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30 Jul 2012

Volume 101, Issue 5, Articles (05xxxx)

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Appl. Phys. Lett. 101, 051103 (2012); http://dx.doi.org/10.1063/1.4738774 (4 pages)

Alec Rose, Da Huang, and David R. Smith
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Analysis of photocurrents in lateral-geometry organic bulk heterojunction devices

Z.-E. Ooi, K. L. Chan, C. J. Lombardo, and A. Dodabalapur

Appl. Phys. Lett. 101, 053301 (2012); http://dx.doi.org/10.1063/1.4739469 (5 pages) | Cited 3 times

Online Publication Date: 30 July 2012

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Lateral-geometry organic bulk heterojunction (BHJ) devices resemble field-effect transistors without a gate and are useful for probing electrical characteristics of BHJ blends. The lateral structure allows physical access to the BHJ during device operation and lends itself well to device modelling. Photocurrent-voltage data from P3HT:PCBM-based lateral devices were found to possess features of space-charge limited (SCL) extraction. Numerical simulations indeed suggest a physical picture of SCL operation, effectively dividing the device into a photogenerative zone and a photoconducting zone. Based on these simulations, an analytical model was created to describe SCL photocurrents that account for channel lengths and bimolecular recombination.
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85.30.Tv Field effect devices
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Electron and hole transport in an anthracene-based conjugated polymer

Nadia Camaioni, Francesca Tinti, Alessandra Degli Esposti, Sara Righi, Özlem Usluer, Sameh Boudiba, and Daniel A. M. Egbe

Appl. Phys. Lett. 101, 053302 (2012); http://dx.doi.org/10.1063/1.4742738 (3 pages) | Cited 2 times

Online Publication Date: 31 July 2012

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The bulk transport properties of an anthracene-containing poly(p-phenylene-ethynylene)-alt-poly(p-phenylene-vinylene) (AnE-PVstat) were investigated by small-signal time-of-flight. AnE-PVstat exhibited a very good ambipolar behaviour, consistent with the high delocalization of both frontier molecular orbitals. The transport of negative carriers was found to be even faster than that of positive carriers, with an electron drift mobility of about six times higher than that evaluated for holes, suggesting that conjugated polymers with carbon-carbon triple bonds could represent a promising platform toward high performance ambipolar materials for plastic electronics.
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72.20.Fr Low-field transport and mobility; piezoresistance
73.50.Dn Low-field transport and mobility; piezoresistance
73.61.Ph Polymers; organic compounds
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Solution processed multilayer cadmium-free blue/violet emitting quantum dots light emitting diodes

Chaoyu Xiang, Wonhoe Koo, Song Chen, Franky So, Xiong Liu, Xiangxing Kong, and Yunjun Wang

Appl. Phys. Lett. 101, 053303 (2012); http://dx.doi.org/10.1063/1.4738375 (4 pages) | Cited 1 time

Online Publication Date: 31 July 2012

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We demonstrate blue/violet emitting devices based on cadmium-free zinc selenide/zinc sulfide core/shell quantum dots. Using poly(N-vinylcarbazole) with a low lying highest occupied molecular orbital energy for the hole transporting layer, enhanced hole injection was observed, resulting in efficient blue/violet emitting devices. The device charge balance was further enhanced by tuning the thicknesses of the hole transporting layer and quantum dot emitting layer.
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85.60.Jb Light-emitting devices
85.30.Kk Junction diodes
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Polarization effects on energy-level alignment at the interfaces of polymer organic semiconductor films

Li-Hong Zhao, Rui-Qi Png, Cavan C. H. Chiam, Han Guo, Jing-Mei Zhuo, Lay-Lay Chua, Andrew T. S. Wee, and Peter K. H. Ho

Appl. Phys. Lett. 101, 053304 (2012); http://dx.doi.org/10.1063/1.4734502 (4 pages)

Online Publication Date: 1 August 2012

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The thickness-dependent evolutions of the Fermi level EF and ionization potential Ip of ultrathin films of regioregular poly(3-hexylthiophene) and poly[2,5-bis(3-tetradecylthiophene-2-yl)thieno[3,2-b]thiophene], deposited as edge-on π-stacked lamellae on gold and on p-doped poly(3,4-ethylenedioxythiophene) electrodes, have been measured by ultraviolet photoemission spectroscopy. The Ip increases by 0.3 eV on going from a monolayer film to a few-nanometer-thick film. Correspondingly, the EF pinning depth increases from 0.2 eV to 0.5 eV. This valence “band bending” which occurs for a constant vacuum level can be quantitatively modeled by microelectrostatic self-consistent polarization field calculations that incorporate both substrate and organic semiconductor film effects. The EF pinning to the first monolayer is relatively shallow.
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61.41.+e Polymers, elastomers, and plastics
71.20.Rv Polymers and organic compounds
73.20.At Surface states, band structure, electron density of states
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Enhancement of the carrier mobility of poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate) by incorporating reduced graphene oxide

Yow-Jon Lin (林祐仲), Jian-Jhou Zeng (曾建洲), and Cheng-Lung Tsai (蔡丞龍)

Appl. Phys. Lett. 101, 053305 (2012); http://dx.doi.org/10.1063/1.4740073 (4 pages) | Cited 3 times

Online Publication Date: 1 August 2012

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The investigation of poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate) (PEDOT:PSS) having the reduced graphene oxide (RGO), denoted RGO-doped PEDOT:PSS shows that conductivity of RGO-doped PEDOT:PSS samples is 27 times higher than that of PEDOT:PSS at 300 K. The improved electrical conductivity is considered to mainly come from the mobility enhancement. The carrier mobility in RGO-doped PEDOT:PSS samples exhibits unexpectedly strong temperature dependence, implying the domination of tunneling (hopping) at low (high) temperatures. An exhibition of high mobility of RGO-doped PEDOT:PSS samples is attributed to the increased spacing between molecules.
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72.20.Fr Low-field transport and mobility; piezoresistance
72.80.Rj Fullerenes and related materials
73.40.Gk Tunneling
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High-brightness organic light-emitting diodes based on a simplified hybrid structure

R. Acharya and X. A. Cao

Appl. Phys. Lett. 101, 053306 (2012); http://dx.doi.org/10.1063/1.4740272 (4 pages)

Online Publication Date: 1 August 2012

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High-brightness green phosphorescent hybrid inorganic-organic light-emitting diodes (HyLEDs) based on a simplified WO3/organic layer structure have been demonstrated. The WO3 layer greatly improved hole injection and enabled facile hole transport, leading to balanced charge injection and significant performance enhancement. The HyLEDs reached a luminance of 104 cd/m2 at 7.3 V. At this brightness level, their current efficiency was 55.2 cd/A, 57% higher than that of all-organic LEDs with a N,N-bis-(1-naphthyl)-N,N’-diphenyl-1,1′-biphenyl-4,4′-diamine hole transport layer. The HyLEDs also exhibited markedly improved reliability under constant-current stressing due to the stable indium-tin-oxide/WO3 interface.
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85.60.Jb Light-emitting devices
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Identifying the magnetoconductance responses by the induced charge transfer complex states in pentacene-based diodes

Wei-Shun Huang, Tsung-Hsun Lee, Tzung-Fang Guo, J. C. A. Huang, and Ten-Chin Wen

Appl. Phys. Lett. 101, 053307 (2012); http://dx.doi.org/10.1063/1.4742135 (5 pages) | Cited 1 time

Online Publication Date: 1 August 2012

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We investigate the magnetoconductance (MC) responses in photocurrent, unipolar injection, and bipolar injection regimes in pentacene-based diodes. Both photocurrent and bipolar injection contributed MC responses show large difference in MC line shape, which are attributed to triplet-polaron interaction modulated by the magnetic field dependent singlet fission and the intersystem crossing of the polaron pair, respectively. By blending 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane into pentacene, all the MC responses are suppressed but the MC response at unipolar injection regime is enhanced, which is attributed to the induced charge transfer complex states (CT complex states). This work identify the MC responses between single carrier contributed MC and exciton related MC by the induced CT complex states.
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85.30.-z Semiconductor devices
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Near-infrared photoresponse sensitization of solvent additive processed poly(3-hexylthiophene)/fullerene solar cells by a low band gap polymer

Zhongjian Hu, Simon Tang, Anne Ahlvers, Saiful I. Khondaker, and Andre J. Gesquiere

Appl. Phys. Lett. 101, 053308 (2012); http://dx.doi.org/10.1063/1.4742143 (4 pages) | Cited 1 time

Online Publication Date: 2 August 2012

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With the aim of extending the photoresponse of the poly(3-hexylthiophene)/[6,6]-phenyl-C61-butyric acid methyl ester photovoltaic system into the near-infrared region, a low band gap polymer poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene-2,6-diyl]] was incorporated to make ternary blend solar cells. Solvent additive 1,8-diiodooctane (DIO) was applied to optimize the phase separation in a one-step active layer preparation. Devices fabricated with DIO additive demonstrate an improvement of device power conversion efficiency by 17%, which can be attributed to favorable microscopic structure of the ternary blend with 1:1:0.2 composition processed with DIO and extended absorption on the red side of the visible spectrum up to 900 nm.
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88.40.hj Efficiency and performance of solar cells
78.40.-q Absorption and reflection spectra: visible and ultraviolet
88.40.J- Types of solar cells
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Solution processed sodium chloride interlayers for efficient electron extraction from polymer solar cells

Felix Nickel, Manuel Reinhard, Zhenhao Zhang, Andreas Pütz, Siegfried Kettlitz, Uli Lemmer, and Alexander Colsmann

Appl. Phys. Lett. 101, 053309 (2012); http://dx.doi.org/10.1063/1.4742161 (3 pages)

Online Publication Date: 2 August 2012

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We investigate a solution-processable, non-toxic sodium chloride (NaCl) interlayer for aluminum cathodes in organic solar cells. The electron extraction at the cathode interface is strongly improved upon the insertion of NaCl leading to power conversion efficiencies of up to 2.9% as compared to 1.8% efficient devices without interlayer. Scanning electron and kelvin probe force microscopy studies reveal that the formation of NaCl crystals causes a decrease of the aluminum work function by more than 300 mV. By impedance spectroscopy, we found evidence that the NaCl crystals improve the energetic alignment at the polymer/metal interface.
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88.40.jr Organic photovoltaics
73.30.+y Surface double layers, Schottky barriers, and work functions
88.40.hj Efficiency and performance of solar cells
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High efficient white organic light-emitting diodes based on triplet multiple quantum well structure

Bo Zhao, Zisheng Su, Wenlian Li, Bei Chu, Fangming Jin, Xingwu Yan, Feng Zhang, Di Fan, Tianyou Zhang, Yuan Gao, and Junbo Wang

Appl. Phys. Lett. 101, 053310 (2012); http://dx.doi.org/10.1063/1.4742739 (4 pages)

Online Publication Date: 3 August 2012

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We demonstrate the high efficient triplet multiple quantum well (MQW) structure white organic light-emitting diodes (WOLEDs) (MQW-WOLEDs) by introducing phosphorescent Ir-complex material. In the triplet MQW-WOLEDs, 1,3,5-tris(N-phenyl-benzimidazol-2-yl)benzene functions as the potential barrier layer; fluorescent dopant layer and Ir-complex phosphorescent dopant layer act as the blue-emitting and orange-emitting potential well layer, respectively. The maximum luminance, current efficiency, and power efficiency are 19 000 cd/m2, 14.5 cd/A, and 5.4 lm/W, respectively; and the efficiency roll-off is especially improved by ∼15% over reference device. The emission spectra almost do not change with the changing drive voltage; that is, Commission Internationale de I’Eclairage coordinates x and y vary only ±0.002 and ±0.008 from 8 to 14 V. The improvement of electroluminescence performances is attributed to the balanced electron-hole injection and transport, confinement of carriers and excitions within potential well; as well more detail of improvement mechanism is discussed.
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85.60.Jb Light-emitting devices
78.55.Kz Solid organic materials
78.60.Fi Electroluminescence
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)
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Investigation of polysilsesquioxane as a gate dielectric material for organic field-effect transistors

M. Kawamura, Yoshio Nakahara, Mitsuhiro Ohse, Maki Kumei, K. Uno, Hidefumi Sakamoto, Keiichi Kimura, and Ichiro Tanaka

Appl. Phys. Lett. 101, 053311 (2012); http://dx.doi.org/10.1063/1.4742891 (4 pages)

Online Publication Date: 3 August 2012

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We fabricated pentacene field-effect transistors using polysilsesquioxane (PSQ) as a gate dielectric material that can be cured at sufficiently low temperatures for low cost plastic substrates. The surface roughness of the PSQ films were reduced by the introduction of 3-methacryloxypropyl groups, and their surface energy was controlled to match with that of the pentacene layers by the introduction of phenyl groups. Consequently, the carrier mobility was improved by two orders of magnitude and reached 0.38 cm2 V−1 s−1.
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85.30.Tv Field effect devices
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