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February 2013

Volume 6, Issue 2

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High-performance and air-processed polymer solar cells by room-temperature drying of the active layer

Ziyang Hu, Jianjun Zhang, and Yuejin Zhu

Appl. Phys. Lett. 102, 043307 (2013); http://dx.doi.org/10.1063/1.4789522 (4 pages)

Online Publication Date: 1 February 2013

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High device performance is demonstrated in air-processed polymer solar cells made from an active layer of poly(3-hexylthiophene) and [6,6]-phenyl-C61-butyric acid methyl ester, with optimized efficiency and fill factor as high as 4.71% and 0.71, respectively. The degree of self-organization of the active layer can be varied by controlling the solvent evaporation rate at different room temperature (298–292 K). Device performance improvement originates from an increased absorption and increased charge-carrier mobility in the active layer. This free-annealing process compatible with flexible substrates contributes to a flexible cell with an efficiency of 4.06%.
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88.40.jr Organic photovoltaics
88.40.hj Efficiency and performance of solar cells
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Two-photon induced excited-state absorption and optical limiting properties in a chiral polymer

Yi Zeng, Changshun Wang, Fuli Zhao, Mu Qin, Yan Zhou, and Xiaobo Huang

Appl. Phys. Lett. 102, 043308 (2013); http://dx.doi.org/10.1063/1.4790391 (4 pages)

Online Publication Date: 1 February 2013

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The nonlinear absorption and optical limiting properties of a chiral polymer were investigated by employing Z-scan technique in femtosecond regime. Reverse saturable absorption was observed in the polymer at 800 nm and the nonlinear absorption coefficient of 5.97 cm/GW was obtained at the irradiance of 2.75 GW/cm2. The nonlinear absorption coefficient versus the input irradiance was measured to meet a linear increasing function, giving evidence of two-photon induced excited-state absorption existing. Particularly, the chiral polymer was shown to possess a large ratio (∼251) of excited-state to ground-state absorption cross-section and a remarkable optical limiting behavior was achieved in it.
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42.50.Md Optical transient phenomena: quantum beats, photon echo, free-induction decay, dephasings and revivals, optical nutation, and self-induced transparency
42.65.Re Ultrafast processes; optical pulse generation and pulse compression
42.70.Jk Polymers and organics
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
42.50.Gy Effects of atomic coherence on propagation, absorption, and amplification of light; electromagnetically induced transparency and absorption
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Efficient electron injection from solution-processed cesium stearate interlayers in organic light-emitting diodes

G. A. H. Wetzelaer, A. Najafi, R. J. P. Kist, M. Kuik, and P. W. M. Blom

Appl. Phys. Lett. 102, 053301 (2013); http://dx.doi.org/10.1063/1.4790592 (4 pages)

Online Publication Date: 4 February 2013

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The electron-injection capability of solution-processed cesium stearate films in organic light-emitting diodes is investigated. Cesium stearate, which is expected to exhibit good solubility and film formation due to its long hydrocarbon chain, is synthesized using a straightforward procedure. Light-emitting diodes are fabricated using orange-, yellow-, and blue-emitting conjugated polymers, comprising a cesium stearate electron-injection layer deposited from ethanol solution. It is demonstrated that these devices perform as well as benchmark devices using vacuum-deposited barium as electron-injection layer, without loss of color purity.
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85.60.Jb Light-emitting devices
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Enhanced performance in polymer solar cells by the use of a halogenated indium tin oxide anode

Shizhao Zheng and King Y. Wong

Appl. Phys. Lett. 102, 053302 (2013); http://dx.doi.org/10.1063/1.4790624 (4 pages)

Online Publication Date: 4 February 2013

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Use of a halogenated, surface-modified indium tin oxide (ITO) anode was found to enhance the photovoltaic performance of a bulk heterojunction (BHJ) polymer solar cell using poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) by as much as 29% but produced no enhancement in a BHJ device using poly(3-hexylthiophene). The position of the positive polaronic state of the polymer was found to be crucial to the enhancement. A power conversion efficiency of 6.27% for a PCDTBT-based BHJ device was achieved by using a chlorinated ITO anode without the needs of using any modifying interlayer or optical spacer at the cathode.
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88.40.jr Organic photovoltaics
88.40.hj Efficiency and performance of solar cells
88.40.jp Multijunction solar cells
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Elucidation of ambient gas effects in organic nano-floating-gate nonvolatile memory

Xiao-Jian She, Chang-Hai Liu, Jing-Yu Zhang, Xu Gao, and Sui-Dong Wang

Appl. Phys. Lett. 102, 053303 (2013); http://dx.doi.org/10.1063/1.4790186 (5 pages)

Online Publication Date: 5 February 2013

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Pentacene-based organic field-effect transistor nonvolatile memories employing nano-floating-gate show high performance in vacuum, typically with field-effect mobility of 0.6 cm2/Vs, memory window of 45 V, reading ON/OFF ratio over 106, and excellent retention ability and programming/erasing endurance. The memory performance is unstable in air, which is demonstrated to result mainly from the device operation instability in O2. The O2-induced acceptor-like trap states reduce the electron supply in pentacene during programming, limiting the electron trapping into the nano-floating-gate and thus suppressing the positive threshold voltage shift. The corresponding hole trapping during erasing is not effectively influenced by the ambient gas effects.
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84.30.Sk Pulse and digital circuits
85.30.Tv Field effect devices
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A trilayer architecture for polymer photoconductors

Zhiwen Jin and Jizheng Wang

Appl. Phys. Lett. 102, 053304 (2013); http://dx.doi.org/10.1063/1.4791595 (4 pages)

Online Publication Date: 7 February 2013

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A trilayer architecture is designed for polymer photoconductors. In such a structure, photogenerated electrons in poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid ethyl ester (P3HT:PCBM) blend film will flow into a TiOx layer beneath and then diffuse into a underlying PCBM layer. Photogenerated holes and electrons are thus efficiently separated by the spacer TiOx, and, respectively, transport in P3HT and PCBM films, carrier recombination is thereby greatly suppressed. As a result, photocurrent of the PCBM/TiOx/P3HT:PCBM trilayer structure increases more than 200 times over that of the conventional P3HT:PCBM single layer device.
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85.60.Bt Optoelectronic device characterization, design, and modeling
81.05.Fb Organic semiconductors
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths
73.50.Pz Photoconduction and photovoltaic effects
73.61.Ph Polymers; organic compounds
81.05.Lg Polymers and plastics; rubber; synthetic and natural fibers; organometallic and organic materials
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An efficient non-Lambertian organic light-emitting diode using imprinted submicron-size zinc oxide pillar arrays

S. W. Liu, J. X. Wang, Y. Divayana, K. Dev, S. T. Tan, H. V. Demir, and X. W. Sun

Appl. Phys. Lett. 102, 053305 (2013); http://dx.doi.org/10.1063/1.4791786 (4 pages)

Online Publication Date: 8 February 2013

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We report phosphorescent organic light-emitting diodes with a substantially improved light outcoupling efficiency and a wider angular distribution through applying a layer of zinc oxide periodic nanopillar arrays by pattern replication in non-wetting templates technique. The devices exhibited the peak emission intensity at an emission angle of 40° compared to 0° for reference device using bare ITO-glass. The best device showed a peak luminance efficiency of 95.5 ± 1.5 cd/A at 0° emission (external quantum efficiency—EQE of 38.5 ± 0.1%, power efficiency of 127 ± 1 lm/W), compared to that of the reference device, which has a peak luminance efficiency of 68.0 ± 1.4 cd/A (EQE of 22.0 ± 0.1%, power efficiency of 72 ± 1 lm/W).
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85.60.Jb Light-emitting devices
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Identifying the efficient inter-conversion between singlet and triplet charge-transfer states by magneto-electroluminescence study

Ping Chen, Qiming Peng, Liang Yao, Na Gao, and Feng Li

Appl. Phys. Lett. 102, 063301 (2013); http://dx.doi.org/10.1063/1.4791562 (4 pages)

Online Publication Date: 11 February 2013

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Using the magneto-electroluminescence (MEL) as a tool, we demonstrated the efficient inter-conversion between singlet and triplet charge-transfer (CT) states in exciplex-based organic light-emitting diodes (OLEDs). Results show that the MEL of exciplex-based device is larger than that of exciton-based device by a factor of 3.2. The emission of exciplex-based devices comes from the direct intermolecular electron-hole pair recombination and their spin exchange energy is much smaller, which causes the efficient inter-conversion between singlet and triplet states. This argument was supported by the consistent evolutions of the MEL and EL spectra versus applied bias and donor concentrations. Finally, the bandgap effects on the MEL as well as the external quantum efficiency of exciplex-based devices were discussed. Our findings of MEL may offer a feasible way to unravel underlying mechanisms that limit the EL efficiency in the OLEDs.
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85.60.Jb Light-emitting devices
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Study of the fast photoswitching of spin crossover nanoparticles outside and inside their thermal hysteresis loop

G. Gallé, C. Etrillard, J. Degert, F. Guillaume, J.-F. Létard, and E. Freysz

Appl. Phys. Lett. 102, 063302 (2013); http://dx.doi.org/10.1063/1.4792527 (4 pages)

Online Publication Date: 13 February 2013

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We have studied the low spin to high spin phase transition induced by nanosecond laser pulses outside and within the thermal hysteresis loop of the [Fe(Htrz)2 trz](BF4)2-H2O spin crossover nanoparticles. We demonstrate that, whatever the temperature of the compound, the photo-switching is achieved in less than 12.5 ns. Outside the hysteresis loop, the photo-induced high spin state remains up to 100 μs and then relaxes. Within the thermal hysteresis loop, the photo-induced high spin state remains as long as the temperature of the sample is kept within the thermal loop. A Raman study indicates that the photo-switching can be completed using single laser pulse excitation.
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42.65.Pc Optical bistability, multistability, and switching, including local field effects
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
78.30.-j Infrared and Raman spectra
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Electronic and interface properties of polyfluorene films on GaN for hybrid optoelectronic applications

G. Itskos, X. Xristodoulou, E. Iliopoulos, S. Ladas, S. Kennou, M. Neophytou, and S. Choulis

Appl. Phys. Lett. 102, 063303 (2013); http://dx.doi.org/10.1063/1.4792211 (5 pages)

Online Publication Date: 13 February 2013

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Electronic and interface properties of spin-coated poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO) films on GaN have been investigated in terms of their potential for optoelectronic applications. The PFO/GaN interface was studied by photoemission spectroscopy showing a type-II energy alignment with band offsets suitable for efficient photocurrent generation. The light harvesting potential is further supported by fluorescence experiments that show evidence of photo-induced electron transfer from PFO to GaN. The impact of polymer film thickness was probed using emission anisotropy and ellipsometry, indicating the presence of an ordered planar phase of PFO. The study has implications to hybrid optoelectronic devices employing the two important materials.
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85.60.Bt Optoelectronic device characterization, design, and modeling
73.20.At Surface states, band structure, electron density of states
78.55.-m Photoluminescence, properties and materials
78.66.-w Optical properties of specific thin films
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Cadmium-free copper indium gallium diselenide hybrid solar cells comprising a 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole buffer layer

Manuel Reinhard, Christoph Simon, Johannes Kuhn, Linda Bürkert, Marco Cemernjak, Bernhard Dimmler, Uli Lemmer, and Alexander Colsmann

Appl. Phys. Lett. 102, 063304 (2013); http://dx.doi.org/10.1063/1.4792507 (4 pages)

Online Publication Date: 14 February 2013

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Copper indium gallium diselenide (CIGS) solar cells are the most efficient thin film photovoltaic devices today. In this work, we investigate CIGS/organic hybrid solar cells comprising a semi-transparent metal top electrode and a wide band gap organic semiconductor as buffer layer. Depositing the organic semiconductor from solution, we fabricate Cd-free solar cells exhibiting about the same efficiency as their counterparts comprising CdS and significantly higher open-circuit voltages as compared to buffer-free devices. Although the organic molecules do not cover the CIGS surface homogeneously, their use enables prolonged charge carrier lifetimes according to impedance spectroscopy measurements.
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88.40.jn Thin film Cu-based I-III-VI2 solar cells
88.40.jr Organic photovoltaics
88.40.hj Efficiency and performance of solar cells
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Real-time dynamic hologram in photorefractive ferroelectric liquid crystal with two-beam coupling gain coefficient of over 800 cm–1 and response time of 8 ms

Takeo Sasaki, Masashi Ikegami, Tatsuya Abe, Daisuke Miyazaki, Satoshi Kajikawa, and Yumiko Naka

Appl. Phys. Lett. 102, 063306 (2013); http://dx.doi.org/10.1063/1.4792735 (4 pages)

Online Publication Date: 14 February 2013

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The photorefractive effect in photoconductive ferroelectric liquid crystals (FLCs) that contain photoconductive chiral compounds was investigated. Terthiophene compounds with chiral structure were chosen as the photoconductive chiral compounds and mixed with an achiral smectic C liquid crystal. The mixture exhibited the ferroelectric chiral smectic C phase and photoconductivity. The photorefractivity of the mixture was investigated by two-beam coupling experiment. It was found that the FLC containing the photoconductive chiral compound exhibits a large gain coefficient of over 800 cm−1 and a fast response time of 8 ms. The high photorefractive performance was considered to be due to enhanced charge mobility.
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42.40.-i Holography
42.70.Df Liquid crystals
42.70.Gi Light-sensitive materials
42.70.Ln Holographic recording materials; optical storage media
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Self-contact thin-film organic transistors based on tetramethyltetrathiafulvalene

Sumika Tamura, Tomofumi Kadoya, Tadashi Kawamoto, and Takehiko Mori

Appl. Phys. Lett. 102, 063305 (2013); http://dx.doi.org/10.1063/1.4792704 (4 pages)

Online Publication Date: 14 February 2013

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Carrier injection from organic contacts to tetramethyltetrathiafulvalene (TMTTF) is investigated in the thin-film transistors. When 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) is patterned on a TMTTF film, the resulting (TMTTF)(TCNQ) works as highly conducting source and drain electrodes. Such self-contact transistors, in which the organic material constructing the active layer is selectively transformed to the contacts, have achieved low contact resistance and high performance.
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85.30.Tv Field effect devices
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Efficiency enhancement of organic photovoltaic devices using a Sm:Al compound electrode

Bin-Bin Yang, Dan-Dan Zhang, Shuit-Tong Lee, Yan-Qing Li, and Jian-Xin Tang

Appl. Phys. Lett. 102, 073301 (2013); http://dx.doi.org/10.1063/1.4793414 (4 pages)

Online Publication Date: 19 February 2013

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An effective cathode consisting of samarium (Sm) doped aluminum (Al) layer and a pure Al layer is reported for application in organic photovoltaic cells (OPVs). Standard copper phthalocyanine (CuPc)/C60 OPVs using this bilayer cathode show dramatically increased short-circuit current density and power conversion efficiency, which are 64% increased by employing a appropriate ratio of 1:3 of Sm:Al layer as compared with that of control devices with pure Al cathode. The photoelectric properties reveal that the improved efficiency is mainly related to the balance of the enhanced electron collection ability and the optimized optical reflection of a Sm doped Al layer.
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85.60.Bt Optoelectronic device characterization, design, and modeling
84.60.Jt Photoelectric conversion
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A hybrid planar-mixed tetraphenyldibenzoperiflanthene/C70 photovoltaic cell

Xin Xiao, Jeramy D. Zimmerman, Brian E. Lassiter, Kevin J. Bergemann, and Stephen R. Forrest

Appl. Phys. Lett. 102, 073302 (2013); http://dx.doi.org/10.1063/1.4793195 (4 pages)

Online Publication Date: 19 February 2013

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We describe a hybrid planar-mixed heterojunction (PM-HJ) organic photovoltaic cell based on tetraphenyldibenzoperiflanthene (DBP) and C70 with a power conversion efficiency of up to 6.4% ± 0.3%. Optimized cells consist of a DBP:C70 mixed layer at a volume ratio of 1:8 and a 9-nm thick C70 cap layer. The external quantum efficiency (EQE) in the visible of the PM-HJ cell is up to 10% larger than the mixed-HJ cell that lacks a C70 acceptor cap layer. The improvement in EQE is attributed to reduced exciton quenching at the MoO3 anode buffer layer surface. This leads to an internal quantum efficiency >90% between the wavelengths of λ = 450 nm and 550 nm, suggesting efficient exciton dissociation and carrier extraction in the PM-HJ cell. The power conversion efficiency under simulated AM 1.5G, 1 sun irradiation increases from 5.7% ± 0.2% for the mixed-HJ cell to 6.4% ± 0.3% for the PM-HJ cell, with a short-current density of 12.3 ± 0.3 mA/cm2, open circuit voltage of 0.91 ± 0.01 V, and fill factor of 0.56 ± 0.01.
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85.60.-q Optoelectronic devices
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Enhancement of optical gain and amplified spontaneous emission due to waveguide geometry in the conjugated polymer poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene]

Zach E. Lampert, John M. Papanikolas, and C. Lewis Reynolds, Jr.

Appl. Phys. Lett. 102, 073303 (2013); http://dx.doi.org/10.1063/1.4793422 (5 pages)

Online Publication Date: 22 February 2013

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We report enhanced amplified spontaneous emission (ASE) and optical gain performance in a conjugated polymer (CP)-based thin film waveguide (WG) Si(100)/SiO2/poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV) by encapsulating the active layer with a transparent dielectric film of poly(methyl methacrylate) (PMMA). With index matched SiO2 and PMMA claddings, symmetric WGs are formed that exhibit increased mode confinement and reduced propagation loss enabling lower ASE threshold (40%) and higher optical gain (50%) compared to Si(100)/SiO2/MEH-PPV/air asymmetric WGs. An extremely large net gain coefficient of 500 cm−1 is achieved under picosecond pulse excitation, which is >4× larger than values previously reported in the literature. Fabrication of symmetric WGs requires no complex processing techniques, thus offering a simple, low-cost approach for effectively controlling the ASE behavior of CP-based WGs and related optical devices.
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42.79.Gn Optical waveguides and couplers
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
78.45.+h Stimulated emission
78.66.Qn Polymers; organic compounds
42.65.Re Ultrafast processes; optical pulse generation and pulse compression
42.70.Jk Polymers and organics
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Ambipolar organic light-emitting electrochemical transistor based on a heteroleptic charged iridium(III) complex

Ye Zhou, Suting Han, Guijiang Zhou, Wai-Yeung Wong, and V. A. L. Roy

Appl. Phys. Lett. 102, 083301 (2013); http://dx.doi.org/10.1063/1.4792842 (4 pages)

Online Publication Date: 25 February 2013

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High performance organic light-emitting electrochemical transistor (OLECT) based on a phosphorescent heteroleptic charged iridium(III) complex has been developed with low-cost solution processing technique. The new OLECT showed good ambipolar behavior with balanced hole and electron mobilities of 0.20 cm2 V–1 s–1 and 0.22 cm2 V–1 s–1, respectively. Furthermore, light emission has been observed from the OLECT device and modulated by the gate. All these results suggest that charged iridium(III) complexes could be a promising candidate for single-component multifunctional organic light-emitting field-effect transistor.
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85.30.Tv Field effect devices
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Highly efficient inverted polymer solar cells with a solution-processable dendrimer as the electron-collection interlayer

Vajjiravel Murugesan, Kuan Sun, and Jianyong Ouyang

Appl. Phys. Lett. 102, 083302 (2013); http://dx.doi.org/10.1063/1.4794065 (4 pages)

Online Publication Date: 27 February 2013

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Poly(amido amine) (PAMAM, generation 2), a solution-processable dendrimer, is used as the electron-collection interlayer on indium tin oxide (ITO) for inverted polymer solar cells (PSCs) with poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester as the active materials. The inverted PSCs exhibited a power conversion efficiency of 3.53% under AM1.5G illumination, which is significantly higher than that of the control inverted PSCs with blank ITO and comparable to that of the control PSCs with normal architecture. The high-performance of the inverted PSCs is attributed to the reduction of the work function of ITO by 0.75 eV by PAMAM.
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88.40.jr Organic photovoltaics
88.40.hj Efficiency and performance of solar cells
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Elastomeric transparent capacitive sensors based on an interpenetrating composite of silver nanowires and polyurethane

Weili Hu, Xiaofan Niu, Ran Zhao, and Qibing Pei

Appl. Phys. Lett. 102, 083303 (2013); http://dx.doi.org/10.1063/1.4794143 (5 pages)

Online Publication Date: 28 February 2013

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Highly flexible transparent capacitive sensors have been demonstrated for the detection of deformation and pressure. The elastomeric sensors employ a pair of compliant electrodes comprising silver nanowire networks embedded in the surface layer of polyurethane matrix, and a highly compliant dielectric spacer sandwiched between the electrodes. The capacitance of the sensor sheets increases linearly with strains up to 60% during uniaxial stretching, and linearly with externally applied transverse pressure from 1 MPa down to 1 kPa. Stretchable sensor arrays consisting of 10 × 10 pixels have also been fabricated by patterning the composite electrodes into X-Y addressable passive matrix.
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07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
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