+ with a small amount of 5+, the lower oxidation states appear to severely reduce device performance. Hole injection is believed to be due to transfer of holes from the conduction band of the molybdenum oxide into the HOMO level of the polymer. As for OPV devices they initially begin to work at 15% oxygen concentration, as the oxygen during deposition is increased a sharp rise in device performance is observed where at 40% a peak efficiency of 3.7% is reached. Upon further increase of the oxygen content device performance drops rapidly and at 50% the efficiency was 1.9%, a drop in total efficiency of around 1.8%. The observed changes in the efficiency of these devices are believed to be due to small amounts of dopant states lying just below the Fermi level of molybdenum oxide. These dopant states are attributed to a mixture of impurities and also small amounts of metallic molybdenum oxidation states that still remain, these occupied states allow for the extraction of holes from the HOMO level of the polymer.
9:00 AM - J14.8
New Zinc Bis-terpyridine Complexes for Possible Use in Light Emitting Electrochemical Cells (LEECs)
Nik Hostettler 1 Edwin C Constable 1 Catherine E Housecroft 1 Markus Neuburger 1
1University of Basel Basel Switzerland
Show Abstract
Finding ways of building low cost and energy efficient lighting devices is a very important present-day issue. Therefore zinc(II) bis-terpyridine complexes have been investigated for their emissive properties. On the one hand, Zn(II) has an advantage of being a d10 transition metal and so is coordinatively flexible and suffers from no unwanted redox processes. On the other hand, 2,2':6',2''-terpyridine is a well-studied chelating ligand which allows for both facile functionalisation and straightforward coordination to a metal centre, forcing Zn(II) into an {M(tpy)2} domain with a coordination number of 6. A series of homoleptic Zn(II) complexes has been synthesised using 4'-(4-RC6H4)-2,2':6',2''-terpyridine ligands, with different R substituents (R = Me, OMe, OH, SMe, NO2 and NPh2). The emissive behaviour of the complexes has been studied and a red-shift in the emission has been observed upon altering the substituents from poorly to highly electron donating. Electronic absorption, electrochemical and lifetime properties of the complexes will be presented, along with computational studies.
9:00 AM - J14.9
Experimental Determination of the Exciton Recombination Zone Width in Double- and Graded-emissive Layer Organic Light-emitting Devices
Nicholas C Erickson 1 Russell J Holmes 2
1University of Minnesota Minneapolis USA2University of Minnesota Minneapolis USA
Show AbstractIn this work, the width of the exciton recombination zone in conventional double-emissive layer organic light-emitting devices (OLEDs) is compared to that of devices containing a single-layer graded-composition emissive layer (G-EML). The G-EML consists of nearly 100% hole-transporting material (HTM) at the anode and nearly 100% electron-transport material (ETM) at the cathode, having a continuously varying HTM:ETM composition across the active layer. Electroluminescence originates from a phosphorescent guest that is uniformly doped throughout the G-EML. Conventional OLED emissive layer architectures suffer from a reduction in efficiency at high current density. This roll-off is often attributed to bimolecular exciton quenching and it has been previously suggested that a wide exciton recombination zone may reduce its severity. Here, the exciton recombination zone of each emissive layer architecture is probed through the use of a fluorescent sensitizer capable of quenching excitons formed on the emissive guest. The red-emitting fluorescent material, tetraphenyltetrabenzoporphyrin (TPTBP) is used to quench excitons formed on the archetypical green phosphor tris(2-phenylpyridine)iridium(III) (Ir(ppy)3). Triplet excitons formed on Ir(ppy)3 may undergo a Dexter-type energy transfer to TPTBP and recombine nonradiatively, leading to a decrease in phosphorescence from Ir(ppy)3. It is found that thin, lightly-doped sensitizer strips may be inserted into both double- and graded-emissive layer OLEDs without impacting electronic transport. By comparing the overall emission intensity of sensitized devices to control structures containing no sensitizer, a â?~mapâ?T of the local exciton density may be determined. Here, it is found that the single-layer G-EML architecture exhibits a broad exciton recombination zone, while analogous double-emissive layer devices show a significantly narrower width centered on the HTM:ETM interface. The wider recombination zone of the G-EML comes as a result of the intrinsically broad overlap between the electron and hole densities, which also leads to high efficiency in these structures. The ability to engineer the exciton recombination zone by varying the emissive layer architecture may ultimately permit devices with a reduction in the efficiency roll-off to be realized.
J11: Displays II
Session Chairs
Thursday AM, April 12, 2012
Moscone West, Level 2, Room 2008
9:30 AM - J11.1
Vapor Jet Direct Printing of a Multicolor Organic Light Emitting Diode Array
Gregory J McGraw 3 Stephen R Forrest 1 2 3
1University of Michigan Ann Arbor USA2University of Michigan Ann Arbor USA3University of Michigan Ann Arbor USA
Show AbstractOrganic Vapor Jet Printing (OVJP) is a small molecule organic thin film deposition technique that combines growth and device patterning into a single process. It eliminates the need for shadow masking steps in the fabrication of multicolor organic light emitting diode (OLED) arrays.[1] The OVJP process uses an inert carrier gas to drive the vapors of host and dopant materials into a heated microfluidic print head. The evaporants are mixed in microchannels and collimated into jets by an array of micronozzles. The jets produce stripes of doped organic thin film on a chilled substrate as it translates below the print head. Multicolor printing can be achieved in a single pass by incorporating multiple mixing channels into a print head. Alternating printed stripes of different electroluminescent mixtures can form the sub-pixels of a full color OLED display. In this work we discuss a print head fabricated using a double anodic bond that seals the silicon nozzle arrays, to the microfluidic channels etched in glass, to a metal print head support. This assembly technique eliminates sealants that can outgas at the high temperatures required for the evaporation and transport of the organic molecules. OVJP has demonstrated the ability to grow phosphorescent OLEDs (PHOLEDs) with an external quantum efficiency of 8.0%, which is comparable to analogous devices grown entirely by vacuum thermal evaporation. Analytical and computational models for organic vapor and carrier gas transport within the print head have been developed to help understand the OVJP process and have been experimentally confirmed. An array of 30 µm wide, alternating stripes of red and green PHOLED emissive layers separated by 10 µm are patterned by a two color print head. The capability to print features with size and spacing comparable to those found in a high definition color display is evaluated. Cross contamination of electroluminescent materials between stripes can reduce the color rendering ability of the OLED array, and must be minimized. Methods of mitigating cross contamination, and for registering and addressing individual pixels will be discussed. [1] G. J. McGraw, D. L. Peters, and S. R. Forrest, Appl. Phys. Lett. 98, 013302 (2011).
9:45 AM - J11.2
Multi-electrochromic Materials: Devices and Characterization
Rupasree Ragini Das 1 Chang Ho Noh 1 Seog-Jin Jeon 1 Hong Shik Kim 1 Woo Sung Jeon 1 Jung Woo Kim 1 Sang Yoon Lee 1
1Samsung Advanced Institute of Technology, Samsung Electronics Yongin-Si Republic of Korea
Show AbstractElectrochromic materials have been the current research focus for the application in the electrochromic displays for Color e-Paper due to their vivid color, good visibility in bright sunlight, low power consumption, and wide viewing angle among various candidates such as LCD, OLED, and particle-type display (electrophoretic) with color filter. We will discuss the multielectrochromic materials and devices comprising small organic molecules and viologens demonstrating RGB along with black (K) colors in the oxidized and reduced states. The compounds responsible for the oxidative colored states are small organic molecules with an anchor to help adsorbtion on TiO2, whereas the reductive colors are produced by small molecules as well as viologens. Our devices made by ink-jet printing show good reflectivity, contrast ratio, color gamut, and bistability, important for the reflective displays. The structural modulation for color tuning, color enhancement, fast coloring and bleaching strategy will also be emphasized.
10:00 AM - J11.3
Developments in Low Voltage, Three-color Carbon Nanotube Enabled Vertical Organic Light Emitting Transistors for AMOLED Displays
Bo Liu 1 Mitchell A McCarthy 1 Evan P Donoghue 1 Do Y Kim 2 Franky So 2 Andrew G. Rinzler 1
1University of Florida Gainesville USA2University of Florida Gainesville USA
Show AbstractCarbon nanotube enabled vertical organic light emitting transistors (CN-VOLETs) show promise to solve some of the major problems limiting widespread adoption of AMOLED technology. The novel device architecture results in three important benefits over the traditional side-by-side TFT and OLED active matrix design. Firstly, the vertical architecture permits sub-micron transistor channel lengths without high resolution patterning. Such short channel lengths permit comparatively low mobility organic semiconductors to compete against polycrystalline silicon in providing the high drive currents required by OLEDs in AMOLED displays, at a low power burden. Secondly, the high transparency of the dilute nanotube source electrode yields an aperture ratio (excluding the switching TFT and address lines) that approaches unity. A higher aperture ratio allows the OLED to be driven at a lower intensity for a given display brightness, prolonging the lifetime of the OLEDâ?"a major problem still faced by OLEDs today. Thirdly, the CN-VOLET architecture simplifies the fabrication process because it combines the driving transistor, storage capacitor and OLED into a single device, eliminating two of the four components in the most basic conventional AMOLED design.
J12: Sensors
Session Chairs
Thursday AM, April 12, 2012
Moscone West, Level 2, Room 2008
10:15 AM - J12.1
Near-infrared Photodetector Consisting of J-Aggregating Cyanine Dye and Metal Oxide Thin Films
Timothy P Osedach 1 2 Antonio Iacchetti 3 Richard R Lunt 4 Trisha L Andrew 1 Patrick R Brown 5 Gleb M Akselrod 5 Vladimir Bulovicacute; 1
1Massachusetts Institute of Technology Cambridge USA2Harvard University Cambridge USA3Politecnico di Milano Milan Italy4Michigan State University East Lansing USA5Massachusetts Institute of Technology Cambridge USA
Show Abstract
We demonstrate a photodetector structure that employs metal-oxide charge transport layers and that is sensitized at near-infrared wavelengths by a thin film of a J-aggregating cyanine dye. The high absorption coefficient of the J-aggregate film, combined with the use of a reflective anode and optical spacer layer, enables an external quantum efficiency (EQE) of 16.1 ± 0.1% (λ = 756 nm) to be achieved at zero-bias in a device consisting of an 8.1 ± 0.3 nm-thick dye film. The specific detectivity (D*) and response speed (f3dB) of the fully-optimized device are measured to be (4.3 ± 0.1)Ã-1011 cm Hz1/2 W-1 and 91.5 kHz, respectively. Modeling of our structure reveals that the photocurrent is limited by the diffusion of photo-generated excitons to the ZnO/J-aggregate hetero-interface and we determine the exciton diffusion length in the J-aggregate film to be LD = 2.0 ± 0.4 nm. This work provides insights relevant to the use of J-aggregating cyanine dyes in photodetector and photovoltaic applications and highlights the importance of engineering the optical field profile within such structures in order to maximize performance.
10:30 AM - *J12.2
Colloidal Quantum Dot Light Sensors and Solar Cells
Edward Sargent 1
1University of Toronto Toronto Canada
Show AbstractColloidal quantum dots are, like semiconducting organics and polymers, synthesized in and processed from the solution phase. They offer wide and highly-controlled tunability of their bandgap via the quantum size effect, including in the range 400 nm â?" 3000 nm. I will review progress in understanding electron and hole transport and recombination in solution-cast CQD films and at interfaces with metals and transparent conductive oxides. I will discuss how these advances have been applied in achieving photodetectors and image sensors having high sensitivity and excellent response × and in building solar cells that harvest both the sunâ?Ts visible and infrared spectral regions.
11:30 AM - *J12.3
Organic and Hybrid Photodiodes for Industrial and Medical Applications
Sandro Francesco Tedde 1 Arca Franceso 1 Maria Sramek 1 Joachim Wecker 1
1Siemens AG Erlangen Germany
Show Abstract
Organic semiconductors are attractive materials as active elements for a variety of optoelectronic devices, such as light emitting diodes, solar cells, and photodiodes. In particular the high charge separation efficiency obtained with an interpenetrating donor-acceptor heterojunction [1,2] was key for cost-efficient and large scale fabrication of organic solar cells and organic photodiodes (OPDs).Here, we will discuss the performance of OPDs with spectral sensitivities that can be tailored to cover the range from the visible to the infrared in combination with amorphous Silicon (a-Si) thin film transistor (TFT) backplane technology to create a pixelated photodetector for X-ray or short-wavelength infrared (SWIR) imaging. The detector consists of 256x256 squared pixels with 154 μm pitch. On top of the backplane a photoactive bulk heterojunction (BHJ) layer is solution-processed [3] consisting of a blend of semiconducting polymers as absorber and electron donor material for the visible, fullerenes as electron acceptor material, and additional inorganic absorber for SWIR. The most interesting processing technique of OPDs is effortless sprayâ?"coating, which is a high as well as a low-throughput fabrication method. Despite the surface roughness of spray-coated BHJ layers, the OPDs show superior current-voltage characteristics [4]. Furthermore, no lithographic steps are necessary for the photoactive layer and the top electrode to achieve very low crosstalk levels between neighbouring pixels. Pixelated OPDs show high external quantum efficiency up to 60% in the visible spectrum, low dark currents densities of 5x10-6 mA/cm2 at -5 V reverse bias, as well as high linearity and dynamic range. With a 20 μm thin film encapsulation the detector life time exceeded 1000 h at accelerated aging conditions of 85°C and 85% relative humidity. For applications beyond the silicon bandgap limit hybrid BHJ photodiodes containing PbS colloidal quantum dots as tunable absorber have been applied to extend the spectral sensitivity up to 1.8 μm. Rectification ratios of ~6,000, minimum lifetimes of one year and external quantum efficiencies of up to 51% have been demonstrated [5]. By integration of the solution-processed devices on the a-Si TFT backplane, it has been even possible to show for the first time near-infrared imaging and videos at 1300 nm with hybrid photodiodes. References [1] G. Yu, J. Gao, J. C. Hummelen, F. Wudl, A. J. Heeger, Science 270, 1789 (1995). [2] N. S. Sariciftci, L. Smilowitz, A. J. Heeger, F. Wudl, Science 258, 1474, (1992). [3] C. J. Brabec, S. Sariciftci, J.C. Hummelen, Adv. Funct. Mater., 11, No.1, (2001). [4] S. Tedde, J. Kern, J. Fürst, P. Lugli, O. Hayden, Nano Letters 9, 980-983 (2009). [5] T. Rauch et al, Nature Photonics 3, 332-336 (2009).
12:00 PM - J12.4
Silicon/Organic Hybrid Heterojunction Infrared Photodetector Operating at Room Temperature in Telecommunication Range
Mateusz Bednorz 1 Thomas Fromherz 1 Gebhard J Matt 2 Eric D Glowacki 3 Christoph J Brabec 2 Markus Scharber 4 Helmut Sitter 1 Niyazi S Sariciftci 3
1Johannes Kepler University Linz Austria2Friedrich- Alexander-Universit#127;aet Erlangen-N#127;urnberg Erlangen Germany3Johannes Kepler University Linz Austria4Konarka Austria Linz Austria
Show Abstract
The authors report on the fabrication and performance of a silicon/organic heterojunction based IR photodetector. Perylene derivative N,N'-Dimethyl-3,4,9,10-Perylenetetracarboxylic diimide (DiMe-PTCDI) thin film (thickness around 25nm) is grown by the Hot-wall epitaxy technique on boron doped p-Si substrate. As a result, recorded with scanning electron microscope, a closed layer followed by crystallites with lateral dimensions of several 100nm is formed. The ohmic contacts to both silicon and organic layer are formed by thermal deposition of 100nm Al contact. The back Al/Si contact is initially alloyed to provide ohmicity. Such a device exhibits for a bias variation from -1V to +1V a rectification ratio of 104 and 1011 at 300K and at 77K respectively. The presence of barrier for charge carriers at the interface is additionally confirmed by the thermal activation measurement of reverse dark current, resulting in the value of around 0.5eV. It is demonstrated that although both materials composing the device do not absorb light in the NIR range the Al/p-Si/DiMe-PTCDI/Al heterostructure exhibits a room temperature photovoltaic effect up to 2.3μm (0.54eV). It will be outlined that the short circuit photocurrent in the sub-band gap range originates from an absorption mechanism at the organic/inorganic interface, similarly to fullerene based devices reported in [1]. Nevertheless the achieved responsivity of 0.2mA/W at 1.55μm and 300K [2] is two orders of magnitude higher than in our previously presented report [1], in spite of the fact that the device has not yet been fully optimized. A detailed study of the performance of photodetector presented in [2] will be shown, including the response time analysis and the measurement of specific detectivity. The influence of surface treatment, organic layer thickness and electrode area will be presented with the scope of possible improvement of the device performance. The room temperature operation, powerless operation mode and most of all the non-invasive deposition of the organic interlayer onto the Si that results in compatibility with the CMOS process, make the presented approach a potential alternative to all inorganic device concepts. [1] Fullerene Sensitized Silicon for near- to Mid-Infrared Light Detection G. Matt, T. Fromherz, M. Bednorz, S. Zamiri, G. Goncalves, C. Lungenschmied, D. Meissner, H. Sitter, N.S. Sariciftci, C. Brabec, G. Bauer Adv. Mat. 22 (2010), 647 [2] Silicon/organic hybrid heterojunction IR photodetector operating in the telecom regime M.Bednorz, G. Matt, E.Glowacki, T.Fromherz, C.J.Brabec, M.Scharber, H.Sitter, submitted to Advanced Functional Materials
12:15 PM - J12.5
Ultrasensitive Nanocomposite Ultraviolet Detector Enabled by Interfacial Trap-controlled Charge Injection
Jinsong Huang 1 Fawen Guo 1
1University of Nebraska Lincoln Lincoln USA
Show AbstractUltraviolet photodetectors (UV-PDs) are under investigation for many applications in the medical, industrial, communication, defense, and research fields. Today, semiconductor solid UV-PDs are generally made of single crystalline silicon, silicon carbide (SiC) or gallium nitride (GaN) p-n junction photodiodes. However, these single crystalline PDs have limitations that make them unsuitable for many applications due to their high cost, low quantum efficiency of less than 40% (responsivity of less than 0.2 ampere per watt (A/W)), and demand of cooling for very weak light sensing. Solution processed PDs based on organic- and/or nano-materials represent a direction to significantly reduce the cost but always leading to a lower performance than inorganic counterpart. Here, we report a solution processed UV-PD with a nanocomposite active layer composed of zinc oxide (ZnO) nanoparticles (NPs) blended with semiconducting polymers which significantly outperforms the inorganic PDs. This class of PDs shows a self-adaptive transition from a Schottky contact in the dark to ohmic contact under irradiation, and a huge internal gain of 4,080 enabled by the trap-controlled charge injection. The combination of low dark current of the Schottky diode and high responsivity of 1,165 A/W under a relatively low bias below ten volts leads to a very high detectivity of 1.9 Ã- 10^15 Jones at 340 nm at room temperature. It is one to two orders of magnitude higher than that of the existing inorganic semiconductor UV-PDs. The gain-bandwidth product of these PDs is over 10^5 larger than the colloidal quantum dot PDs. These low cost thin-film photodetectors have great potential for the existing applications of very weak UV and visible light detection and can potentially open new application opportunities because of their flexibility, light weight and printability.
12:30 PM - J12.6
Charge Carrier Trapping in NO2 Sensors Based on Field-effect Transistors
Anne-Marije Andringa 1 2 Nynke Vlietstra 1 2 Juliaan Meijboom 2 Edsger Smits 3 Simon Mathijssen 2 4 Paul Blom 1 3 Dago de Leeuw 1 2
1University of Groningen Groningen Netherlands2Philips Research Laboratories Eindhoven Netherlands3Holst Centre Eindhoven Netherlands4Eindhoven University of Technology Eindhoven Netherlands
Show Abstract
Detection of nitrogen dioxide, NO2, is required to monitor the air-quality for human health and safety. Commercial sensors are typically chemiresistors, however field-effect transistors are being explored for their reported sensitivity gain.[1] Although numerous investigations have been reported using conjugated monomers and polymers, carbon nanotubes and inorganic nanowires, the operating mechanism of NO2 sensing is not clear. Here, we show that a positive threshold voltage shift is observed upon exposure to NO2 using semiconductors as polytriarylamine, a quinquethiophene self-assembled monolayer, perylene and zinc oxide. The generic response is further investigated using zinc oxide as a model semiconductor.[2]
Upon NO2 exposure and application of a positive gate bias, the current through the semiconductor gradually decreases. The gate induced electrons are immobilized in traps formed by NO2. The fixed interface charges yield a shift of the threshold voltage towards the applied gate bias. With time, mobile electrons get trapped until the gate bias is completely compensated by immobile trapped charges. We show that the temporal behavior of the threshold voltage shift can phenomenologically be described by a stretched-exponential function. This description yields two parameters: a characteristic relaxation time (Ï") and a dispersion parameter β equal to
T/T0, where
T0 is a temperature describing the exponential distribution of trap sites. The obtained characteristic relaxation time is inversely proportional to the partial NO2 pressure. A shift of the threshold voltage is observed for extremely low NO2 concentrations down to 10 ppb.
Crucial for the sensor application is the temperature dependence of charge trapping and recovery. At room temperature no recovery is observed, however charge detrapping can be achieved at elevated temperatures. We present the activation energy for trapping and detrapping extracted from temperature dependent stress and recovery measurements. The energy level of the trap species is determined by means of thermally stimulated current measurements. A tentative interpretation of the microscopic mechanism will be presented and the consequences for NO2 TFT sensors will be discussed.
[1] Torsi, L.; Dodabalapur, A.; Anal. Chem. 2005, 77, 380A.
[2] Andringa, A.; Meijboom, J. R.; Smits, E. C. P.; Mathijssen, S. G. J.; Blom, P. W. M.; Leeuw, D. M. de; Adv. Funct. Mater. 2011, 21, 100.
12:45 PM - J12.7
Study of Organic Magnetoresistance in MEHPPV with Admittance Spectroscopy
Thaddee Kamdem Djidjou 1 Tho D Nguyen 1 Z. V Vardeny 1 Andrey Rogachev 1
1University of Utah Salt Lake City USA
Show AbstractThe organic magnetoresistance (OMAR) in organic light emitting diodes (OLED) made of poly(2-methoxy,5-(2'-ethylheloxy)-1, 4-phenylene vinylene) (MEH-PPV) was investigated by means of DC transport and the admittance spectroscopy in the range of 1Hz to 10MHz at room temperature. The measurements were carried out on unipolar and bipolar OLEDs made of pristine MEH-PPV as well as MEH-PPV with traps introduced by the UV light irradiation. We found that in bipolar UV-exposed OLEDs, the magnitude of the magnetoresistance effect in real part of the admittance increases with DC bias, reaching a very high value of 35% (in the field of 30mT) at bias of 4.8V and decreases at higher bias voltages. Also, we observed that the cutoff frequency of OMAR effect monotonically increases with DC bias voltage. The cutoff has extrinsic origin and is likely caused by a dissipative process related to the reorientation of permanent dipoles. At the highest tested bias voltage of 6.7V, we were able to detect the OMAR effect at the highest frequency of our system, 10MHz. We have found that the imaginary part of the admittance is also affected by the the magnetic field. The effect of the magnetic field on dynamical capacitance of the device at low frequencies is very strong and opens up a possibility of using these devices as magnetic field sensors.