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23 Aug 2010

Volume 97, Issue 8, Articles (08xxxx)

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Appl. Phys. Lett. 97, 081901 (2010); http://dx.doi.org/10.1063/1.3457448 (3 pages)

Zhaofeng Li, Rongkuo Zhao, Thomas Koschny, Maria Kafesaki, Kamil Boratay Alici, Evrim Colak, Humeyra Caglayan, Ekmel Ozbay, and C. M. Soukoulis
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Device characteristics of short-channel polymer field-effect transistors

Takeshi Hirose, Takashi Nagase, Takashi Kobayashi, Rieko Ueda, Akira Otomo, and Hiroyoshi Naito

Appl. Phys. Lett. 97, 083301 (2010); http://dx.doi.org/10.1063/1.3480549 (3 pages) | Cited 4 times

Online Publication Date: 23 August 2010

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The influence of channel materials on the electrical characteristics of organic field-effect transistors (OFETs) with short-channel lengths ranging from 1 μm to 30 nm is investigated using polymer semiconductors. The current-voltage characteristics of short-channel OFETs strongly depend on the electrode/organic semiconductor contacts, and the parabolic output current due to space-charge limited current can be reduced by increasing the ionization potential of organic semiconductors. Transistor operations with a high on/off ratio over 103 are achieved in OFETs with 30 nm length channels.
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85.30.Tv Field effect devices
81.05.Fb Organic semiconductors
85.30.Fg Bulk semiconductor and conductivity oscillation devices (including Hall effect devices, space-charge-limited devices, and Gunn effect devices)
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Thermal annealing-induced vertical phase separation of copper phthalocyanine: Fullerene bulk heterojunction in organic photovoltaic cells

H. X. Wei, J. Li, Z. Q. Xu, Y. Cai, J. X. Tang, and Y. Q. Li

Appl. Phys. Lett. 97, 083302 (2010); http://dx.doi.org/10.1063/1.3483761 (3 pages) | Cited 9 times

Online Publication Date: 24 August 2010

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The effect of thermal annealing treatment on the morphology change in bulk heterojunction (BHJ) in organic photovoltaic cells was studied by photoemission spectroscopy. The results reveal that vertical phase separation upon annealing occurs in typical BHJ layer formed between planar molecule CuPc and spherical C60 with inhomogeneous concentration distribution in profile, varying from CuPc-rich near the air surface to C60-rich adjacent to the substrate interface. The morphology variation is associated with the difference in surface energy of CuPc and C60, leading to the accumulation of CuPc to the interface with air to lower the overall energy of the free surface.
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85.60.-q Optoelectronic devices
84.60.Jt Photoelectric conversion
81.40.Gh Other heat and thermomechanical treatments
65.40.gp Surface energy
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Enhancement of the hole injection into regioregular poly(3-hexylthiophene) by molecular doping

Yuan Zhang and Paul W. M. Blom

Appl. Phys. Lett. 97, 083303 (2010); http://dx.doi.org/10.1063/1.3464560 (3 pages) | Cited 9 times

Online Publication Date: 24 August 2010

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The hole injection in Schottky barriers formed between p-type doped regioregular poly(3-hexylthiophene-2,5-diyl) (rr-P3HT) and silver (Ag) is investigated. The rr-P3HT is controllably doped using the acceptor 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ). We demonstrate that only one order of magnitude increase in the background hole density p0, from 2×1016 to 2×1017 cm−3, enhances the hole injection with two orders of magnitude. The hole injection barrier is lowered by 0.5 eV and exhibits a linear dependence on p0, which can be explained by doping induced surface charges.
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73.30.+y Surface double layers, Schottky barriers, and work functions
73.61.Ph Polymers; organic compounds
85.30.Kk Junction diodes
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Enhanced efficiency and reduced roll-off in nondoped phosphorescent organic light-emitting devices with triplet multiple quantum well structures

Shumei Liu, Bin Li, Liming Zhang, Hang Song, and Hong Jiang

Appl. Phys. Lett. 97, 083304 (2010); http://dx.doi.org/10.1063/1.3483131 (3 pages) | Cited 11 times

Online Publication Date: 24 August 2010

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Highly efficient nondoped phosphorescent organic light-emitting devices (NPOLEDs) with triplet multiple quantum well structures are fabricated by using 4,4′-N,N′-dicarbazole-biphenyl and an iridium(III) complex as the potential barrier layer and the potential well layer/light-emitting layer, respectively. Remarkably, such NPOLED with an optimized device configuration achieves reduced current efficiency roll-off, which slightly decreases from its peak value of 31.5 cd/A at 19.8 mA/cm2 to 29.2 cd/A at 100 mA/cm2. We attribute this improvement to the efficient triplet exciton confinement effect and the suppression of triplet-triplet annihilation which occurs via single-step long range (Förster-type) energy transfer between excited molecules.
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85.60.Jb Light-emitting devices
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)
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Organic solar cells with remarkable enhanced efficiency by using a CuI buffer to control the molecular orientation and modify the anode

Chuan Hui Cheng, Jin Wang, Guo Tong Du, Shao Hua Shi, Zhi Jie Du, Zhao Qi Fan, Ji Ming Bian, and Min Shuai Wang

Appl. Phys. Lett. 97, 083305 (2010); http://dx.doi.org/10.1063/1.3483159 (3 pages) | Cited 19 times

Online Publication Date: 24 August 2010

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CuI is used to control the molecular orientation of copper phthalocyanine (CuPc) and modify the anodes in organic solar cells based on CuPc/C60. By introducing a CuI buffer between indium tin oxide and CuPc, the power conversion efficiency is significantly enhanced by a factor of ∼ 70%. Because of the strong interactions between the CuI and CuPc, the stacking orientation of CuPc molecules is changed, resulting in a ∼ 65% increase in absorption coefficient, a larger carrier mobility and a smoother film surface. The anode work function is raised by the formation of a dipole layer.
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88.40.jr Organic photovoltaics
88.40.hj Efficiency and performance of solar cells
82.45.Fk Electrodes
73.30.+y Surface double layers, Schottky barriers, and work functions
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Enhancement of the short circuit current in organic photovoltaic devices with microcavity structures

Jane Lee, Sei-Yong Kim, Changsoon Kim, and Jang-Joo Kim

Appl. Phys. Lett. 97, 083306 (2010); http://dx.doi.org/10.1063/1.3480612 (3 pages) | Cited 11 times

Online Publication Date: 24 August 2010

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We demonstrate that the use of a microcavity structure can overcome the trade-off between exciton diffusion and optical absorption in planar heterojunction organic photovoltaic cells. Optical simulation based on the copper phthalocyanine (CuPc)-fullerene donor-acceptor system showed that the microcavity device with the spacer layer confines a large electric field inside the cavity so that high external quantum efficiency can be achieved even with a 10-nm-thick CuPc layer, which is comparable to the exciton diffusion length of the layer. The optimized microcavity device leads to an enhancement of the short circuit current of up to 51.6% compared with the conventional device.
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85.60.Bt Optoelectronic device characterization, design, and modeling
84.60.Jt Photoelectric conversion
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Photoluminescence spectroscopy and lifetime measurements from self-assembled semiconductor-metal nanoparticle hybrid arrays

M. Haridas, J. K. Basu, D. J. Gosztola, and G. P. Wiederrecht

Appl. Phys. Lett. 97, 083307 (2010); http://dx.doi.org/10.1063/1.3483162 (3 pages) | Cited 3 times

Online Publication Date: 26 August 2010

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We present results of photoluminescence spectroscopy and lifetime measurements on thin film hybrid arrays of semiconductor quantum dots and metal nanoparticles embedded in a block copolymer template. The intensity of emission as well as the measured lifetime would be controlled by varying the volume fraction and location of gold nanoparticles in the matrix. We demonstrate the ability to both enhance and quench the luminescence in the hybrids as compared to the quantum dot array films while simultaneously engineering large reduction in luminescence lifetime with incorporation of gold nanoparticles.
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78.55.-m Photoluminescence, properties and materials
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
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Highly efficient, single-layer organic light-emitting devices based on a graded-composition emissive layer

Nicholas C. Erickson and Russell J. Holmes

Appl. Phys. Lett. 97, 083308 (2010); http://dx.doi.org/10.1063/1.3481426 (3 pages) | Cited 15 times

Online Publication Date: 27 August 2010

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We demonstrate highly efficient electrophosphorescence from devices comprising a single organic layer. High efficiency is realized by combining both hole- and electron-transporting host materials (HTMs and ETMs, respectively) into a single, graded-composition emissive layer with the green phosphorescent emitter fac-tris(2-phenylpyridine) iridium (III). The composition is continuously graded to realize nearly 100% HTM at the anode and nearly 100% ETM at the cathode. Peak external quantum and power efficiencies of ηEQE = (19.3±0.4)% and ηP = (66.5±1.3) lm/W are realized at a luminance level of 600 cd/m2.
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85.60.Jb Light-emitting devices
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