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20 Apr 2009

Volume 94, Issue 16, Articles (16xxxx)

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Appl. Phys. Lett. 94, 161105 (2009); http://dx.doi.org/10.1063/1.3119666 (3 pages)

Artur R. Davoyan, Ilya V. Shadrivov, Andrey A. Sukhorukov, and Yuri S. Kivshar
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Measurement of polar C-plane and nonpolar A-plane InN/ZnO heterojunctions band offsets by x-ray photoelectron spectroscopy

A. L. Yang, H. P. Song, H. Y. Wei, X. L. Liu, J. Wang, X. Q. Lv, P. Jin, S. Y. Yang, Q. S. Zhu, and Z. G. Wang

Appl. Phys. Lett. 94, 163301 (2009); http://dx.doi.org/10.1063/1.3123814 (3 pages) | Cited 9 times

Online Publication Date: 20 April 2009

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The valence band offsets of the wurtzite polar C-plane and nonpolar A-plane InN/ZnO heterojunctions are directly determined by x-ray photoelectron spectroscopy to be 1.76±0.2 eV and 2.20±0.2 eV. The heterojunctions form in the type-I straddling configuration with a conduction band offsets of 0.84±0.2 eV and 0.40±0.2 eV. The difference of valence band offsets of them mainly attributes to the spontaneous polarization effect. Our results show important face dependence for InN/ZnO heterojunctions, and the valence band offset of A-plane heterojunction is more close to the “intrinsic” valence band offset.
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73.20.At Surface states, band structure, electron density of states
71.20.Nr Semiconductor compounds
79.60.Jv Interfaces; heterostructures; nanostructures
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Full-wave simulation of enhanced outcoupling of organic light-emitting devices with an embedded low-index grid

Michael Slootsky and Stephen R. Forrest

Appl. Phys. Lett. 94, 163302 (2009); http://dx.doi.org/10.1063/1.3116644 (3 pages) | Cited 7 times

Online Publication Date: 20 April 2009

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Enhancement of light outcoupling into substrate modes by a grid of low-refractive-index material embedded into the organic layer of an organic light-emitting device (OLED) is analyzed using full-wave electromagnetic simulations. The low-index grid (LIG) redirects modes normally trapped within the high-index organic and indium tin oxide layers (waveguide modes) into the substrate where they can be further extracted into free space using methods such as microlens arrays or roughened surfaces. This increases the external quantum and power efficiencies without affecting the electroluminescent spectrum. The dependence on grid geometry, dimensions, and refractive index is explored to optimize the structure. Simulations show that up to 50% more light can be extracted from the high-index region using an ultralow-index (n = 1.03) grid than a conventional device, and provided efficient substrate-to-air outcoupling, the external quantum efficiencies of LIG OLEDs can reach >50%.
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85.60.Jb Light-emitting devices
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Intramolecular energy transfer between the triplet of ancillary ligand and the metal to ligand charge transfer state existed in heterocyclometalated iridium (III) complexes

Liangliang Han, Dongfang Yang, Wenlian Li, Bei Chu, Yiren Chen, Zisheng Su, Dongyu Zhang, Fei Yan, Shuanghong Wu, Junbo Wang, Zhizhi Hu, and Zhiqiang Zhang

Appl. Phys. Lett. 94, 163303 (2009); http://dx.doi.org/10.1063/1.3122938 (3 pages) | Cited 2 times

Online Publication Date: 20 April 2009

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Higher efficiency red organic light emitting devices (OLEDs) were obtained by employing Ir (III) complexes with 1-phenylbutane-1, 3-dione (ba) as the second ligand than that with acetylacetone (acac) as the second ligand, which were attributed to the intramolecular energy transfer existed in these complexes. The reason were attributed to the intramolecular energy transfer existed in these complexes and were approved by their photoluminescence characteristics at 77 K, as well the phosphorescence decay lifetime. Because of the lower triplet energy level of ba (Tba,19 230 cm−1) whose energy was suitable to transfer to the triplet of the metal to ligand charge transfer (MLCT) (3MLCT,16 260 cm−1) state compared to Tacac (25 500 cm−1), more efficient OLEDs were obtained (7.0 cd/A) using Ir[2-(4′-methanoxy-styryl)-benzothiazole]2ba (ba-2) than Ir[2-(4′-methanoxy-styryl)-benzothiazole]2acac acac-2 (6.1 cd/A).
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85.60.Jb Light-emitting devices
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Molecular orientation of CuPc thin films on C60/Ag(111)

Han Huang, Wei Chen, Shi Chen, Dong Chen Qi, Xing Yu Gao, and Andrew Thye Shen Wee

Appl. Phys. Lett. 94, 163304 (2009); http://dx.doi.org/10.1063/1.3122940 (3 pages) | Cited 11 times

Online Publication Date: 20 April 2009

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The molecular orientation of copper(II) phthalocyanine (CuPc) thin films on monolayer C60 on Ag(111) was studied by in situ near-edge x-ray absorption fine structure measurements and low-temperature scanning tunneling microscopy. It is found that in densely packed thin films, CuPc molecules adopt a standing-up configuration with the molecular π-plane tilting slightly from the surface normal on monolayer C60.
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68.55.ag Semiconductors
78.70.Dm X-ray absorption spectra
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Highly phosphorescent organic mixed films: The effect of aggregation on triplet-triplet annihilation

Sebastian Reineke, Gregor Schwartz, Karsten Walzer, Meiken Falke, and Karl Leo

Appl. Phys. Lett. 94, 163305 (2009); http://dx.doi.org/10.1063/1.3123815 (3 pages) | Cited 22 times

Online Publication Date: 21 April 2009

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The efficiency roll-off at high brightness levels is a key factor limiting the application of organic light emitting diodes. We investigate triplet-triplet annihilation in an archetype phosphorescent host-guest system. We show that the currently used host-guest systems are not at the physical limit set by intrinsic annihilation, but have an increased roll-off due to aggregate formation. The existence of these aggregates is directly proven by transmission electron microscopy.
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78.55.Kz Solid organic materials
78.66.Qn Polymers; organic compounds
85.60.Jb Light-emitting devices
68.37.Lp Transmission electron microscopy (TEM)
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Determining exciton bandwidth and film microstructure in polythiophene films using linear absorption spectroscopy

Jenny Clark, Jui-Fen Chang, Frank C. Spano, Richard H. Friend, and Carlos Silva

Appl. Phys. Lett. 94, 163306 (2009); http://dx.doi.org/10.1063/1.3110904 (3 pages) | Cited 49 times

Online Publication Date: 21 April 2009

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We analyze the linear absorption spectrum of regioregular poly(3-hexylthiophene) films spun from a variety of solvents to probe directly the film microstructure and how it depends on processing conditions. We estimate the exciton bandwidth and the percentage of the film composed of aggregates quantitatively using a weakly interacting H-aggregate model. This provides a description of the degree and quality of crystallites within the film and is in turn correlated with thin-film field-effect transistor characteristics.
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71.35.-y Excitons and related phenomena
68.55.am Polymers and organics
81.15.Lm Liquid phase epitaxy; deposition from liquid phases (melts, solutions, and surface layers on liquids)
81.05.Lg Polymers and plastics; rubber; synthetic and natural fibers; organometallic and organic materials
78.66.Qn Polymers; organic compounds
78.40.Me Organic compounds and polymers
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Analysis of hole transport in a polyfluorene-based copolymer— evidence for the absence of correlated disorder

R. J. de Vries, S. L. M. van Mensfoort, V. Shabro, S. I. E. Vulto, R. A. J. Janssen, and R. Coehoorn

Appl. Phys. Lett. 94, 163307 (2009); http://dx.doi.org/10.1063/1.3119317 (3 pages) | Cited 15 times

Online Publication Date: 21 April 2009

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The presence of spatial correlation between the disordered transport site energies in semiconducting polymers used in organic electronic devices is known to affect the mobility. However, it is not established whether such a correlation is present in relevant polymers. We study hole transport in a polyfluorene-based copolymer and provide evidence for the absence of spatially correlated disorder in this material, based on an analysis of the current-voltage characteristics of sandwich-type devices. Distinguishing correlated from uncorrelated disorder, which we achieve on the basis of the hopping site density, is shown to be highly relevant for the development of quantitative device models.
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72.20.Fr Low-field transport and mobility; piezoresistance
72.80.Le Polymers; organic compounds (including organic semiconductors)
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A hybrid encapsulation method for organic electronics

N. Kim, W. J. Potscavage, Jr., B. Domercq, B. Kippelen, and S. Graham

Appl. Phys. Lett. 94, 163308 (2009); http://dx.doi.org/10.1063/1.3115144 (3 pages) | Cited 17 times

Online Publication Date: 22 April 2009

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We report a thin-film encapsulation method for organic electronics that combines the deposition of a layer of SiOx or SiNx (100 nm) by plasma enhanced chemical vapor deposition followed by a layer of Al2O3 (10–50 nm) by atomic layer deposition and a 1-μm-thick layer of parylene by chemical vapor deposition. The effective water vapor transmission rates of the encapsulation was (2±1)×10−5 g/m2 day at 20 °C and 50% relative humidity (RH). The encapsulation was integrated with pentacene/C60 solar cells, which showed no decrease in conversion efficiency after 5800 h of exposure to air demonstrating the effectiveness of the encapsulation methodology.
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84.60.Jt Photoelectric conversion
52.77.Dq Plasma-based ion implantation and deposition
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
68.55.-a Thin film structure and morphology
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
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A high ON/OFF ratio organic film for photo- and electro-dual-mode recording

Heng Li, Yongqiang Wen, Pengwei Li, Rongming Wang, Gang Li, Ying Ma, Lianming Yang, Yanlin Song, Qinglin Yang, and Daoben Zhu

Appl. Phys. Lett. 94, 163309 (2009); http://dx.doi.org/10.1063/1.3125257 (3 pages) | Cited 5 times

Online Publication Date: 24 April 2009

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To improve the stability of organic multifunctional materials and enhance the ON/OFF ratio, a donor-π-acceptor molecule with the triphenylamine (TPA) unit is designed and synthesized. The TPA unit can effectively stabilize the charge transferred state, and the thin film shows excellent bistable electronic switching behavior with high ON/OFF current ratio of about 105. Meanwhile, reversible and stable nanometer-scale data storage is realized on the thin film by applying pulsed voltages, and optical information storage is achieved simultaneously by utilizing fluorescence changes under UV irradiation.
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42.70.Ln Holographic recording materials; optical storage media
42.79.Vb Optical storage systems, optical disks
78.66.Qn Polymers; organic compounds
73.61.Ph Polymers; organic compounds
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
78.55.Kz Solid organic materials
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