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18 Feb 2008

Volume 92, Issue 7, Articles (07xxxx)

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Appl. Phys. Lett. 92, 073101 (2008); http://dx.doi.org/10.1063/1.2840574 (3 pages)

N. W. Gong, M. Y. Lu, C. Y. Wang, Y. Chen, and L. J. Chen
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Lithium cobalt oxide as electron injection material for high performance organic light-emitting diodes

Deqiang Zhang, Yang Li, Guohui Zhang, Yudi Gao, Lian Duan, Liduo Wang, and Yong Qiu

Appl. Phys. Lett. 92, 073301 (2008); http://dx.doi.org/10.1063/1.2828859 (3 pages) | Cited 7 times

Online Publication Date: 19 February 2008

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Lithium cobalt oxide was introduced into organic light-emitting diodes as the electron injection layer. The device with tris(8-hydroxyquinolato) aluminum doped with 10-(2-benzothiazolyl)-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H,11H-[1]benzopyrano[6,7,8-ij]quinolizin-11-one as the light-emitting layer and LiCoO2 as the electron injection layer showed promising efficiency (10.74 cd/A at 11 V) and longer lifetime (2.8 times as much as LiF/Al control device). Lithium cobalt oxide proved to be thermally decomposed in vacuum to form lithium oxide, which was responsible for the enhanced electron injection.
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85.60.Jb Light-emitting devices
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Light scattering from an ordered array of needle-shaped organic nanoaggregates: Evidence for optical mode launching

J. Fiutowski, V. G. Bordo, L. Jozefowski, M. Madsen, and H.-G. Rubahn

Appl. Phys. Lett. 92, 073302 (2008); http://dx.doi.org/10.1063/1.2839396 (3 pages) | Cited 1 time

Online Publication Date: 19 February 2008

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Pronounced peaks in both photoluminescence and light scattering from an array of almost parallel oriented, needle-shaped organic nanoaggregates on mica are observed as a function of angle of incidence. Within the framework of a simple theoretical model, we identify those peaks as originating from the launching of normal modes in the nanofibers, both radiative and waveguiding. Quantitative information is obtained about the mode spectrum and morphology of nanoscaled objects from a simple far field scattering measurement.
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78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
78.55.Kz Solid organic materials
81.07.Nb Molecular nanostructures
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Passivated p-type silicon: Hole injection tunable anode material for organic light emission

W. Q. Zhao, G. Z. Ran, W. J. Xu, and G. G. Qin

Appl. Phys. Lett. 92, 073303 (2008); http://dx.doi.org/10.1063/1.2857543 (3 pages) | Cited 2 times

Online Publication Date: 19 February 2008

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We find that hole injection can be enhanced simply by selecting a lower-resistivity p-Si anode to match an electron injection enhancement for organic light emitting diodes with ultrathin-SiO2-layer-passivated p-Si anode (Si-OLED). For a Si-OLED with ordinary AlQ electron transport layer, the optimized resistivity of the p-Si anode is 40 Ω cm; for that with n-doped Bphen electron transport layer, it decreases to 5 Ω cm. Correspondingly, the maximum power efficiency increases from 0.3 to 1.9 lm/W, even higher than that of an indium tin oxide control device (1.4 lm/W). This passivated p-type silicon is a hole injection tunable anode material for OLED.
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85.60.Jb Light-emitting devices
81.65.Rv Passivation
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Electroluminescence imaging and microstructure of organic light-emitting field-effect transistors

Jana Zaumseil, R. Joseph Kline, and Henning Sirringhaus

Appl. Phys. Lett. 92, 073304 (2008); http://dx.doi.org/10.1063/1.2836790 (3 pages) | Cited 13 times

Online Publication Date: 20 February 2008

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The effect of morphology and microstructure on the emission characteristics of ambipolar light-emitting field-effect transistors is studied using the polyfluorene copolymer F8BT [poly(9,9-di-n-octylfluorene-alt-benzothiadiazole)] as a model system. Different intensity distributions of the emission zones of amorphous, polycrystalline, and aligned F8BT films are demonstrated. Electroluminescence maps of the channel region are produced by overlaying a series of images recorded during gate voltage sweeps. They show a correlation to the microcrystalline structure of the F8BT and are assumed to visualize the current density distribution within the transistor channel.
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85.60.Jb Light-emitting devices
85.30.Tv Field effect devices
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Star-configured carbazole as an efficient near-ultraviolet emitter and hole-transporting material for organic light-emitting devices

Hai-Feng Xiang, Zong-Xiang Xu, V. A. L. Roy, Chi-Ming Che, P. T. Lai, Peng-Ju Zeng, Fang-Fang Niu, Ya-Wei Liu, Wei-Qun Tang, Cai-Jie He, and Han-Ben Niu

Appl. Phys. Lett. 92, 073305 (2008); http://dx.doi.org/10.1063/1.2841063 (3 pages) | Cited 2 times

Online Publication Date: 20 February 2008

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A novel organic material, 9-methyl-1,3,6,8-tetraphenyl-carbazole (MTPC-Me), for use in organic electroluminescent devices has been developed. This star-configured carbazole gives a strong near-ultraviolet (n-UV) emission (λmax = 389 nm) with a high emission quantum efficiency of 47% and a narrow full width half maximum of 40 nm. Two types of high-performance organic light-emitting devices were obtained using MTPC-Me as a n-UV emitter and hole-transporting material with maximum external quantum efficiency, brightness, and turn-on voltage of 1.2%, 1040 cd/m2, and 3.5 V for the former and 1.1%, 18 000 cd/m2, and 2.4 V for the latter, respectively.
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78.60.Fi Electroluminescence
85.60.Jb Light-emitting devices
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Direct analysis of monomer diffusion times in polyvinyl/acrylamide materials

Sergi Gallego, Andrés Márquez, David Méndez, Cristian Neipp, Manuel Ortuño, Augusto Beléndez, Elena Fernández, and Inmaculada Pascual

Appl. Phys. Lett. 92, 073306 (2008); http://dx.doi.org/10.1063/1.2836948 (3 pages)

Online Publication Date: 21 February 2008

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We propose an alternative, direct method to calculate monomer diffusion times in photopolymer materials. Very long period gratings are recorded in the photopolymer, and the real time variation in diffraction efficiency due to monomer diffusion is then analyzed. In photopolymers, monomer diffusion plays a fundamental role in hologram formation. However, since many interrelated parameters affect hologram formation, the usual techniques do not allow for an independent estimation of monomer diffusion times. The direct method proposed is applied to two polyvinyl based photopolymers with different molecular weights.
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42.70.Jk Polymers and organics
42.79.Dj Gratings
42.40.-i Holography
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Performance improvement of TiO2/P3HT solar cells using CuPc as a sensitizer

Liang Shen, Guohui Zhu, Wenbin Guo, Chen Tao, Xindong Zhang, Caixia Liu, Weiyou Chen, Shengping Ruan, and Zhicheng Zhong

Appl. Phys. Lett. 92, 073307 (2008); http://dx.doi.org/10.1063/1.2884270 (3 pages) | Cited 8 times

Online Publication Date: 21 February 2008

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In this work, a new type of TiO2/polymer solar cells was fabricated. The device structure was indium tin oxide titanium dioxide (TiO2)/ copper phthalocyanine (CuPc)/poly(3-hexylthiophene) (P3HT)/Au. In this architecture, TiO2 was designed to act as electron acceptor, and P3HT was electron donor. CuPc was used as a sensitizer to enhance photon absorption. The results show that by inserting CuPc between P3HT and TiO2 layers, the light absorption, excitons separation, and photocurrent under illumination are dramatically improved. The device has a short current density (JSC) of 1.15 mA/cm2 and power conversion efficiency (η) of 0.28% without CuPc layer. However, JSC and η turn to be 2.22 mA/cm2 and 0.66%, respectively, with a 20 nm thickness CuPc layer under air mass 1.5 global (AM1.5G) illumination with the intensity of 100 mW/cm2. The performance improvement can be attributed to the higher carrier mobility and the stronger photon absorption using CuPc as a sensitizing layer.
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84.60.Jt Photoelectric conversion
72.20.Fr Low-field transport and mobility; piezoresistance
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Distinct band gaps and isotropy combined in icosahedral band gap materials

D. Sutter-Widmer, P. Neves, P. Itten, R. Sainidou, and W. Steurer

Appl. Phys. Lett. 92, 073308 (2008); http://dx.doi.org/10.1063/1.2884319 (3 pages) | Cited 1 time

Online Publication Date: 21 February 2008

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Icosahedral band gap materials (BGMs) optimally combine the distinct band gaps of periodic BGMs with the high rotational symmetry of quasiperiodic structures. This is shown experimentally for longitudinal and transverse polarized elastic waves in a phononic crystal based on the three-dimensional Penrose tiling (3D-PT) and applies equally to photonic crystals. The ability of icosahedral BGMs to form Bragg-type band gaps follows from the similarity between the 3D-PT and the face-centered cubic structure (its periodic average structure). The 3D quasiperiodic BGM lacks bands of strong transmission like random or disordered BGMs but shows clear band gaps like periodic BGMs do.
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42.70.Qs Photonic bandgap materials
61.44.Br Quasicrystals
62.30.+d Mechanical and elastic waves; vibrations
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