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28 Jan 2013

Volume 102, Issue 4, Articles (04xxxx)

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Appl. Phys. Lett. 102, 041101 (2013); http://dx.doi.org/10.1063/1.4777564 (5 pages)

K. Winkler, C. Schneider, J. Fischer, A. Rahimi-Iman, M. Amthor, A. Forchel, S. Reitzenstein, S. Höfling, and M. Kamp
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Improved cathode buffer layer to decrease exciton recombination in organic planar heterojunction solar cells

Bregt Verreet, Pawel E. Malinowski, Bjoern Niesen, David Cheyns, Paul Heremans, Andre Stesmans, and Barry P. Rand

Appl. Phys. Lett. 102, 043301 (2013); http://dx.doi.org/10.1063/1.4789852 (5 pages)

Online Publication Date: 29 January 2013

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We show that an advanced cathode buffer design, consisting of bathocuproine/3,4,9,10-perylenetetracarboxylic bis-benzimidazole/Ag, increases the short-circuit current of organic planar heterojunction cells and reduces the J-V slope at reverse voltages. We study the physical origin of these effects by measuring reflectivity, voltage dependent external quantum efficiency, and voltage dependent photoluminescence. Our findings suggest that the observed effects are mainly associated with a voltage dependent polaron-induced exciton quenching in the C60 layer. Finally, this improved cathode buffer design is applied to a diindeno[1,2,3-cd:1′,2′,3′-lm]perylene/C70 based cell, leading to a considerable planar heterojunction efficiency of 5.7%.
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88.40.jr Organic photovoltaics
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Photoelectron spectroscopic study of band alignment of polymer/ZnO photovoltaic device structure

T. Nagata, S. Oh, Y. Yamashita, H. Yoshikawa, N. Ikeno, K. Kobayashi, T. Chikyow, and Y. Wakayama

Appl. Phys. Lett. 102, 043302 (2013); http://dx.doi.org/10.1063/1.4790298 (4 pages) | Cited 1 time

Online Publication Date: 30 January 2013

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Using x-ray photoelectron spectroscopy, we investigated the band alignment of a Ag/poly(3-hexylthiophene-2,5-diyl) (P3HT)/ZnO photovoltaic structure. At the P3HT/ZnO interface, a band bending of P3HT and a short surface depletion layer of ZnO were observed. The offset between the highest occupied molecular orbital of P3HT and the conduction band minimum of ZnO at the interface contributed to the open circuit voltage (Voc) was estimated to be approximately 1.5 ± 0.1 eV, which was bigger than that of the electrically measured effective Voc of P3HT/ZnO photovoltaic devices, meaning that the P3HT/ZnO photovoltaic structure has the potential to provide improved photovoltaic properties.
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79.60.Jv Interfaces; heterostructures; nanostructures
81.40.Lm Deformation, plasticity, and creep
82.80.Pv Electron spectroscopy (X-ray photoelectron (XPS), Auger electron spectroscopy (AES), etc.)
62.20.F- Deformation and plasticity
73.20.At Surface states, band structure, electron density of states
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A plasmonically enhanced charge generation layer for tandem organic light emitting device

Fei Yan and Xiao Wei Sun

Appl. Phys. Lett. 102, 043303 (2013); http://dx.doi.org/10.1063/1.4789979 (4 pages) | Cited 1 time

Online Publication Date: 30 January 2013

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We studied silver nanoparticles (Ag NPs) as an n-type dopant in the charge generation connector of tandem organic light-emitting diodes. Due to the localized surface plasmonic resonance and quantum size effects, Ag NPs are more active and easy to lose electron comparing to bulk or film forms of Ag, and can work as excellent n-type dopant. For the tandem device with Ag NPs in the charge generation layer, the current efficiency is more than doubled and the driving voltage is less than twice compared to one single emission unit device.
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85.60.Jb Light-emitting devices
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Efficient organic solar cells with solution-processed carbon nanosheets as transparent electrodes

Seok-In Na, Yong-Jin Noh, Su-Young Son, Tae-Wook Kim, Seok-Soon Kim, Sungho Lee, and Han-Ik Joh

Appl. Phys. Lett. 102, 043304 (2013); http://dx.doi.org/10.1063/1.4789980 (5 pages)

Online Publication Date: 30 January 2013

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We demonstrate that solution-processed carbon nanosheet (CNS) films can efficiently serve as transparent electrodes for organic solar cells (OSCs). The CNS was obtained by spin-coating of polyacrylonitrile (PAN) dissolved in dimethylformamide on quartz substrates, followed by stabilization and carbonization processes to convert polymer into CNS. The thickness of the newly developed CNS films was easily controlled by varying the PAN solution concentration. The polymer-converted CNS films were intensively examined for the feasibility of the use as transparent anodes in solar cells. This approach could be highly desirable for all-solution-processed or printed OSCs.
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88.40.jr Organic photovoltaics
81.15.Lm Liquid phase epitaxy; deposition from liquid phases (melts, solutions, and surface layers on liquids)
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Kinetics of light induced defect creation in organic solar cells

R. A. Street and D. M. Davies

Appl. Phys. Lett. 102, 043305 (2013); http://dx.doi.org/10.1063/1.4789993 (3 pages)

Online Publication Date: 30 January 2013

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The kinetics of light-induced recombination centers in bulk heterojunction organic solar cells are measured as a function of exposure time, intensity, and the illumination photon energy. The density of induced centers increases with exposure but stabilizes partially due to self-annealing. UV exposure is roughly 50 times more effective for defect creation than white light or yellow-filtered white light. Light-induced breaking of C-H bonds to create H-related localized states is proposed as the underlying mechanism.
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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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Enhancing crystallinity of C60 layer by thickness-control of underneath pentacene layer for high mobility C60/pentacene ambipolar transistors

Kwangseok Ahn, Jong Beom Kim, Hyunjun Park, Hyunjung Kim, Moo Hyung Lee, Beom Joon Kim, Jeong Ho Cho, Moon Sung Kang, and Dong Ryeol Lee

Appl. Phys. Lett. 102, 043306 (2013); http://dx.doi.org/10.1063/1.4789873 (5 pages)

Online Publication Date: 31 January 2013

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We present systematic control of the crystallinity and electrical transport properties of C60 films that are deposited onto pentacene layers, based on simple tuning of the underneath pentacene layer thickness. With increasing the pentacene layer thickness from 0 to 2 monolayers, we observed improvement in crystallinity and grain size of the C60 layer, which led to dramatic enhancement in electron conduction. Also, hole transport in this bilayer structure could be generated when the thickness of the pentacene layer was above one monolayer. The resulting ambipolar transport thin-film transistors yielded electron and hole mobilities as high as 2.8 and 0.3 cm2 V−1 s−1, respectively, and complementary inverters with gain value above 20.
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81.05.Fb Organic semiconductors
73.61.Cw Elemental semiconductors
73.61.Ph Polymers; organic compounds
73.61.Wp Fullerenes and related materials
85.30.Tv Field effect devices
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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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