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29 Aug 2011

Volume 99, Issue 9, Articles (09xxxx)

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Appl. Phys. Lett. 99, 094101 (2011); http://dx.doi.org/10.1063/1.3629783 (3 pages)

Tomasz Szymborski, Piotr M. Korczyk, Robert Hołyst, and Piotr Garstecki
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Chemical vapor deposition graphene as structural template to control interfacial molecular orientation of chloroaluminium phthalocyanine

Hong Ying Mao, Rui Wang, Yu Wang, Tian Chao Niu, Jian Qiang Zhong, Ming Yang Huang, Dong Chen Qi, Kian Ping Loh, Andrew Thye Shen Wee, and Wei Chen

Appl. Phys. Lett. 99, 093301 (2011); http://dx.doi.org/10.1063/1.3629812 (3 pages) | Cited 5 times

Online Publication Date: 29 August 2011

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Chemical vapor deposition (CVD) graphene has been used as an effective structural template to manipulate molecular orientation of organic thin film of chloroaluminium phthalocyanine (ClAlPc) on indium tin oxide (ITO) electrode. As revealed by in-situ near-edge x-ray adsorption fine structure measurement, ClAlPc molecules on the CVD graphene modified ITO electrode adopt a well-aligned lying-down configuration with their molecular π-plane nearly parallel to the electrode surface, in contrast to the random orientation of ClAlPc molecules on the bare ITO electrode. This lying-down configuration results in an optimized stacking of the molecular π-plane perpendicular to the electrode surface and hence facilitates efficient charge transport along this direction.
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68.55.ag Semiconductors
78.70.Dm X-ray absorption spectra
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Highly stable Al-doped ZnO transparent conductors using an oxidized ultrathin metal capping layer at its percolation thickness

T. L. Chen, D. S. Ghosh, D. Krautz, S. Cheylan, and V. Pruneri

Appl. Phys. Lett. 99, 093302 (2011); http://dx.doi.org/10.1063/1.3631674 (3 pages) | Cited 7 times

Online Publication Date: 30 August 2011

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Metal capping layer can be used to enhance the physical properties of thin films. We propose a transparent conductor structure made of Al-doped ZnO (AZO) and an oxidized Ni capping layer, the latter with a thickness in proximity of its percolation threshold (2.5 nm). The capping layer inhibits the penetration of oxygen and water into the AZO’s grain boundaries thus significantly increasing the stability of the combined structure, as it is shown by its resistance in damp heat testing at 95 °C and 95% humidity. In addition, the oxidized Ni capping layer increases the performance of AZO transparent anodes in organic light emitting diodes by producing efficiencies as high as those of indium-tin-oxide based devices.
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68.55.ag Semiconductors
61.72.Mm Grain and twin boundaries
68.60.Wm Other nonelectronic physical properties
78.66.Hf II-VI semiconductors
81.05.Dz II-VI semiconductors
81.65.Mq Oxidation
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Efficient bulk heterojunction photovoltaic cells with a pre-organized poly(3-hexylthiophene) phase

D. Moerman, R. Lazzaroni, and O. Douhéret

Appl. Phys. Lett. 99, 093303 (2011); http://dx.doi.org/10.1063/1.3628658 (3 pages) | Cited 2 times

Online Publication Date: 31 August 2011

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This paper presents the controlled generation of nanostructured poly(3-hexylthiophene) (P3HT) from solution to fully fibrillate solid-state films. The fibrillate morphology of P3HT is further maintained when the deposition is carried out from a mixed solution with the acceptor material ([6,6]-phenyl-C61-butyric acid methyl ester, PCBM). This enables the fabrication of active layers for bulk heterojunction organic photovoltaic devices with a controlled morphology. The contribution of these nanostructures to the photovoltaic performances is evidenced. High-resolution electrical characterization with conductive atomic force microscopy (C-AFM) confirms an improved charge transport throughout the fibrillate P3HT matrix. A local hole mobility value of 0.9 × 10−4 cm2/V s is derived from the C-AFM measurements.
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84.60.Jt Photoelectric conversion
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Numerical impedance analysis for organic semiconductors with exponential distribution of localized states

Evelyne Knapp and Beat Ruhstaller

Appl. Phys. Lett. 99, 093304 (2011); http://dx.doi.org/10.1063/1.3633109 (3 pages) | Cited 5 times

Online Publication Date: 1 September 2011

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We present a comprehensive numerical impedance spectroscopy analysis of an organic semiconductor device. A physical model that considers localized states is combined with a space- and frequency-resolved numerical framework. We study the details of the frequency-dependent capacitance of an electron-only device and distinguish different trapping regimes depending on the parameters. Depending on the choice of the trapping parameters, a capacitance rise at low frequency is observed. The extraction of the characteristic temperature of the exponential of the trap density of states (DOS) by a simplified method by T. Okachi et al. [Appl. Phys. Lett. 94, 043301(2009)] is investigated.
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72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
72.80.Le Polymers; organic compounds (including organic semiconductors)
71.20.Rv Polymers and organic compounds
71.20.Nr Semiconductor compounds
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MoO3 as combined hole injection layer and tapered spacer in combinatorial multicolor microcavity organic light emitting diodes

Rui Liu, Chun Xu, Rana Biswas, Joseph Shinar, and Ruth Shinar

Appl. Phys. Lett. 99, 093305 (2011); http://dx.doi.org/10.1063/1.3623482 (3 pages) | Cited 1 time

Online Publication Date: 1 September 2011

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Multicolor microcavity (μC) organic light-emitting diode (OLED) arrays were fabricated simply by controlling the hole injection and spacer MoO3 layer thickness. The normal emission was tunable from ∼490 to 640 nm and can be further expanded. A compact, integrated spectrometer with two-dimensional combinatorial arrays of μC OLEDs was realized. The MoO3 yields more efficient and stable devices, revealing a new breakdown mechanism. The pixel current density reaches ∼4 A/cm2 and a maximal normal brightness ∼140 000 Cd/m2, which improves photoluminescence-based sensing and absorption measurements.
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85.60.Jb Light-emitting devices
78.55.-m Photoluminescence, properties and materials
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High-performance space-charge-limited transistors with well-ordered nanoporous aluminum base electrode

Kun-Yang Wu, Yu-Tai Tao, Chi-Chih Ho, Wei-Li Lee, and Tsong-Pyng Perng

Appl. Phys. Lett. 99, 093306 (2011); http://dx.doi.org/10.1063/1.3632045 (3 pages) | Cited 1 time

Online Publication Date: 1 September 2011

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A large-area and periodically patterned nanoporous aluminum grid with controlled pore size was fabricated by poly(ethylene oxide)-assisted self-assembly of polystyrene nanospheres. The grid layer was used as the shadow mask for the creation of nanochannels in a polymeric dielectric layer, as well as the base electrode in a space-charge limited transistor prepared thereafter. A high performance device with poly(3-hexylthiophene) as conducting semiconductor was achieved, yielding a high on-current output of ∼12 mA/cm2 and a high on-off ratio of ∼2 × 104 at a collector voltage of −2.0 V.
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85.30.Fg Bulk semiconductor and conductivity oscillation devices (including Hall effect devices, space-charge-limited devices, and Gunn effect devices)
85.30.Tv Field effect devices
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The design of efficient surface-plasmon-enhanced ultra-thin polymer-based solar cells

Adam Williamson, Éadaoin McClean, David Leipold, Dominic Zerulla, and Erich Runge

Appl. Phys. Lett. 99, 093307 (2011); http://dx.doi.org/10.1063/1.3633349 (3 pages) | Cited 5 times

Online Publication Date: 2 September 2011

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Polymer based solar cells are particularly attractive because of their mechanical flexibility and potential for low-cost fabrication. Although significant progress has been made, their efficiency is reduced strongly due to recombination processes that scale with the thickness of the active layer. A theoretical study of periodic plasmonic solar cell enhancement is presented, including a design for demonstrating high efficiency while using a significantly reduced active layer thicknesses. This is achieved through the superposition of toothgrating structures of multiple periodicities along a silver reflecting layer. Through finite-difference time-domain calculations, it was possible to optimize the overall spectral response of the cell yielding surface plasmon resonances at predetermined wavelengths. The improved solar cell design results in a system with increased absorption, allowing for the desired reduction in active layer thickness while also enhancing the performance of the cell over a wide wavelength range.
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88.40.jr Organic photovoltaics
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
02.70.Bf Finite-difference methods
78.68.+m Optical properties of surfaces
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Thermoelectric properties of n-type C60 thin films and their application in organic thermovoltaic devices

Mao Sumino, Kentaro Harada, Masaaki Ikeda, Saburo Tanaka, Koji Miyazaki, and Chihaya Adachi

Appl. Phys. Lett. 99, 093308 (2011); http://dx.doi.org/10.1063/1.3631633 (3 pages) | Cited 2 times

Online Publication Date: 2 September 2011

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The high performance of n-type organic thin-film thermoelectric elements utilizing a bilayer structure composed of C60 and Cs2CO3 was demonstrated. By employing an underlying layer, the electrical conductivity and the power factor of the n-type thermoelectric elements were significantly improved, and a maximum power factor of 20.5 μW m−1 K−2 at room temperature was demonstrated. In addition, an organic p-n prototype thermovoltaic device was demonstrated.
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85.80.Fi Thermoelectric devices
72.20.Pa Thermoelectric and thermomagnetic effects
73.50.Lw Thermoelectric effects
73.61.Wp Fullerenes and related materials
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Origin of photocurrent generation and collection losses in large area organic solar cells

Ajay K. Pandey and Jean-Michel Nunzi

Appl. Phys. Lett. 99, 093309 (2011); http://dx.doi.org/10.1063/1.3637041 (3 pages) | Cited 3 times

Online Publication Date: 2 September 2011

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Loss of performance in large area devices is a major concern for organic solar cells. We present studies highlighting up-scaling loss mechanisms caused by interfacial resistance at organic-metal interface in pentacene/C60 heterojunction solar cells. Devices with different active areas and electrodes are investigated, revealing the origin of higher series resistance in large area devices. We also simulate influence of spatial non-uniformity and anisotropic charge collection in active layer through selective coverage using customized electrode geometries and identify grain boundaries as the dominating link behind loss in photocurrent generation and collection.
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88.40.jr Organic photovoltaics
88.40.jp Multijunction solar cells
88.40.hj Efficiency and performance of solar cells
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Optimizing one-dimensional photonic-crystals based semitransparent organic solar cells by tailoring reflection phase shift within photonic bandgap

Yongbing Long (龙拥兵)

Appl. Phys. Lett. 99, 093310 (2011); http://dx.doi.org/10.1063/1.3631675 (3 pages) | Cited 2 times

Online Publication Date: 2 September 2011

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Phase adjustment is employed to redistribute the optical electric field and improve the optical absorption for semitransparent organic solar cells with one-dimensional photonic-crystals. Simulations reveal that tailoring the reflection phase shift of the one-dimensional photonic-crystals can shift the active layer into the electrical maximum region where it is most beneficial for photon absorption. With appropriate adjustment of the reflection phase shift, the optical absorption within the blue and near-infrared wavelength ranges can be improved simultaneously and yield an improvement of 112.5% in the total absorbed photons for the devices. In addition, the average transmission remains 71.1% across the visible wavelength range of 470 nm-700 nm.
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88.40.jr Organic photovoltaics
42.70.Qs Photonic bandgap materials
88.40.H- Solar cells (photovoltaics)
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