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14 Jan 2008

Volume 92, Issue 2, Articles (02xxxx)

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Appl. Phys. Lett. 92, 022509 (2008); http://dx.doi.org/10.1063/1.2807274 (3 pages)

Sang-Koog Kim, Ki-Suk Lee, Young-Sang Yu, and Youn-Seok Choi
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High-efficiency endothermic energy transfer in polymeric light-emitting devices based on cyclometalated Ir complexes

Hong-Mei Liu, Peng-Fei Wang, Jian He, Caijun Zheng, Xiao-Hong Zhang, Siew-Ling Chew, Chun-Sing Lee, Jack Chang, and Shuit-Tong Lee

Appl. Phys. Lett. 92, 023301 (2008); http://dx.doi.org/10.1063/1.2830618 (3 pages)

Online Publication Date: 14 January 2008

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We report polymeric light-emitting diodes (PLEDs) made from pinene-substituted iridium(III) phosphorescent dopants: tris(5-(4-difluoro phenyl)-10,10-dimethyl-4-aza-tricycloundeca-2,4,6-triene) iridium (III) [Ir(F-pppy)3] and tris(5-(2,4-difluorophenyl)-10,10-dimethyl-4-aza-tricycloundeca-2,4,6-triene) iridium (III) [Ir(F2-pppy)3]. The pinene substitution introduces steric hindrance to molecular structure of the dopant that reduces triplet-triplet annihilation between dopants and consequently enhances device performance. Via endothermic energy transfer from poly(vinylcarbazole) to Ir(F-pppy)3 and Ir(F2-pppy)3, a peak electroluminescent efficiency of 32.8 cd/A or 12.3 cd/A at 12 wt % Ir(F-pppy)3 or 15 wt % Ir(F2-pppy)3 doped and solution-processed PLEDs have been obtained. These values represent significant improvement in performance over previously reported endothermic energy-transfer based electrophosphorescent devices.
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85.60.Jb Light-emitting devices
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Energy level alignment and injection barriers at spin injection contacts between La0.7Sr0.3MnO3 and organic semiconductors

M. Grobosch, K. Dörr, R. B. Gangineni, and M. Knupfer

Appl. Phys. Lett. 92, 023302 (2008); http://dx.doi.org/10.1063/1.2829391 (3 pages) | Cited 14 times

Online Publication Date: 14 January 2008

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We have determined the energy level alignment at interfaces between La0.7Sr0.3MnO3 and two typical organic semiconductors, copper-phthalocyanine and α-sexithiophene. La0.7Sr0.3MnO3 thin films have been grown using pulsed laser deposition and subsequently ex situ cleaned before the organic materials have been deposited. This procedure is often applied in the fabrication of organic devices. We show that under these conditions the interfaces are free from chemical interaction and characterized by a short range interface dipole and large charge injection barriers.
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73.20.At Surface states, band structure, electron density of states
73.40.-c Electronic transport in interface structures
72.25.Hg Electrical injection of spin polarized carriers
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Gamma-ray irradiated polymer optical waveguides

Cheng-Chih Lai, Tso-Yun Wei, Chin-Yu Chang, Way-Seen Wang, and Yuan-Yaw Wei

Appl. Phys. Lett. 92, 023303 (2008); http://dx.doi.org/10.1063/1.2829862 (3 pages)

Online Publication Date: 14 January 2008

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Optical waveguides fabricated by gamma-ray irradiation on polymer through a gold mask are presented. The gamma-ray induced index change is found almost linearly dependent on the dose of the irradiation. And the measured propagation losses are low enough for practical application. Due to the high penetrability of gamma ray, uniform refractive index change in depth can be easily achieved. Moreover, due to large-area printing, the uniformity of waveguide made by gamma-ray irradiation is much better than that by e-beam direct writing.
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42.79.Gn Optical waveguides and couplers
42.86.+b Optical workshop techniques
61.80.Ed γ-ray effects
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X-ray stability and response of polymeric photodiodes for imaging applications

Panagiotis E. Keivanidis, Neil C. Greenham, Henning Sirringhaus, Richard H. Friend, James C. Blakesley, Robert Speller, Mariano Campoy–Quiles, Tiziano Agostinelli, Donal D. C. Bradley, and Jenny Nelson

Appl. Phys. Lett. 92, 023304 (2008); http://dx.doi.org/10.1063/1.2834364 (3 pages) | Cited 18 times

Online Publication Date: 16 January 2008

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The x-ray stability of photodiodes made of poly(9,9-di-n-octylfluorene-co-benzothiadiazole):perylene diimide, poly[2,7-(9,9-di-n-octylfluorene)-co-(1,4-phenylene-[(4-sec-butylphenyl)imino]-1,4-phenylene)]:perylene diimide and poly(3-hexylthiophene):([6,6]-phenylC61-butyric acid methyl ester) (P3HT:PCBM) blends has been examined up to lifetime doses equivalent to those used in medical x-ray digital imaging applications. Dark currents and external quantum efficiencies (EQEs) are not significantly affected after exposure to 500 Gy. Only in the case of P3HT:PCBM is a significant loss in EQE (17% of the initial value) observed. Possible reasons for the observed changes are proposed. When a scintillation layer is attached to the devices, a linear dependence of the photocurrent on the x-ray dose rate is observed for the three material systems.
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85.60.Dw Photodiodes; phototransistors; photoresistors
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Contact resistance of dibenzotetrathiafulvalene-based organic transistors with metal and organic electrodes

Koji Shibata, Ken Ishikawa, Hideo Takezoe, Hiroshi Wada, and Takehiko Mori

Appl. Phys. Lett. 92, 023305 (2008); http://dx.doi.org/10.1063/1.2834374 (3 pages) | Cited 10 times

Online Publication Date: 16 January 2008

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Thin-film transistors of dibenzotetrathiafulvalene (DBTTF) are investigated by changing the source and drain (S/D) electrode materials. Not only the mobility but also the contact resistance, estimated from the transfer line method, changes depending on the metal work functions. Nonetheless, S/D electrodes made of a metallic organic charge-transfer salt, (tetrathiafulvalene) (tetracyanoquinodimethane) [(TTF)(TCNQ)] exhibits much smaller contact resistance, which is attributed to small potential shift on the organic/organic interface compared with the organic/metal interface. A thin film of (DBTTF)(TCNQ) works as an active layer of air-stable n-channel organic transistors when (TTF)(TCNQ) is used as the S/D electrodes.
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85.30.Tv Field effect devices
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Transparent organic light-emitting diodes consisting of a metal oxide multilayer cathode

Seung Yoon Ryu, Joo Hyon Noh, Byoung Har Hwang, Chang Su Kim, Sung Jin Jo, Jong Tae Kim, Hyeon Seok Hwang, Hong Koo Baik, Hee Seong Jeong, Chang Ho Lee, Seung Yong Song, Seung Ho Choi, and Si Young Park

Appl. Phys. Lett. 92, 023306 (2008); http://dx.doi.org/10.1063/1.2835044 (3 pages) | Cited 11 times

Online Publication Date: 16 January 2008

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The authors have developed a semitransparent, multilayered cathode of indium tin oxide (ITO)/Ag/tungsten oxide (WO3) for transparent organic light-emitting diodes. The device showed a weak negative differential resistance (NDR), until the operating voltage of 8 V was reached. NDR was due to the resonant tunneling by both the quantum barrier and quantum well. The silver oxide (Ag2O) on the Ag metal was confirmed by x-ray photoelectron spectroscopy, and the energy levels of Ag2O were quantized due to the quantum size effect and this produced the resonant tunneling channels. The device using ITO/Ag/WO3 with a LiF/Al bilayer was superior to those devices which only used ITO or WO3, mainly because the out coupling was enhanced by employing a WO3 material, which is much more transparent than ITO.
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85.60.Jb Light-emitting devices
79.60.Jv Interfaces; heterostructures; nanostructures
73.40.Gk Tunneling
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Submicron-scale manipulation of phase separation in organic solar cells

Fang-Chung Chen, Yi-Kai Lin, and Chu-Jung Ko

Appl. Phys. Lett. 92, 023307 (2008); http://dx.doi.org/10.1063/1.2835047 (3 pages) | Cited 17 times

Online Publication Date: 17 January 2008

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This paper describes a method for controlling the submicron-scale phase separation of poly(3-hexylthiophene) and (6,6)-phenyl-C61-butyric acid methyl ester in organic solar cells. Using microcontact printing of self-assembled monolayers on the device buffer layer to divide the surface into two regimes having different surface energies, an interdigitated structure aligned vertical to the substrate surface is achieved after spontaneous surface-directed phase separation. The power conversion efficiency increases upon decreasing the grating spacing, reaching 2.47%. The hole mobility increased as a consequence of improved polymer chain ordering, resulting in higher device efficiency, while smaller pattern sizes were used.
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84.60.Jt Photoelectric conversion
85.40.Hp Lithography, masks and pattern transfer
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