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11 Aug 2008

Volume 93, Issue 6, Articles (06xxxx)

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Appl. Phys. Lett. 93, 062101 (2008); http://dx.doi.org/10.1063/1.2968206 (3 pages)

K. Shibata and K. Hirakawa
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Dissociation of excitons in the C60 film studied by transient photovoltage measurements

X. Y. Sun, B. F. Ding, Q. L. Song, X. Y. Zheng, X. M. Ding, and X. Y. Hou

Appl. Phys. Lett. 93, 063301 (2008); http://dx.doi.org/10.1063/1.2957677 (3 pages) | Cited 5 times

Online Publication Date: 11 August 2008

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The dissociation of excitons at indium tin oxide (ITO)/C60 interface is studied by means of transient photovoltage measurements. An abnormal polarity change of transient photovoltage from positive to negative upon pulsed laser irradiation is observed, indicating that the exciton dissociation at ITO/C60 interface results in holes injected into ITO and electrons left in the C60 film, opposite to that occurring at ITO/NPB and ITO/CuPc interfaces. It is confirmed that C60 has a moderately strong ability of donating holes to ITO during the dissociation process of the excitons at the ITO/C60 interface. Moreover the long term transient photovoltage (t>10 ns) and its polarity can be tuned by applying external bias on the device, which further proves the validity of the model proposed to explain the polarity change of the transient photovoltage.
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71.35.-y Excitons and related phenomena
73.40.-c Electronic transport in interface structures
72.40.+w Photoconduction and photovoltaic effects
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Solution-processed small molecule-based blue light-emitting diodes using conjugated polyelectrolytes as electron injection layers

Bright Walker, Arnold Tamayo, Jihua Yang, Jacek Z. Brzezinski, and Thuc-Quyen Nguyen

Appl. Phys. Lett. 93, 063302 (2008); http://dx.doi.org/10.1063/1.2968189 (3 pages) | Cited 10 times

Online Publication Date: 12 August 2008

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Organic blue light-emitting diodes were studied using the solution processable small molecule 2,7-dipyrenyl-9,9′-dioctyl-fluorene (DPF) as the light-emitting material. The devices were fabricated in two simple structures: indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS)/DPF/LiF/Al and ITO/PEDOT:PSS/DPF/PFN-BIm4/Al, where PFN-BIm4 is poly[9,9′-bis[6″-(N,N,N-trimethylammonium)hexyl]fluorene-alt-co-phenylene] with tetrakis(imidazolyl)borate counterions. The LiF or PFN-BIm4 act as electron injection layers. The ITO/PEDOT:PSS/DPF/PFN-BIm4/Al device, in which all organic layers are cast from solution, has a turn-on voltage of 3.8 V, a luminance of 2000 cd/m2, and an efficiency of 0.6 cd/A. Using the PFN-BIm4 layer shows a significant improvement of the device performance when compared to the LiF layer.
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85.60.Jb Light-emitting devices
82.35.Cd Conducting polymers
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Low roll-off efficiency green phosphorescent organic light-emitting devices with simple double emissive layer structure

Woo Sik Jeon, Tae Jin Park, Sun Young Kim, Ramchandra Pode, Jin Jang, and Jang Hyuk Kwon

Appl. Phys. Lett. 93, 063303 (2008); http://dx.doi.org/10.1063/1.2969040 (3 pages) | Cited 23 times

Online Publication Date: 12 August 2008

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Using an Ir(ppy)3 metal complex doped in 4,4′4″-tris(N-carbazolyl)-triphenylamine hole and bis[2-(2-hydroxyphenyl)-pyridine]beryllium electron transport host materials, simple three layer green phosphorescent organic light-emitting devices comprising double emissive layers have been fabricated. A low driving voltage value of 3.3 V to reach a luminance of 1000 cd/m2 and maximum current- and power-efficiency values of 58.7 cd/A and 65.1 lm/W, and maximum external quantum efficiency (EQE) value of 18.6% are reported in this device. EQE exceeding 15% over a wide range of current density from 0.03 to 25 mA/cm2 is noticed. We demonstrate a low roll-off current efficiency value of 12% at a luminance of 10 000 cd/m2 in a highly efficient simple double emissive layer device, imperative to high brightness applications.
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85.60.Jb Light-emitting devices
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Polymer infrared proximity sensor

En-Chen Chen, Shin-Rong Tseng, Jia-Hong Ju, Chia-Ming Yang, Hsin-Fei Meng, Sheng-Fu Horng, and Ching-Fong Shu

Appl. Phys. Lett. 93, 063304 (2008); http://dx.doi.org/10.1063/1.2949069 (3 pages) | Cited 6 times

Online Publication Date: 13 August 2008

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A proximity sensor that combines a polymer light-emitting diode and a polymer photodiode is presented. The operation wavelength is in the near infrared from 700 to 850 nm. The infrared emission is obtained by adding a color conversion film of polyvinylpyrrolidone polymer matrix blended with infrared dye 1,1-diethyl-2,2-dicarbocyanine iodide to a red polymer light-emitting diode. The photodetector relies on the direct charge-transfer exciton generation in a donor-acceptor polymer blend of poly(3-hexylthiophene) and (6,6)-phenyl-C61-butyric acid methyl ester. The detection distance is up to 19 cm for objects with various colors and roughness under ambient indoor lighting.
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85.60.Jb Light-emitting devices
85.60.Dw Photodiodes; phototransistors; photoresistors
85.60.Gz Photodetectors (including infrared and CCD detectors)
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Lateral organic bilayer heterojunction photoconductors

John C. Ho, Alexi Arango, and Vladimir Bulović

Appl. Phys. Lett. 93, 063305 (2008); http://dx.doi.org/10.1063/1.2949317 (3 pages) | Cited 6 times

Online Publication Date: 14 August 2008

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We demonstrate a two-terminal, lateral organic bilayer photoconductor that generates an external quantum efficiency of (12±1)%, with an internal quantum efficiency of (140±2)% indicative of photon-to-electron conversion gain. The photoconductor incorporates a heterojunction between N,N′-bis(3-methylphenyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (TPD) and 3,4,9,10-perylenetetracarboxylic bisbenzimidazole (PTCBI). Excitons generated with photoexcitation of PTCBI dissociate at the PTCBI/TPD interface and raise the charge carrier concentration in TPD, increasing device conductance. The exposed top surface enables interaction with chemical analytes in the environment, motivating the use of the photoconductor as a chemical sensor that transduces chemical signals into amplified changes in the electrical response.
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72.40.+w Photoconduction and photovoltaic effects
71.35.-y Excitons and related phenomena
72.80.Le Polymers; organic compounds (including organic semiconductors)
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
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100% internal quantum efficiency and stable efficiency roll-off in phosphorescent light-emitting diodes using a high triplet energy hole transport material

Soon-Ok Jeon, Kyoung Soo Yook, Chul Woong Joo, Jun Yeob Lee, Kwang-Youn Ko, Jong-Yek Park, and Yong Gu Baek

Appl. Phys. Lett. 93, 063306 (2008); http://dx.doi.org/10.1063/1.2969786 (3 pages) | Cited 18 times

Online Publication Date: 14 August 2008

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Highly efficient green phosphorescent organic light-emitting diodes were developed by using high triplet energy hole transport materials. The quantum efficiency of green devices could be improved from 12% to 20% at 1000 cd/m2 by using a phenylcarbazole based hole transport material. In addition, the high quantum efficiency could be stably maintained up to a high luminance of 10 000 cd/m2.
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85.60.Jb Light-emitting devices
42.70.Jk Polymers and organics
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Sharp thermal emission and absorption from conformally coated metallic photonic crystal with triangular lattice

R. Biswas, D. Zhou, I. Puscasu, E. Johnson, A. Taylor, and W. Zhao

Appl. Phys. Lett. 93, 063307 (2008); http://dx.doi.org/10.1063/1.2971168 (3 pages) | Cited 4 times

Online Publication Date: 15 August 2008

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A metallic photonic crystal consisting of a triangular lattice of holes in a silicon layer coated with gold is fabricated at a lattice pitch of 3.75 μm using conventional lithographic methods. The photonic crystal exhibits a deep reflection minimum and sharp thermal emission peak near the lattice spacing. Scattering matrix simulations agree well with measurements. This simple structure with a single patterned metallic layer has no emission sidebands and can be scaled to other lattice spacings to tune the wavelength of the absorption and emission peak.
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42.70.Qs Photonic bandgap materials
78.30.-j Infrared and Raman spectra
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Efficiency enhancement of polymer photovoltaic devices hybridized with ZnO nanorod arrays by the introduction of a vanadium oxide buffer layer

Kazuko Takanezawa, Keisuke Tajima, and Kazuhito Hashimoto

Appl. Phys. Lett. 93, 063308 (2008); http://dx.doi.org/10.1063/1.2972113 (3 pages) | Cited 57 times

Online Publication Date: 15 August 2008

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The introduction of an electron-blocking layer of vanadium oxide between the organic layer and the Ag electrode was studied as a means of improving the performance of bulk heterojunction photovoltaic devices based on poly(3-hexylthiophene): (6,6)-phenyl C61 butyric acid methyl ester hybridized with vertically aligned ZnO nanorod arrays. The power conversion efficiency was further improved to 3.9% by applying a short circuit current density of 10.4 mA cm−2, an open circuit voltage of 0.58 V, and a fill factor of 65% under AM1.5 100 mW cm−2 irradiation, owing to the electron collection “antenna” effect of the ZnO nanorod arrays.
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85.60.-q Optoelectronic devices
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