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6 Oct 2008

Volume 93, Issue 14, Articles (14xxxx)

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Appl. Phys. Lett. 93, 141901 (2008); http://dx.doi.org/10.1063/1.2990662 (3 pages)

J. H. Han, K. B. Kim, S. Yi, J. M. Park, S. W. Sohn, T. E. Kim, D. H. Kim, J. Das, and J. Eckert
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Approaches for achieving highly efficient exciplex-based organic light-emitting devices

S. L. Lai, M. Y. Chan, Q. X. Tong, M. K. Fung, P. F. Wang, C. S. Lee, and S. T. Lee

Appl. Phys. Lett. 93, 143301 (2008); http://dx.doi.org/10.1063/1.2993326 (3 pages) | Cited 9 times

Online Publication Date: 6 October 2008

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We studied the performance of exciplex-based organic light-emitting devices (OLEDs) made of different electron transporting materials (ETMs) with similar electron affinities to minimize the effect of the lowest unoccupied molecular orbital levels. A strong correlation was observed between the intensity of exciplex emission and the choice of ETMs. The intensity of exciplex emission relied on interfacial charge accumulation densities at organic/organic contacts, which in turn determined device color and efficiency. Contrary to common belief, highly efficient exciplex-based OLEDs can be achieved, provided that the involved organic materials have high carrier mobility, high photoluminescence quantum yield, and suitable electron energy levels.
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85.60.Jb Light-emitting devices
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Artificial electrical dipole in polymer multilayers for nonvolatile thin film transistor memory

Chia-Chieh Chang, Zingway Pei, and Yi-Jen Chan

Appl. Phys. Lett. 93, 143302 (2008); http://dx.doi.org/10.1063/1.2996260 (3 pages) | Cited 11 times

Online Publication Date: 6 October 2008

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In this letter, an organic nonvolatile thin film transistor (TFT) memory on a plastic substrate is reported. The cross-linked poly-4-vinyl phenol (PVP) is used as a polymer dielectric layer in the form of a triple layer structure to achieve the memory function. Two interfaces between the PVP triple layers are the main trapping centers for electrons and holes, respectively, which are verified by the capacitance-voltage analysis. The electric dipole is established by the separated electrons and holes in the two interfaces of the PVP triple layer structure and results in an 11 V memory window for the TFT nonvolatile memory.
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84.30.Sk Pulse and digital circuits
85.30.Tv Field effect devices
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Trimolecular recombination in polythiophene: fullerene bulk heterojunction solar cells

G. Juška, K. Genevičius, N. Nekrašas, G. Sliaužys, and G. Dennler

Appl. Phys. Lett. 93, 143303 (2008); http://dx.doi.org/10.1063/1.2996588 (3 pages) | Cited 31 times

Online Publication Date: 7 October 2008

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In this work, reduced Langevin recombination in poly(3-hexylthiophene): 1-(3-methoxycarbonyl) propyl-1-phenyl[6,6]C61 blends has been studied with various methods. Using photogenerated charge carrier extraction by linearly increasing voltage technique, it was observed that charge carrier recombination transient corresponds to trimolecular recombination. By double injection current transient technique, it was shown that the influence of deep charge carrier trapping is insignificant and that trimolecular recombination coefficient γ is independent of the electric field. Finally, the temperature dependence of γ was measured by integral time-of-flight method.
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84.60.Jt Photoelectric conversion
36.20.-r Macromolecules and polymer molecules
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Postpolymerization alignment of bulk conjugated polymers

Tiffany M. S. Wilson, Douglas A. Chinn, David B. Robinson, and F. P. Doty

Appl. Phys. Lett. 93, 143304 (2008); http://dx.doi.org/10.1063/1.2978247 (3 pages) | Cited 1 time

Online Publication Date: 7 October 2008

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We have demonstrated postpolymerization stretch alignment of substituted conjugated polymers as evidenced by polarized infrared spectroscopy, with a Hermans orientation function value of 0.66 for a stretch ratio of 3.7. The effects of stretch rate and substrate on the material order are examined. We further show that the stretch-induced increase in long-range order leads to an increase in photoconductive response parallel to the stretch direction, and a decrease in the orthogonal direction. The introduction of small amounts of substituted fullerenes does not interfere with the ordering of the polymer on a bulk scale.
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82.35.Cd Conducting polymers
61.41.+e Polymers, elastomers, and plastics
72.40.+w Photoconduction and photovoltaic effects
78.30.Jw Organic compounds, polymers
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Thin pentacene interlayer for polymer bulk-heterojunction solar cell

Ju-Hyung Kim, Sung-Yoon Huh, Tae-il Kim, and Hong H. Lee

Appl. Phys. Lett. 93, 143305 (2008); http://dx.doi.org/10.1063/1.2998583 (3 pages) | Cited 12 times

Online Publication Date: 9 October 2008

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We demonstrate that introducing a thin pentacene layer between metal cathode and photoactive layer results in a better than 50% improvement in the power conversion efficiency of polymer bulk-heterojunction solar cell when compared with the cell without the interlayer. An additional path for electron transfer provided by the thin pentacene layer appears to be mainly responsible for the improvement along with the increase in the open circuit voltage that occurs with increasing pentacene thickness. The concepts of utilizing additional charge transfer route and exploiting possible change in open circuit voltage with the inserted layer could widen avenues for enhancing the device performance.
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84.60.Jt Photoelectric conversion
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Cavity-enhanced stimulated emission cross section in polymer microlasers

Martin Djiango, Takeyuki Kobayashi, and Werner J. Blau

Appl. Phys. Lett. 93, 143306 (2008); http://dx.doi.org/10.1063/1.2999584 (3 pages) | Cited 5 times

Online Publication Date: 9 October 2008

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We report a cavity quantum electrodynamic enhancement of stimulated emission in polymer microlasers. The enhancement of the stimulated emission cross section is shown through the demonstration of laser action in the 40 μm diameter cylindrical microcavities whose total molecular population density is below that theoretically required for the onset of lasing. The lower bound of the enhancement factor is estimated to be 3.9 based on the noncavity value of the stimulated emission cross section and the measured cavity quality factor.
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42.55.Rz Doped-insulator lasers and other solid state lasers
42.50.Pq Cavity quantum electrodynamics; micromasers
78.45.+h Stimulated emission
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High efficiency blue phosphorescent organic light-emitting device

Neetu Chopra, Jaewon Lee, Ying Zheng, Sang-Hyun Eom, Jiangeng Xue, and Franky So

Appl. Phys. Lett. 93, 143307 (2008); http://dx.doi.org/10.1063/1.3000382 (3 pages) | Cited 77 times

Online Publication Date: 10 October 2008

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We have demonstrated a substantial enhancement in the efficiency of iridium (III) bis[(4,6-di-fluorophenyl)-pyridinate-N,C2′]picolinate based blue phosphorescent organic light-emitting devices (PHOLEDs). The efficiencies of PHOLEDs with conventional electron transport materials are low due to their low electron mobilities as well low triplet energies. High triplet energy electron transporting material with high electron mobility was used as a hole blocker to achieve efficient exciton confinement and good charge balance in the device thereby achieving a high current efficiency of 49 cd/A and an external quantum efficiency of 23%.
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85.60.Jb Light-emitting devices
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Correlation between the glass-rubber transition and ionic conductivity in poly(3-hexylthiophene)

M. Lada

Appl. Phys. Lett. 93, 143308 (2008); http://dx.doi.org/10.1063/1.2992570 (3 pages) | Cited 2 times

Online Publication Date: 10 October 2008

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Using impedance spectroscopy and metal-insulator-semiconductor structures, the small-signal bulk conductivity of annealed poly(3-hexylthiophene) (P3HT) has been extracted in the temperature range of 150–420 K. A faster-than-Arrhenius increase in the conductivity of P3HT observed near and above room temperature is shown to be a result of the glass-rubber transition and is closely related to the amorphous phase of the polymer. The super-Arrhenius conductivity can be modeled and interpreted as ionic, arising from the thermal motion of the polymer segments. In addition, a percolative conduction mechanism induced by the glass transition can phenomenologically model the conductivity increase.
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64.70.pj Polymers
66.30.hk Polymers
61.72.Cc Kinetics of defect formation and annealing
82.35.Cd Conducting polymers
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