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6 Aug 2012

Volume 101, Issue 6, Articles (06xxxx)

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Appl. Phys. Lett. 101, 062401 (2012); http://dx.doi.org/10.1063/1.4730997 (4 pages)

Mahdi Jamali, Kulothungasagaran Narayanapillai, Jae Hyun Kwon, and Hyunsoo Yang
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Self-cavity laser oscillations with very low threshold from a symmetric organic crystal waveguide

Xianjie Li, Na Gao, Yuanxiang Xu, Feng Li, and Yuguang Ma

Appl. Phys. Lett. 101, 063301 (2012); http://dx.doi.org/10.1063/1.4742734 (3 pages) | Cited 1 time

Online Publication Date: 6 August 2012

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A symmetric waveguide structure of air/crystal/air comprising a slab crystal of cyano substituted oligo(p-phenylenevinylene) (OPV) has been developed for organic crystal lasers. The OPV crystals exhibit high photoluminescence efficiency of 30% and have smooth surfaces and edges, which ensures strong self-cavity optical confinement and provides optical feedback for the laser oscillation. A single-mode laser oscillation is observed around 573 nm with a threshold of 12.8 kW/cm2 that is one of the lowest values for organic crystal lasers. The leaky loss caused by the substrate is prevented by the air/crystal/air waveguide structure.
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42.79.Gn Optical waveguides and couplers
78.55.-m Photoluminescence, properties and materials
42.55.-f Lasers
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Angular selective backreflector for semitransparent photovoltaics

D. M. N. M. Dissanayake, B. Roberts, and P.-C. Ku

Appl. Phys. Lett. 101, 063302 (2012); http://dx.doi.org/10.1063/1.4742174 (4 pages)

Online Publication Date: 6 August 2012

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A nanoporous anodized aluminum oxide template is used as an incident-angle selective backreflector to lift the transparency vs. light absorption tradeoff in semi-transparent photovoltaics. Light incident at angles greater-than 45° from the template normal is backscattered at greater-than 35% intensity to a semitransparent photovoltaic increasing absorption and photocurrent. Meanwhile, light incident orthogonal to the template is transmitted with minimum attenuation (>85% at 600 nm), limited only by a single-pass absorption within the active layer. Using a vertical semitransparent photovoltaic proposed here, efficient capture of direct sunlight is possible while maintaining transparency at viewing angles close to the substrate normal.
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88.40.jr Organic photovoltaics
88.40.hj Efficiency and performance of solar cells
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Tandem organic photovoltaics using both solution and vacuum deposited small molecules

Brian E. Lassiter, Jeramy D. Zimmerman, Anurag Panda, Xin Xiao, and Stephen R. Forrest

Appl. Phys. Lett. 101, 063303 (2012); http://dx.doi.org/10.1063/1.4742921 (4 pages) | Cited 6 times

Online Publication Date: 7 August 2012

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We demonstrate a tandem organic photovoltaic cell incorporating solution- and vacuum-deposited small molecules as the active layers. A blue and green-absorbing boron subphthalocyanine chloride:C70 graded heterojunction (HJ) sub-cell is combined with a green and red-absorbing functionalized squaraine/C70 bilayer HJ sub-cell, resulting in a tandem cell with a wavelength response from 350 nm to 800 nm. The efficiency of the cells depends on process conditions such as solvent annealing, resulting in nanocrystalline morphology that leads to improved charge and exciton transport compared with un-annealed cells. The incorporation of C70 in both sub-cells leads to an increase of short-circuit current by at least 30% compared to analogous cells using C60. The optimized power conversion efficiency of the tandem cell is 6.6% ± 0.1%, with an open-circuit voltage of 1.97 ± 0.1 V under simulated 1 sun, AM 1.5G illumination. The tandem cell voltage is equal to the sum of the constituent sub-cells, indicating that the transparent, Ag nanoparticle/MoO3 compound charge recombination layer interposed between the cells is nearly lossless.
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88.40.hj Efficiency and performance of solar cells
88.40.jr Organic photovoltaics
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Engineering the metal gate electrode for controlling the threshold voltage of organic transistors

Yoonyoung Chung, Olasupo Johnson, Michael Deal, Yoshio Nishi, Boris Murmann, and Zhenan Bao

Appl. Phys. Lett. 101, 063304 (2012); http://dx.doi.org/10.1063/1.4739511 (4 pages)

Online Publication Date: 7 August 2012

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For practical applications of organic field-effect transistors (OFETs), the control of threshold voltage (VTH) is important as different circuits require different electrical characteristics. Here, we demonstrate two types of gate electrode structures to achieve this control, namely, via dual-metal gates and bilayer metal gates. The first approach uses different metallic materials, titanium, and platinum, while the second approach uses different thicknesses in a metal bilayer composed of aluminum and platinum. Our results show that the VTH is varied by more than 20% of the supply voltage without affecting the field-effect mobility values for both pentacene p-channel and C60 n-channel OFETs.
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85.30.Tv Field effect devices
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Enhanced ultraviolet response using graphene electrodes in organic solar cells

Zhouying Zhao, John D. Fite, Pradeep Haldar, and Ji Ung Lee

Appl. Phys. Lett. 101, 063305 (2012); http://dx.doi.org/10.1063/1.4742928 (4 pages) | Cited 1 time

Online Publication Date: 8 August 2012

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We demonstrate enhanced ultraviolet (UV) response in organic photovoltaic devices using few-layer graphene as the transparent and conducting electrode. This is compared to the same device stack fabricated on a conventional indium tin oxide (ITO) electrode. The enhanced UV response was confirmed by examining the photocurrent spectra over a wide spectral range and is consistent with the excellent UV transmittance of graphene over ITO. The results render the graphene electrode a promising alternative to ITO to enable applications for UV photodetectors or other optoelectronic devices with a wider optical window.
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88.40.jr Organic photovoltaics
88.40.hj Efficiency and performance of solar cells
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High-efficiency and low-efficiency-roll-off single-layer white organic light-emitting devices with a bipolar transport host

Yongming Yin, Xuecheng Piao, Yang Li, Yan Wang, Jian Liu, Kai Xu, and Wenfa Xie

Appl. Phys. Lett. 101, 063306 (2012); http://dx.doi.org/10.1063/1.4745507 (4 pages) | Cited 2 times

Online Publication Date: 9 August 2012

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High-efficiency single-layer (SL) white organic light-emitting devices (WOLEDs) with a bipolar transport host were fabricated. The SL WOLEDs were achieved by combining blue and orange emission. Compared with the corresponding multilayer WOLEDs, the SL WOLEDs alleviated the efficiency roll-off without compromise of current efficiency due to the broader exciton formation zone and balance of carrier injection and transport. For example, The power efficiency of the SL WOLED based on Iridium (III) bis(4-phenylthieno[3,2-c]pyridinato-N,C2′) acetylacetonate orange emission could reach 20.9 lm/W at 1000 cd/m2, which also could reach 14.5 lm/W at a very high brightness of 5000 cd/m2.
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85.60.Jb Light-emitting devices
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