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28 Jul 2008

Volume 93, Issue 4, Articles (04xxxx)

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Appl. Phys. Lett. 93, 043101 (2008); http://dx.doi.org/10.1063/1.2963352 (3 pages)

Xin Fu, Jun Jiang, Wenzheng Zhang, and Jun Yuan
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High-mobility spin-cast organic thin film transistors

Sung Kyu Park, Devin A. Mourey, Sankar Subramanian, John E. Anthony, and Thomas N. Jackson

Appl. Phys. Lett. 93, 043301 (2008); http://dx.doi.org/10.1063/1.2952769 (3 pages) | Cited 22 times

Online Publication Date: 28 July 2008

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We report high-mobility organic thin film transistors (OTFTs) with good uniformity using the small molecule organic semiconductor 2,8-difluoro-5,11-bis(triethylsilylethynyl) anthradithiophene (diF-TESADT). diF-TESADT was spin cast to form OTFTs with a carrier mobility of more than 1.5 cm2/V s and shows a useful differential microstructure on or near pentaflorobenzenethiol (PFBT) treated Au source/drain electrodes compared to untreated Au or gate dielectric areas with improved molecular order observed on PFBT treated Au electrodes. For short channel length devices diF-TESADT crystal grains extend between the source and drain electrodes, resulting in increasing OTFT field effect mobility for decreasing gate length.
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85.30.Tv Field effect devices
72.80.Le Polymers; organic compounds (including organic semiconductors)
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Crystallinity improvement of benzodithiophene-dimer films for organic field-effect transistors

K. Yamaguchi, S. Takamiya, M. Minami, Y. Doge, Y. Nishide, H. Osuga, K. Uno, and Ichiro Tanaka

Appl. Phys. Lett. 93, 043302 (2008); http://dx.doi.org/10.1063/1.2958341 (3 pages) | Cited 6 times

Online Publication Date: 28 July 2008

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We have significantly improved the crystallinity of vacuum-evaporated benzodithiophene (BDT)-dimer films using pentacene crystallinity control layers. The pentacene molecules evaporated on hexamethyldisilazane-treated SiO2 surfaces first form many small islands. Then, the islands grow two dimensionally. We used thin pentacene films with surface coverage of approximately 90% as crystallinity control layers, and deposited 30-nm-thick BDT-dimer films on them. Micron-scale planar grains are observed in atomic force microscopy images of the BDT-dimer films. The highly anisotropic shape of the grains indicates that the BDT-dimer films of good crystallinity have been obtained. The field-effect transistors based on the BDT-dimer films showed excellent hole mobility of 1.2 cm2 V−1 s−1 at maximum.
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85.30.Tv Field effect devices
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
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Direct inkjet printing of silver electrodes on organic semiconductors for thin-film transistors with top contact geometry

Yoshiaki Noguchi, Tsuyoshi Sekitani, Tomoyuki Yokota, and Takao Someya

Appl. Phys. Lett. 93, 043303 (2008); http://dx.doi.org/10.1063/1.2959728 (3 pages) | Cited 22 times

Online Publication Date: 28 July 2008

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We have fabricated high-performance organic thin-film transistors with top contact geometry using silver source and drain electrodes that are directly patterned on pentacene film by inkjet printing. Ink droplets containing Ag nanoparticles were ejected from inkjet nozzles. By controlling wave forms applied to piezoelectric actuators in these nozzles, the volume of the droplets can be changed from 17 to 1.4 pl. When the volume of the droplets is reduced, dc characteristics of manufactured transistors are improved significantly. The transistors manufactured with droplets of 1.4 pl exhibit a mobility of 0.3 cm2/Vs and an on-off current ratio that exceeding 106.
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85.30.Tv Field effect devices
85.50.-n Dielectric, ferroelectric, and piezoelectric devices
81.16.Rf Micro- and nanoscale pattern formation
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The use of thermal initiator to make organic bulk heterojunction solar cells with a good percolation path

S. W. Tong, C. F. Zhang, C. Y. Jiang, Q. D. Ling, E. T. Kang, D. S. H. Chan, and Chunxiang Zhu

Appl. Phys. Lett. 93, 043304 (2008); http://dx.doi.org/10.1063/1.2965468 (3 pages) | Cited 9 times

Online Publication Date: 29 July 2008

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A simple method is developed to make an interpenetrating network of poly(3-hexylthiophene-2,5-diyl) (P3HT) and fullerene (C60) by mixing P3HT solution with a thermal initiator 2,2′-azobis(isobutyronitrile) (AIBN). After mild annealing, the release of nitrogen from AIBN increases the roughness of P3HT dramatically. Significant photoluminescence quenching between the roughened donor P3HT and overlaying acceptor C60 is related to the significant increment of donor-acceptor interfacial areas. Based on this interpenetrated network of P3HT/C60, more than threefold increase in the photovoltaic efficiency of devices is achieved compared with bilayer structure. Fill factor is also improved, implying good percolation path in this heterojunction structure.
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84.60.Jt Photoelectric conversion
81.05.Lg Polymers and plastics; rubber; synthetic and natural fibers; organometallic and organic materials
78.55.Kz Solid organic materials
81.40.Gh Other heat and thermomechanical treatments
61.41.+e Polymers, elastomers, and plastics
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Improvement of transparent organic thin film transistor performance by inserting a lithium fluoride buffer layer

Yu-Ju Lin, Yu-Chang Li, Ten-Chin Wen, Li-Ming Huang, Ying-Kuo Chen, Hong-Jian Yeh, and Yeong-Her Wang

Appl. Phys. Lett. 93, 043305 (2008); http://dx.doi.org/10.1063/1.2959076 (3 pages) | Cited 10 times

Online Publication Date: 31 July 2008

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Pentacene-based highly transparent thin-film transistors which are fabricated by inserting a thin insulating lithium fluoride layer between pentacene and transparent source/drain electrodes are presented. Through this method, device performance can be enhanced dramatically with an average transmittance of as high as 69.72% in the visible region, indicating that the LiF layer is not responsible for optical transmission. For example, there is a significant improvement of a few times with the introduction of the 1-nm-thick LiF layer, the maximum saturation drain current and the field-effect mobility. These improvements are attributed to the energy band realignment and the tunneling process.
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85.30.Tv Field effect devices
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Low voltage organic light-emitting devices with triphenylphosphine oxide layer

Mi-Young Ha and Dae-Gyu Moon

Appl. Phys. Lett. 93, 043306 (2008); http://dx.doi.org/10.1063/1.2960348 (3 pages) | Cited 3 times

Online Publication Date: 1 August 2008

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We have developed low voltage driving organic light-emitting devices using triphenylphosphine oxide (Ph3PO) layers. The devices with a Ph3PO layer show high current density at a low voltage. For example, the current density of 20 mA/cm2 is achieved at a low voltage of 2.9 V for the device consisted of 4,4′,4″-tris[N-(2-naphthyl)-N-phenyl-amino]-triphenylamine (2-TNATA), 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl (DPVBi), and Ph3PO layers. Due to the good electron conduction property of Ph3PO, a luminance of 1017 cd/m2 is achieved at a low voltage of 3.0 V in a device with a structure of ITO/2-TNATA/DPVBi:rubrene (1%,10 nm)/DPVBi (30 nm)/Ph3PO (60 nm)/LiF/Al.
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72.80.Le Polymers; organic compounds (including organic semiconductors)
73.61.Ph Polymers; organic compounds
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Efficient multilayer organic solar cells using the optical interference peak

Chunfu Zhang, S. W. Tong, Changyun Jiang, E. T. Kang, D. S. H. Chan, and Chunxiang Zhu

Appl. Phys. Lett. 93, 043307 (2008); http://dx.doi.org/10.1063/1.2962986 (3 pages) | Cited 23 times

Online Publication Date: 1 August 2008

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A multilayer structure of copper phthalocyanine/poly(3-hexylthiophene-2,5-diyl): [6,6]-phenyl-C61-butyric acid methyl ester (CuPc/P3HT:PCBM) is used to extend the light absorption spectrum covering almost the entire visible spectrum. To maximize the light absorption, the total number of excitons created in the multilayer structure as a function of layer thickness of both CuPc and P3HT:PCBM is simulated by using the optical transfer matrix formalism. The solar cells with a device structure of ITO/PEDOT:PSS/CuPc/P3HT:PCBM/Al are fabricated with different layers thicknesses. The optimized solar cell with a high short circuit current density of 12.54 mA/cm2 and power conversion efficiency as high as 4.13% is achieved, owing to the utilization of the second optical interference peak in the multilayer structure for the enhanced light absorption.
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84.60.Jt Photoelectric conversion
82.45.Wx Polymers and organic materials in electrochemistry
42.25.Hz Interference
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
42.30.Lr Modulation and optical transfer functions
78.67.Pt Multilayers; superlattices; photonic structures; metamaterials
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The origin of the hole injection improvements at indium tin oxide/molybdenum trioxide/N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl- 4,4′-diamine interfaces

Hyunbok Lee, Sang Wan Cho, Kyul Han, Pyung Eun Jeon, Chung-Nam Whang, Kwangho Jeong, Kwanghee Cho, and Yeonjin Yi

Appl. Phys. Lett. 93, 043308 (2008); http://dx.doi.org/10.1063/1.2965120 (3 pages) | Cited 49 times

Online Publication Date: 1 August 2008

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We investigated the interfacial electronic structures of indium tin oxide (ITO)/molybdenum trioxide (MoO3)/N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) using in situ ultraviolet and x-ray photoemission spectroscopy to understand the origin of hole injection improvements in organic light-emitting devices (OLEDs). Inserting a MoO3 layer between ITO and NPB, the hole injection barrier was remarkably reduced. Moreover, a gap state in the band gap of NPB was found which assisted the Ohmic hole injection at the interface. The hole injection barrier lowering and Ohmic injection explain why the OLED in combination with MoO3 showed improved performance.
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73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
79.60.Jv Interfaces; heterostructures; nanostructures
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Optimum channel thickness of rubrene thin-film transistors

Jeong-M. Choi and Seongil Im

Appl. Phys. Lett. 93, 043309 (2008); http://dx.doi.org/10.1063/1.2966354 (3 pages) | Cited 4 times

Online Publication Date: 1 August 2008

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We report the influence of channel thickness on the field effect mobility of rubrene-based thin-film transistors (TFTs). Prior to crystallization annealing, amorphous (α) rubrene film was deposited under thickness conditions of 40, 50, 80, 120, and 160 nm by thermal evaporation on self-assembled-monolayer treated SiO2/p+-Si. Field effect mobility of the TFTs increased from almost 0 to 0.01 cm2/V s with the rubrene channel thickness until it reaches to 120 nm because the rubrene crystallization on our substrate would not be perfect below 120 nm. The mobility decreased with the thickness over 120 nm due to parasitic resistance. We thus conclude that there exists an optimum channel thickness for rubrene TFTs.
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85.30.Tv Field effect devices
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High-efficiency monochrome organic light emitting diodes employing enhanced microcavities

Rico Meerheim, Robert Nitsche, and Karl Leo

Appl. Phys. Lett. 93, 043310 (2008); http://dx.doi.org/10.1063/1.2966784 (3 pages) | Cited 31 times

Online Publication Date: 1 August 2008

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We demonstrate enhanced light outcoupling from bottom emitting monochrome high-efficiency red, green, and blue organic light emitting diodes by adding silver layers on the indium tin oxide (ITO) anode. The devices contain the phosphorescent emitting dyes tris(2-phenylpyridine)iridium and iridium(III)bis[2-methyldibenzo-(f, h)quinoxaline](acetylacetonate), and the blue singlet emitter 2,2′,7,7′-tetrakis 2,2-(diphenylvinyl)spiro-9,9′-bifluorene. We follow the p-i-n doping concept to increase the power efficiencies. We reach 81 lm/W for red, 101 lm/W for green, and 4.0 lm/W for blue color. These efficiencies are improved by up to a factor of 2.3 compared to standard ITO devices, which we attribute to microcavity amplification between the cathode and the Ag layer.
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85.60.Jb Light-emitting devices
85.30.Kk Junction diodes
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Correlation between grain size and device parameters in pentacene thin film transistors

S. D. Wang, T. Miyadera, T. Minari, Y. Aoyagi, and K. Tsukagoshi

Appl. Phys. Lett. 93, 043311 (2008); http://dx.doi.org/10.1063/1.2967193 (3 pages) | Cited 28 times

Online Publication Date: 1 August 2008

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We develop a general approach to precisely extract the device parameters in top-contact pentacene thin film transistors. The charge trap sites are clarified by analyzing the grain size dependence of the device parameters. The channel mobility and threshold voltage are limited by the charge traps in the channel region, most of which are located not at the grain boundaries but at the organic/insulating-layer interface. The contact resistance decreases by increasing the grain size and is controlled by the charge traps in the contact region, which are suggested to be concentrated at the grain boundaries and at the metal/organic interface.
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85.30.Tv Field effect devices
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