Organic Electronics 2011 Volume 12 №12

Elsevier. — 295 p. — ISSN: 1566-1199.«Organic Electronics» is a journal whose primary interdisciplinary focus is on materials and phenomena related to organic devices such as light emitting diodes, thin film transistors, photovoltaic cells, sensors, memories, etc.Papers suitable for publication in this journal cover such topics as photoconductive and electronic properties of organic materials, thin film structures and characterization in the context of organic devices, charge and exciton transport, organic electronic and optoelectronic devices.«Organic Electronics» provides the forum for applied, fundamental and interdisciplinary contributions spanning the wide range of electronic properties and applications of organic materials. A Letters section is included for rapid publication of short articles announcing significant and highly original results.1973-1979
Kyung-Ryang Wee, Ae-Li Kim, So-Yeong Jeong, Soonnam Kwon, Sang Ook Kang. The relationship between the device performance and hole mobility of host materials in mixed host system for deep blue phosphorescent organic light emitting devices
Abstract:
We report on the systematic investigation of the relationship between hole mobility of host materials and device performance to maximize the power efficiency. Three different materials with different hole mobility were used as host materials of iridium(III)[bis(4,6-difluorophenyl) pyridinato-N,C^2']tetrakis(1-pyrazolyl)borate (FIr6) based deep blue phosphorescent organic light-emitting device (PHOELD). Host material, tris(4-(N-carbazolyl)phenyl)methylsilane (MCBP), with higher hole mobility (1,1 x 10^-3 cm² V^-1 s^-1), can accommodate more electron current carried by an n-type host, bis(4-(4,5-diphenyl-4H-1,2,4-triazol-3-yl)phenyl)dimethylsilane (SiTAZ), than the other p-type hosts with lower hole mobility, maintaining the charge balance between hole and electron current. As a result, the optimized device shows high external quantum efficiency (EQE) of 19,0%, high power efficiency of 23,5 μm/W, low efficiency roll off, and deep blue color coordinates of (0,15, 0,23).1980-1987
Biswanath Mukherjee, Moumita Mukherjee. One-step fabrication of ordered organic crystalline array for novel optoelectronic applications
Abstract:
We report the fabrication of ordered crystalline array of low molar-mass, soluble organic compounds (TCNQ, TTF) in large scale by a simple solution casting technique requiring no additional surface treatment. Different solvents and different processing conditions have been applied to find the optimum conditions for direct growth of the ordered crystalline arrays. Based on these crystalline arrays, organic field-effect transistors (OFETs) and photodetectors (PDs) have been fabricated which exhibited excellent device performances. TCNQ (TTF) based OFETs exhibited a low threshold voltage, high on/off current ratio (~10³) and a field-effect mobility as high as 0.02 (0,05) cm² V^-1 s^-1. The ordered organic crystalline arrays, used in the fabrication of PDs, showed reproducible and reversible photoswitching characteristics with photo to dark current conversion ratio ~
40. This one-step, solution-based, self-assembly method for ordered array of different organic crystals in large area could have significant potential for future large-scale and low-cost optoelectronic devices. In addition, this study may encourage the researchers to synthesize and investigate for different organic materials capable of forming high quality nano/micro crystalline structures for various applications in plastic electronic devices.1988-1991
Alessandro Troisi. The speed limit for sequential charge hopping in molecular materials
Abstract:
Elementary arguments are used to show that there is a maximum charge mobility that can be described by sequential hopping between molecules and that this maximum mobility can be expressed in terms of physical quantities that are all experimentally accessible. The evaluation of the maximum hopping mobility with realistic parameters suggests that sequential charge hopping is not the correct transport mechanism for the best molecular materials used for organic transistors.1992-2002
Ping Shen, Xinping Liu, Shenghui Jiang, Yuanshuai Huang, Ling Yi, Bin Zhao, Songting Tan. Effects of aromatic п-conjugated bridges on optical and photovoltaic properties of N,N-diphenylhydrazone-based metal-free organic dyes
Abstract:
A series of novel D–п–A hydrazone dyes (HB, HP, HF and HT) containing an N,N-diphenylhydrazone donor and 2-cyanoacetic acid acceptor linked by a different aromatic bridge (benzene, pyrrole, furan, and thiophene) have been designed and synthesized to evaluate the aromatic bridge effects on the photophysical, electrochemical and the photovoltaic properties of the hydrazone-sensitized TiO₂ solar cells. Each of the hydrazone-based dyes exhibited different adsorption behavior, frontier molecular orbitals, and photovoltaic performance depending on the identity of aromatic p-conjugated bridges. Specifically, HP-sensitized TiO₂ solar cell showed an obviously higher photocurrent, photovoltage, and power conversion efficiency than other three dyes. We interpret that these results were stemmed from light-harvesting abilities, the charge recombination rate and electron lifetime of different aromatic bridges. Optical spectroscopy, cyclic voltammetry, density functional theory calculations, electrochemical impedance spectra, and photovoltaic measurements were employed to support our proposal. With the addition of 1 mM chenodeoxycholic acid (CDCA) as the coadsorbent, a maximum power conversion efficiency of a DSSC based on HP was 7,74% (J_sc = 16,17 mA/cm², V_oc = 0,69 V, FF = 0,694) under simulated AM 1,5 G solar irradiation (100 mW/cm2). This work suggests that N,N-diphenylhydrazone and N-(2-ethylhexyl) substituted pyrrole moieties can be used as an alternative and effective donor and p-conjugated bridge, respectively, in the construction of efficient D–п–A organic dyes.2003-2011
Chih-Hung Tsai, Sui-Ying Hsu, Tsung-Wei Huang, Yu-Tang Tsai, Yan-Fang Chen, Yuan Hsuan Jhang, Lun Hsieh, Chung-Chih Wua, Yen-Shan Chen, Chieh-Wei Chen, Chung-Chun Li. Influences of textures in fluorine-doped tin oxide on characteristics of dye-sensitized solar cells
Abstract:
We investigated the influences of textures in the transparent conductor fluorine-doped tin oxides (FTO) on characteristics and performances of dye-sensitized solar cells (DSSCs). FTO conductors having varied textures were characterized for their physical, electrical and optical properties and were subjected to device studies to establish the correlation between FTO textures and DSSC characteristics/performances. The results suggest that the highly textured and high-haze FTO can effectively scatter a portion of the incident light into larger angles in the device, enhancing the propagation length and trapping of the incident light within the device and contributing to significant enhancement of absorption (and thus short-circuit current, quantum efficiency, and power conversion efficiency) in the device configuration. As a result, the conversion efficiency of the DSSC was enhanced from 8,18% to 10,1%, an improvement by 24%, with using the highly textured, large-roughness and high-haze FTO as the transparent conductor.2012-2018
Tse Nga Ng, Beverly Russo, Brent Krusor, Rene Kist, Ana Claudia Arias. Organic inkjet-patterned memory array based on ferroelectric field-effect transistors
Abstract:
An inkjet-patterned, flexible organic memory array was demonstrated using non-volatile ferroelectric field-effect transistors which remained functional below 0,6% tensile strain. Each memory cell is comprised of an addressing transistor and a ferroelectric memory transistor. Less than 20% cross-talk was observed between neighboring cells, and binary memory states in a 7 x 8 array were retained for at least 8 h. Variations among the printed memory transistors were characterized and shown to be caused by different rates of charge trapping in the semiconductor–ferroelectric interface.2019-2024
Yong Xu, Takeo Minari, Kazuhito Tsukagoshi, Romain Gwoziecki, Romain Coppard, Francis Balestra, Gerard Ghibaudo. Power transfer-length method for full biasing contact resistance evaluation of organic field-effect transistors
Abstract:
A development of the conventional transfer-length method (TLM) for organic transistors’ contact resistance evaluation is proposed. By adding the dissipated power in access resistance and in the channel, we found that TLM can be extended to the non-linear region, i.e., up to saturation regime, which is widely believed to be a barrier for TLM application now. To improve the extraction accuracy and stability, a modified TLM version is used for the operation at small gate voltages. This modified power TLM is applied to polymer and pentacene field-effect transistors and proved as a useful tool for the contact resistance evaluation as a function of gate and drain voltages from common current–voltage characterizations.2025-2032
Hsin-Hua Chang, Wan-Shan Tsai, Chien-Ping Chang, Nien-Po Chen, Ken-Tsung Wong, Wen-Yi Hung, Shou-Wei Chen. A new tricarbazole phosphine oxide bipolar host for efficient single-layer blue PhOLED
Abstract:
A novel tricarbazole phosphine oxide (POCz₃) with high triplet energy and promising physical properties serves as a bipolar host material of blue-emitting phosphor (FIrpic) to realize highly efficient single-layer blue PhOLED which achieves a maximum external quantum efficiency up to 9% (η_p = 10,4 μm/W, η_c = 21,3 cd/A) and shows low efficiency roll-off effect. POCz₃ was also utilized in a three-layer device with a double confinement effect exhibiting maximum luminance 60098 cd/m² and maximum EQE (power efficiency) of 14,5% (31,3 μm/W).2033-2039
Giorgio Mattana, Piero Cosseddu, Beatrice Fraboni, George G. Malliaras, Juan P. Hinestroza, Annalisa Bonfiglio. Organic electronics on natural cotton fibres
Abstract:
Nanoscale modification of natural cotton fibres with conformal coatings of gold nanoparticles, deposition of thin layers of the conductive polymer poly(3,4-ethylenedioxithiophene) (PEDOT) and a combination of these two processes were employed to increase conductivity of plain cotton yarns. This innovative approach was especially designed to fabricate two classes of devices: passive devices such as resistors obtained from electrically conductive cotton yarns, and two types of active devices, namely organic electrochemical transistors (OECTs) and organic field effect transistors (OFETs). The detailed electrical and mechanical analysis we performed on treated cotton yarns revealed that they can be used as conductors still maintaining a good flexibility. This study opens an avenue for real integration between organic electronics and traditional textile technology and materials.2040-2046
Wentao Xu, Shi-Woo Rhee. Low-temperature self-curable polyacrylate copolymer gate insulator for hysteresis-free organic field-effect transistors
Abstract:
We report on the fabrication of hysteresis-free organic field-effect transistors (OFETs) and metal–insulator–semiconductor (MIS) capacitors that utilize self-curable polyacrylate type copolymer (L-PA) as a gate insulator. Superior to previously reported polymer gate insulators, L-PA for the first time enables hysteresis-free OFETs and MIS capacitors through a low temperature (110–140 °C) curing process, which is compatible with the conventional plastic substrates (T_g 150 °C) for flexible electronics. The dielectric film showed high dielectric strength 4,5 MV/cm and low leakage current density of 5 x 10^-9–10^-8 A/cm² at 1 MV/cm, while the transistors displayed a decent field-effect mobility (l) of 0,91 cm²/V s, on/off current ratio (I_on/I_off) of 10⁶ and inverse subthreshold slope (SS) as low as 1,13 V/dec. Chemical changes were verified by FT-IR, DSC (differential scanning calorimetry) and TGA (thermo-gravimetric analysis) experiments and the surface properties of dielectrics and morphology of pentacene layers were also examined and correlated with OFET device performance.2047-2055
Pamela Schrögel, Nicolle Langer, Christian Schildknecht, Gerhard Wagenblast, Christian Lennartz, Peter Strohriegl. Meta-linked CBP-derivatives as host materials for a blue iridium carbene complex
Abstract:
We present four derivatives of 4,4'-bis(9-carbazolyl)biphenyl (CBP) for the use as host materials in blue phosphorescent organic light emitting diodes. By replacing the para-linkage by a meta-linkage of the carbazole substituents at the central biphenyl unit materials with improved thermal and optical properties are obtained. The triplet energy of the metalinked host materials is significantly increased to more than 2,90 eV compared to 2,58 eV for the para-linked CBP. Moreover, selective methyl substitution of the basic meta-CBP structure leads to materials with high glass transition temperatures up to 120 °C and electrochemical stability of the oxidised species against dimerisation. The high triplet energy allows the use of the meta-CBP derivatives as host materials for the carbene emitter mer-tris(N-dibenzofuranyl-N0-methylimidazole)iridium (III) (Ir(dbfmi)) with a pure blue emission at 450 nm.2056-2060
Hossein Zamani Siboni, Hany Aziz. The influence of the hole blocking layers on the electroluminescence stability of phosphorescent organic light emitting devices
Abstract:
Delayed electroluminescence (EL) measurements are used to investigate electroluminescence stability in phosphorescent organic light emitting devices (PHOLEDs) containing typical hole blocking layers (HBLs). The results show a strong correlation between the extent of hole blockage capacity of the HBL and the rate of deterioration in device EL efficiency, pointing to the major role that the build-up of hole space charges in the emitting layer (EML) plays in EL degradation. In this regard, the use of a strongly blocking material significantly increases the build-up of holes in the EML, and accelerates EL degradation.2061-2064
Silu Tao, Shiu Lun Lai, Chuan Wu, Tsz Wai Ng, Mei Yee Chan, Weiming Zhao, Xiaohong Zhang. Highly efficient blue organic electrophosphorescence devices using a trifluorine-replaced iridium complex
Abstract:
Highly efficient blue phosphorescent organic light-emitting devices have been achieved by using a new emitter Iridium complex, bis[(3,4,5-trifluorophenyl)-pyridinato-N,C^2'] picolinate (F₃Irpic). For a mCP:4%F₃Irpic device, blue emission with a maximum power efficiency of 18,1 μm/W has been realized, which is much higher than those of FIrpic based devices. The emission peak was located at 480 nm with a subpeak at 511 nm. By using an optimized device structure with a better host material, performance of the device can be further increased to give a maximum power efficiency of 33,7 μm/W.2065-2070
Shuming Chen, Hoi-Sing Kwok. Alleviate microcavity effects in top-emitting white organic light-emitting diodes for achieving broadband and high color rendition emission spectra
Abstract:
It is challenging to obtain broadband emission spectra in top-emitting white organic lightemitting diodes (WOLEDs) due to the well known microcavity effects. In this work, we demonstrate that the microcavity effects can be greatly alleviated by employing a low reflection tri-cathode-layer Yb (5 nm)/Au (15 nm)/MoO₃ (35 nm). Top-emitting WOLEDs with evenly separated red, green and blue emission peaks have been achieved. The white emission spectra show weak angle dependence as well. At a luminance of 1000 cd/m2, the top-emitting WOLEDs show an efficiency of 23 cd/A, 10,5 μm/W, a 1931 Commission International de L’Eclairage coordinate of (0,36, 0,42) and a high color rendering index of
85. The top-emitting WOLEDs have been successfully applied in the Si based microdisplay.2071-2075
Andreas Bauer, Jonas Hanisch, Tina Wahl, Erik Ahlswede. ZnO:Al-based recombination layers for polymer tandem solar cells: Influence of acidic or pH-neutral poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) formulations
Abstract:
We report on the fabrication of organic tandem solar cells which utilize sputtered ZnO:Al recombination layers in combination with the common p-type poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) polymer layer. The impact of the layer thickness on the device performance was investigated. Our experiments determined the minimal value of the ZnO:Al layer thickness in order to obtain a fully functional tandem solar cell. Although PEDOT:PSS is acidic and its application will reduce the initial ZnO:Al layer thickness by 10–50 nm, it could be applied to build functional tandem devices when the initial layer was thick enough to compensate for this loss in layer thickness. For comparison, we also investigated an alternative approach using a pH-neutral PEDOT:PSS formulation which lead to similar results regarding device performance. The conductivity and functionality of thicker ZnO:Al layers became sufficient to serve as a middle electrode to characterize the subcells within the tandem device. In this way, the bottom cell was measured after each subsequent top cell coating process step, revealing no alteration.2076-2083
Yumiko Kaji, Keiko Ogawa, Ritsuko Eguchi, Hidenori Goto, Yasuyuki Sugawara, Takashi Kambe, Koki Akaike, Shin Gohda, Akihiko Fujiwara, Yoshihiro Kubozono. Characteristics of conjugated hydrocarbon based thin film transistor with ionic liquid gate dielectric
Abstract:
Thin film transistors (TFTs) with ionic liquid gate dielectrics, [1-ethyl-3-methylimidazolium][bis(trifluoromethanesulfonyl)imide] (emim[TFSI]) and [1-butyl-3-methylimidazolium][hexafluorophosphate] (bmim[PF6]), are fabricated with thin films of one dimensional (1D) hydrocarbon, [7]phenacene. P-channel characteristics are observed for [7]phenacene TFTs with both ionic liquids by use of platinum electrode. The field-effect mobility μ for [7]phenacene TFT with bmim[PF₆] was recorded to be 0,28 cm² V^-1 s^-1. The value of absolute threshold voltage, |V_TH|, was less than 2,5 V, showing low-voltage operation. The accumulation of hole in the [7]phenacene TFTs with ionic liquids was confirmed from the voltage or time dependence of capacitance in metal–insulator-semiconductor structure, which shows that these TFTs operate electrochemically and the carriers are accumulated in the whole of [7]phenacene thin films.2084-2089
Jean-Christophe Bolsée, Wibren D. Oosterbaan, Laurence Lutsen, Dirk Vanderzande, Jean Manca. CAFM on conjugated polymer nanofibers: Capable of assessing one fiber mobility
Abstract:
Poly(3-alkylthiophene) (P3AT) nanofibers (NFs) are interesting as building blocks for future organic electronic devices but also as a model system to examine transport mechanisms in semiconducting polymers. However, due to the technical problem of making good contacts at the nanometer scale, electrical characterizations on one isolated P3AT NF are rare. Here, using Conductive Atomic Force Microscopy (CAFM), charge transport investigations along the length (п–п stacking direction) and the height (alkyl chain direction) of one single NF are reported. Firstly, by developing a transistor structure with one electrode being the CAFM tip, we measured the longitudinal mobility: μ_L = 0,07 ± 0,03 cm² V^-1 s^-1. Secondly, charge transport along the NF height was done in a sandwich structure and led to a transversal mobility of μ_T ~ 10^-5–10^-6 cm² V^-1 s^-1 where μ_T was found to decrease with increasing alkyl side chain length. With CAFM charge transport anisotropy in P3AT NF is thus evidenced at the nanometer scale, since we find at least four orders of magnitude difference between μ_L and μ_T.2090-2094
Min Guan, LinSen Li, GuoHua Cao, Yang Zhang, BaoQiang Wang, XinBo Chu, ZhanPing Zhu, YiPing Zeng. Organic light-emitting diodes with integrated inorganic photo detector for near-infrared optical up-conversion
Abstract:
We report a hybrid up-conversion device integrating a In_0,2Ga_0,8As/GaAs MQWs photodetector with an organic light emitting diode (OLED), that converts input 980 nm infrared light to output 520 nm green light. Devices with different interface layer, used as the hole injection layer (HIL) in OLEDs were fabricated and tested. It was found that the device with an HIL of MoO₃-doped CuPc exhibited a lowest turn-on voltage of 2,6 V. The maximum external up conversion efficiency of 0,81 W/W% is achieved at the bias of 20 V under an input 980 nm NIR power density of 1 mW/mm².2095-2102
Jonghwa Jeong, Debra Mascaro, Steve Blair. Precise pixel patterning of small molecule organic light-emitting devices by spin casting
Abstract:
We demonstrate the fabrication of OLEDs with precisely patterned pixels as small as 10 μm by the spin-casting of Alq₃ thin films. The solution-based patterning technique produces pixels via the segregation of organic molecules into micro-fabricated channels or wells. Segregation is controlled by a combination of weak adsorbing characteristics of aliphatic terminated self-assembled monolayers and by centrifugal force, which directs the organic solution into the channel or well. This novel patterning technique may resolve the limitations of pixel resolution of the method of thermal evaporation using shadow masks, and is applicable to the fabrication of large area displays. Furthermore, the patterning technique has the potential to produce pixel sizes to the limitation of photolithography and micromachining, thereby enabling the fabrication of high-resolution micro-displays. We discuss the patterning method and device fabrication, and characterize the morphological, optical, and electrical properties of the organic pixels.2103-2110
Yong-Jin Pu, Noriaki Iguchi, Naoya Aizawa, Hisahiro Sasabe, Ken-ichi Nakayama, Junji Kido. fac-Tris(2-phenylpyridine)iridium (III)s, covalently surrounded by six bulky host dendrons, for a highly efficient solution-processed organic light emitting device
Abstract:
The fully surrounded complexes by six host dendrons showed high photoluminescence quantum efficiency in a neat film, comparable to in a dilute solution. The surrounding host dendrons efficiently suppressed intermolecular interaction between central Ir complexes and prevented concentration quenching. The complex, (mCP)₆Ir, fully surrounded by six carbazole type hosts showed much better performance, compared with the complex, (DAP)₆Ir, surrounded by arylamine type hosts, because of well balanced charge injection and transporting in the devices.2111-2119
Jaron G. Van Dijken, Michael D. Fleischauer b, Michael J. Brett. Solvent effects on ZnPc thin films and their role in fabrication of nanostructured organic solar cells
Abstract:
Fabrication of interdigitated organic photovoltaic (OPV) devices commonly involves filling a nanostructured thin film using solution-based methods. In these cases, the sensitivity of the nanostructured films to the solvents used needs to be studied in order to optimize the interface. Here, we study the ability of chlorobenzene (CB) and dichlorobenzene (DCB) to dissolve and recrystallize zinc phthalocyanine (ZnPc) thin films, while fabricating interdigitated active layers composed of ZnPc and [6,6]-phenyl-C₆₁ butyric acid methyl ester (PCBM). Using glancing angle deposition (GLAD), we are able to construct slanted nanorod arrays of ZnPc, with nanorod diameters of nominally 40 nm and lengths up to 450 nm. We find that these films are recrystallized upon direct exposure to CB and DCB, regardless of initial morphology, yielding a variety of possible nanoscale formations. The crystallinity and absorbance of these recrystallized films changes as well, depending on solvent choice and exposure time. Through control of initial ZnPc film morphology and solvent choice for PCBM casting, we exploit the sensitivity of ZnPc to these organic solvents to optimize the photovoltaic performance of ZnPc/PCBM devices. Optimal OPV device performance is achieved with a 30 nm thick GLAD-textured ZnPc film while using DCB as the PCBM solvent. In this case, power conversion efficiencies are up to 3,0%, compared to an average of 2,3% when using CB as the solvent on the same film, and 2,3% also for bilayer devices when using DCB as the solvent. A higher degree of material mixing at the ZnPc/PCBM interface is shown when using DCB over CB as the PCBM solvent, which may be the primary mechanism for the photovoltaic improvements seen in these devices.2120-2125
Yongjoon Choi, Yuhee Kim, Sung-Geun Park, Young-Gon Kim, Bong June Sung, Sung-Yeon Jang, Woochul Kim. Effect of the carbon nanotube type on the thermoelectric properties of CNT/Nafion nanocomposites
Abstract:
The effect of different carbon nanotube (CNT) types on the thermoelectric performance of CNT/polymer nanocomposites was studied. Three different kinds of CNTs, single-(SWCNTs), few- (FWCNTs) and multi-walled CNTs (MWCNTs), were effectively dispersed in an aqueous solution of Nafion. The electrical properties of the CNT/Nafion nanocomposites were primarily affected by the CNTs since the Nafion acts as an electrically nonconducting matrix, while the thermal conductivity of the nanocomposites was dominated by the Nafion mainly due to weak van der Waals interaction. In this way, electrical and thermal transport can be separated. In all three types of CNTs, both the electrical conductivity and Seebeck coefficient increased as the concentration of CNTs was increased. While the electrical conductivity depends on the type of CNT, the behavior of the Seebeck coefficient was relatively insensitive of the CNT type at high CNT loading. This indicates that high-energy-charges can participate in transport processes irrespective of the type of CNT. It is suggested that FWCNTs and MWCNTs are preferred over SWCNTs in CNT/Nafion nanocomposites for thermoelectric applications.2126-2130
Vipul Gohri, Simone Hofmann, Sebastian Reineke, Thomas Rosenow, Michael Thomschke, Marieta Levichkova, Björn Lüssem, Karl Leo. White top-emitting organic light-emitting diodes employing a heterostructure of down-conversion layers
Abstract:
We investigate white top-emitting organic light-emitting diodes (OLEDs) based on a heterostructure of down-conversion (DC) layers. The white DCOLED comprises consecutive organic conversion layers of 4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran (DCM) doped in a matrix of tris(8-hydroxy-quinolinato)aluminum (Alq₃), and N4,N41'-bis-(4-tert-butyl-phenyl)-N4,N4'-di-fluoranthen-3-yl-diphenylether-4,4'-diamine (OYSE). The DC layers also function as capping layers to enhance the light outcoupling and optical modification of the underlying blue OLED. White light emission with CIE color coordinates of (0,27, 0,26) and a color rendering index of 60 is achieved. Furthermore, the spectral angular dependence of the white device is examined.2131-2139
Bregt Verreet, Robert Müller, Barry P. Rand, Karolien Vasseur, Paul Heremans. Structural templating of chloro-aluminum phthalocyanine layers for planar and bulk heterojunction organic solar cells
Abstract:
Chloro-aluminum phthalocyanine (ClAlPc) film growth on 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS) and MoO₃ is studied and correlated to ClAlPc/C₆₀ solar cell performance for both planar and bulk heterojunction (HJ) architectures. On top of unheated substrates, ClAlPc films grow amorphous independent of the substrate surface. When heated to 105 °C, ClAlPc grows with a face-on orientation on MoO₃, with a crystalline phase I-like absorption profile. On FDTS, the film is optically characterized as phase II, and adopts an edge-on orientation. Implemented in planar HJ cells, the latter films show a substantially higher current compared to the other growth conditions, leading to 3% efficient cells. This current increase is investigated with spectral response and reflectivity measurements, and is found to be related to a more efficient exciton dissociation. Next, ClAlPc and C₆₀ are co-evaporated on FDTS and MoO₃ modified ITO substrates to fabricate bulk HJ devices. Notably, we find that when a thin pure ‘‘templating’’ layer of ClAlPc is grown first, the subsequently grown ClAlPc:C₆₀ bulk HJ propagates the templating effect, and films show a higher crystallinity than without this templating layer, with higher fill factors as a result. On MoO₃, this approach yields efficiencies above 4%.2140-2143
Takashi Kushida, Takashi Nagase, Hiroyoshi Naito. Mobility enhancement in solution-processable organic transistors through polymer chain alignment by roll-transfer printing
Abstract:
A roll-transfer printing method has been proposed for the uniaxial alignment of the liquid crystalline polymer semiconductor of poly(3,3'''-didodecylquarterthiophene) (PQT). It is shown that the roll-transfer printing of PQT at its liquid crystal phase temperatures yields highly aligned films in which the PQT molecules are aligned parallel to the roll-transfer direction and oriented perpendicular to substrates. The average field-effect mobility in the alignment direction of 0,17 cm²/Vs is 8 times higher than that in the perpendicular direction. The alignment of polymer chains is attributed to the enhancement of shearstress effects during roll-transfer printing in the liquid crystal phase.2144-2150
Yonghun Kim, Gunuk Wang, Minhyeok Choe, Juhwan Kim, Sangchul Lee, Sungjun Park, Dong-Yu Kim, Byoung Hun Lee, Takhee Lee. Electronic properties associated with conformational changes in azobenzene-derivative molecular junctions
Abstract:
The electronic properties of azobenzene-derivative ([4-(phenylazo)phenoxy]hexane-1-thiol) molecular junctions were studied in terms of their molecular configurations with vertical device structure as solid-state device platform. This molecule has two distinct molecular configurations (trans- and cis-isomer) depending on the wavelength of irradiating light, which converts from more thermodynamically stable trans-isomer to cis-isomer under UV exposure (~365 nm) and reversible photoisomerization of cis-isomer to transisomer under visible light (400–500 nm). The two states showed that the conductance of cis-isomer (compact form) was higher than that of trans-isomer (extended form). From the temperature-variable electrical characterization, the main charge conduction mechanism for the two isomers was found to be tunneling. And, from the transition voltage spectroscopy analysis and ultraviolet photoelectron spectroscopy measurement, the origin of such result can be explained by reduction of molecular tunneling distance between two isomers.2151-2158
Jun-Jun Zhu, Zai-Quan Xu, Guo-Qiang Fan, Shuit-Tong Lee, Yan-Qing Li, Jian-Xin Tang. Inverted polymer solar cells with atomic layer deposited CdS film as an electron collection layer
Abstract:
An efficient inverted polymer solar cell (PSC) is reported by employing an atomic layer deposited (ALD) cadmium sulfide (CdS) film between the indium tin oxide (ITO) cathode and the photoactive layer as the electron collection layer (ECL), on which a active layer is composed of a blended poly(3-hexylthiophene) and [6,6]-phenyl C₆₁ butyric acid methyl ester (P3HT:PCBM) bulk heterojunction. As determined by photoelectron spectroscopy, the sulfur vacancy induces an n-type semiconducting property in the ALD-grown CdS films, and suitable energy level alignment at the ITO/CdS/PCBM interface is favorable to electron extraction through CdS to the ITO electrode. With the optimized CdS film thickness, the power conversion efficiency increases to 3,33%, with short-circuit current of 8,94 mA/cm², open-circuit voltage of 0,61 V, and fill factor of 61,1% under AM1,5G 100 mW/cm² irradiation.2159-2164
Tsung-Che Tsai, Hsiu-Cheng Chang, Chun-Hua Chen, Wha-Tzong Whang. Widely variable Seebeck coefficient and enhanced thermoelectric power of PEDOT:PSS films by blending thermal decomposable ammonium formate
Abstract:
The doping effects of thermal decomposable ammonium formate (AF) from 5 to 50 wt.% on the electrical conductivity, Seebeck coefficient, and microstructures of poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) films have been investigated for the first time for modern thermoelectric applications. It has been found that the Seebeck coefficient can be effectively tuned in a very wide range by varying the AF doping concentration, where a maximum value of 436,3 μV/K was obtained, i.e., ~40 times higher in magnitude than the pure PEDOT:PSS films prepared with same processes. The greatly enhanced Seebeck coefficient is considered to be the result of reducing the carrier concentration, as evidenced by the Hall measurement. In addition, AF also plays an important role in the formation of the closed or open pores and channels within the films for phonon scattering, as can be clearly observed in the SEM images. The present work provides a new procedure to effectively control the Seebeck coefficient as well as the microstructures of PEDOT:PSS polymer by a simple blending approach with suitable thermal steps, which has not previously been reported.2165-2173
Jae-hyeong Lee, Takashi Sagawa, Susumu Yoshikawa. Morphological and topographical characterizations in spray coated organic solar cells using an additional solvent spray deposition
Abstract:
In a spray coating process, characteristic performance of organic solar cells (OSCs) is limited by some drawbacks such as isolated droplets, non-uniform surface and pinholes. We have investigated the film formation, surface topography and the morphology of spray coated active layers with additional solvent implemented by using an additional solvent spray deposition. Highly efficient OSCs with improved interconnection among droplets and reduced amount of pinholes were obtained by additionally spraying o-dichlorobenzene as a solvent after the conventional spray coating of the mixed solution of poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester in chlorobenzene to form active layer solution. Moreover, the additional solvent spray causes changes in the matrix of the active layer resulting in the effective thermal annealing in terms of the power conversion efficiency of the OSCs.2174-2179
Hyokyun Ham, Jongwoon Park, Youngmo Kim. Thermal and barrier properties of liquid getter-filled encapsulations for OLEDs
Abstract:
We investigate the thermal and barrier properties of liquid getter-filled glass encapsulations for organic light-emitting diodes (OLEDs). A conventional glass encapsulation has a protective hollow where nitrogen gas resides. Such a configuration stores heat that causes damage to OLED devices. To dissipate heat, we have filled a hollow glass cap with a UV curable liquid getter, transferring heat from the device to the glass cap where a flexible heat sink is attached. With this scheme, however, heat is reduced only by 15,5 °C (from 62,3 to 46,8 °C), even though it is larger than that (7,7 °C) of OLED based on the conventional glass encapsulation. We have found that there still exists a N2 gas-filled gap, which appears during UV-curing of the liquid getter. To eliminate it, we have coated a liquid getter on a flat glass cap and then encapsulated the OLED device with it. This scheme dissipates heat by 27,7 °C (from 60,5 to 32,8 °C), yet it causes some damage to organic layers when the moisture getter absorbs out-gases, generating dark spots and thus shortening the device lifetime. By increasing the thickness of an Al cathode that is in contact with the moisture getter, we have obtained much enhanced thermal and barrier properties. Namely, the maximum surface temperature is reduced by 30,5 °C (from 62,4 to 31,9 °C) and the operating lifetime is comparable with that of the conventional glass-encapsulated OLED.2180-2184
Shuwen Yu, Carola Klimm, Peter Schäfer, Jürgen P. Rabe, Bernd Rech, Norbert Koch. Organic photovoltaic cells with interdigitated structures based on pentacene nanocolumn arrays
Abstract:
Arrays of crystalline and regularly spaced pentacene nanocolumns, with a typical diameter of ca. 150 nm, were fabricated by glancing angle deposition on indium tin oxide substrates. The nanocolumn arrays were used to form large interface area organic heterojunction photovoltaic cells (OPVCs) by spin coating [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) as the acceptor material to fill the voids between nanocolumn. The light to electrical energy conversion efficiency of nanocolumn-based OPVCs was three times higher compared to planar heterojunction OPVCs of the same materials due to a significantly enlarged donor/acceptor interface area where exciton dissociation can occur. Further OPVC performance improvement was achieved through employing a thin pentacene electron-blocking layer before nanocolumn formation, which prevented electron current leakage to the anode.2185-2191
Jinsoo Noh, Minhun Jung, Kyunghwan Jung, Gwangyong Lee, Soyeon Lim, Daae Kim, Sangsig Kim, James M. Tour, Gyoujin Cho. Integrable single walled carbon nanotube (SWNT) network based thin film transistors using roll-to-roll gravure and inkjet
Abstract:
Single walled carbon nanotube network (SWNTn) based thin film transistors (TFTs) were printed using roll-to-roll (R2R) gravure and inkjet on plastic foils to show the integrablility in point of mass production with a low cost. Based on the extracted parameters from printed drive and load TFTs with SWNTn (SWNTn–TFTs), the inverter and ring oscillator were simulated and compared with experimentally measured results of the printed circuits so that the limiting factors to integrate printed SWNTn–TFTs on the plastic foils has been addressed.2192-2197
Wenyu Ji, Letian Zhang, Kai Xu, Wenfa Xie, Hanzhuang Zhang, Guoqiang Liu, Jinbo Yao. Semitransparent white organic light-emitting devices with symmetrical electrode structure
Abstract:
A semitransparent white-light organic light-emitting device (SWOLED) with an (Ag/Alq₃)₂ cathode and (Alq₃/Ag)₂ anode was fabricated. The light emitted from the emitters subjecting to the same propagation process because of the symmetrical electrode structure. The device showed few differences in luminance, power distribution of electroluminescence (EL) spectra, efficiency and chromaticity coordinates from both sides. The maximum total current efficiency of the SWOLED (8,46 cd/A) is comparable to that of the corresponding bottom-emitting OLED (9,2 cd/A). The SWOLEDs have potential uses as tinted thin-film coatings on architectural surfaces, such as windows and walls. In addition, this kind of electrode can be used in flexible OLEDs.2198-2206
Guangjun Nan, Zesheng Li. Modeling of charge transport in polycrystalline sexithiophene from quantum charge transfer rate theory beyond the first-order perturbation
Abstract:
Charge mobility in polycrystalline organic semiconductors is often thermally activated, so a semiclassical Marcus charge transfer rate theory has long been used to investigate the charge transport properties of organic semiconductors. However, the classical treatment for the nuclear degrees of freedom and the first-order perturbative nature of electronic coupling in the semiclassical Marcus charge transfer rate theory is often invalid in organic semiconductors. Furthermore, traps in polycrystalline organic semiconductors are not considered during the simulations with the semiclassical Marcus charge transfer rate theory. In the present work, we propose a model to study charge transport properties in polycrystalline organic semiconductors which consist of trap-free crystallitic grains separated by boundaries between them. The charge transfer rate in grains is evaluated with a quantum charge transfer rate theory without weak electronic coupling approximation while the charge transport at grain boundaries is limited by energy barriers there. We find that a thermally activated mobility can be obtained from the quantum charge transfer rate theory when traps at grain boundaries are considered. Meanwhile, a roughly linear dependence of mobility on grain size is shown for large grain size while a rapid variation of mobility with grain size is observed when the grain size is small, which reconciles the discrepancy of the mobility versus grain size in experiments. In addition, the different mobilities for sexithiophene crystal structures in high- and low-temperature phases show that the mobility in polycrystalline organic semiconductors not only depends on the boundary property between grains but also the molecular packing in grains.2207-2214
P.F. Moonen, B. Vratzov, W.T.T. Smaal, G.H. Gelinck, M. Péter, E.R. Meinders, J. Huskens. A common gate thin film transistor on poly(ethylene naphthalate) foil using step-and-flash imprint lithography
Abstract:
In this paper the fabrication of flexible thin film transistors (TFTs) on poly(ethylene naphthalate) foil is reported, with the source–drain layer patterned by step-and-flash imprint lithography (SFIL) as a first step towards fully UV-imprinted TFTs. The semiconductor was deposited by inkjet printing of a blend of TIPS-pentacene/polystyrene. The bottom contact, bottom gate TFTs were fabricated with the foil reversibly glued to a carrier, enhancing the dimensional stability and flatness of the foil to result in a thinner and more homogeneously distributed residual layer thickness. The obtained performance of the TFT devices, showing a mobility of μ = 0,56 cm² V^-1 s^-1 with an on/off ratio of 10⁷ and near-zero threshold voltage, was found to be in good agreement with similar, photolithographically patterned state-of-the-art devices recently reported in literature. The results presented here show the feasibility of SFIL as a roll-to-roll compatible and down scalable patterning technique on flexible PEN foil for the fabrication of bottom-gate, bottomcontact flexible high-quality TFTs.2215-2224
Tran Thanh Tung, Tae Young Kim, Jong Pil Shim, Woo Seok Yang, Hyeongkeun Kim, Kwang S. Suh. Poly(ionic liquid)-stabilized graphene sheets and their hybrid with poly(3,4-ethylenedioxythiophene)
Abstract:
Hybrid materials of reduced graphene oxide (RG-O) and poly(3,4-ethylenedioxythiophene) (PEDOT) were prepared by poly(ionic liquid)-mediated hybridization. In this hybrid material, poly(ionic liquid)s (PILs) are found to be preferentially physisorbed onto the RG-O platelets, and allow them to be dispersed as a homogeneous colloidal system. In addition to the function as an effective stabilizer, the PIL also promotes PEDOT growth on RG-O platelets through favorable molecular interaction of PIL with PEDOT chains. The resulting material, a hybrid of RG-O and PEDOT showed an electrical conductivity of 18,8 S/cm at a RG-O loading of 0,3 wt.%, and its thin film on glass substrate showed a surface resistivity as low as 1,8 x 10⁴ Ω/sq at an optical transmittance of 91,18%.2225-2229
M.A. Khan, Unnat S. Bhansali, H.N. Alshareef. Fabrication and characterization of all-polymer, transparent ferroelectric capacitors on flexible substrates
Abstract:
All-polymer, transparent ferroelectric devices, based on the functional polymer poly(vinylidene fluoride trifluoroethylene) [P(VDF-TrFE)], have been fabricated on flexible substrates. The performance of the all-polymer devices was studied and compared to devices with metal electrodes. Specifically, poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid) [PEDOT:PSS] and platinum (Pt) electrode effects on the morphology, crystallinity and orientation of P(VDF-TrFE) films were investigated. The devices with PEDOT:PSS electrodes showed similar hysteresis and switching current response compared to Pt electrodes but with tremendously improved fatigue performance. Further, the devices with PEDOT:PSS electrodes showed lower coercive field and better fatigue performance than values reported for other polymer electrodes used with P(VDF-TrFE) on flexible substrates.2230-2235
Xiujin Wang, Shiliang Ji, Haibo Wang, Donghang Yan. Highly sensitive gas sensor enhanced by tuning the surface potential
Abstract:
We report a heterojunction gas sensor with an enhanced relative response to nitrogen dioxide (NO₂) below 5 ppm and a detection limit down to 250 ppb at room temperature. The 1,8 nm titanyl phthalocyanine (TiOPc) film as bottom layer with 1 nm hexadecafluorinated copper phthalocyanine (F₁₆CuPc) film as top layer has constructed an ultrathin bilayer sensor device. Kelvin probe force microscopy (KPFM) results indicated that p–n heterojunction was formed at the interface. The top layer has formed local electron accumulating area and made the film more reductive to analyte. The top layer acts as extra surface active sites in addition to the grain boundaries and these heterostructures can adsorb analytes more effectively.2236-2242
Zujin Zhao, Shanghui Ye, Yanju Guo, Zhengfeng Chang, Liya Lin, Tao Jiang, Jacky W.Y. Lam, Ping Lu, Huayu Qiu, Yunqi Liu, Ben Zhong Tang. 1,3,6,8-Tetrakis[(triisopropylsilyl)ethynyl]pyrene: A highly efficient solid-state emitter for non-doped yellow electroluminescence devices
Abstract:
Simple molecule possesses high power: 1,3,6,8-Tetrakis[(triisopropylsilyl)ethynyl]pyrene (TTIPSEPy), a simple-structured molecule, shows strong solid-state photoluminescence with unity emission efficiency. Highly efficient non-doped organic light-emitting diodes are fabricated using TTIPSEPy as emitter, exhibiting stable yellow light (CIE = 0,41, 0,56) with high luminance and efficiency up to 26,400 cd/m² and 9,2 cd/A, respectively.2243-2252
Marieta Levichkova, David Wynands, Alexandr A. Levin, Karsten Walzer, Dirk Hildebrandt, Martin Pfeiffer, Vytautas Janonis, Mindaugas Pranaitis, Vaidotas Kazukauskas, Karl Leo, Moritz Riede. Dicyanovinyl sexithiophene as donor material in organic planar heterojunction solar cells: Morphological, optical, and electrical properties
Abstract:
We study the morphology and optical properties of vacuum deposited films of the a-x-bis-(dicyanovinylen)-sexithiophene, comprising four butyl side chains DCV6T-Bu(1,2,5,6) (DCV6T-Bu). An absorption band showing vibronic substructure indicates ordered molecular arrangement in the solid state. The room temperature (RT) self-organization is confirmed by X-ray diffraction (XRD). For films grown on heated substrates, XRD analysis and atomic force microscopy display increased crystallinity with larger domain size. In correlation to the XRD data, with increasing substrate temperature the absorption of the heated films becomes more structured and continuously shifts to longer wavelengths. Further, the hole mobility in DCV6T-Bu/C60 planar heterojunction (PHJ) devices, utilizing DCV6T-Bu films grown at RT and elevated substrate temperature is investigated using the charge extraction by linearly increasing voltage method. The derived values of the activation energy are consistent with the corresponding DCV6T-Bu film morphology. However, the charge carrier mobility does not increase with improving molecular order, as is evident by the obtained mobility values of 1,0 x 10^-6 cm²/Vs for the RT and 3,1 x 10^-7 cm²/Vs for the heated device, respectively. Finally, DCV6T-Bu/C₆₀ PHJ solar cells consisting of absorber layers deposited on heated and unheated substrates are compared.