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14 Jun 2010

Volume 96, Issue 24, Articles (24xxxx)

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Appl. Phys. Lett. 96, 241101 (2010); http://dx.doi.org/10.1063/1.3449576 (3 pages)

Rui Chen, H. D. Sun, T. Wang, K. N. Hui, and H. W. Choi
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Electrical instability of polymer thin-film transistors using contact film transfer methods

Qingshuo Wei, Keisuke Tajima, and Kazuhito Hashimoto

Appl. Phys. Lett. 96, 243301 (2010); http://dx.doi.org/10.1063/1.3454242 (3 pages) | Cited 7 times

Online Publication Date: 14 June 2010

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The effects of molecular packing on electrical instability in poly(3-hexylthiophene) thin-film transistors were investigated by comparing two devices prepared using the same polymer solution and dielectric layer but by different processes. One device, which was prepared by a contact film transfer method by Wei et al. [ACS Appl. Mater. Interfaces 1, 2660 (2009)] , utilized the structure formed at the air/solution interface and exhibited high performance, the absence of hysteresis and excellent electrical stability under prolonged continuous bias stress, in contrast to the other device prepared by conventional spin coating, which utilized structures formed at the solution/dielectric interface. Neither thermal nor solvent annealing was applied to control the molecular packing, and thus these results indicate that the highly ordered molecular packing formed at the air/solution interface can simultaneously contribute to enhanced electrical stability.
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85.30.Tv Field effect devices
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
68.55.am Polymers and organics
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Enhancing outcoupling efficiency of indium-tin-oxide-free organic light-emitting diodes via nanostructured high index layers

Boris Riedel, Julian Hauss, Ulf Geyer, Johanna Guetlein, Uli Lemmer, and Martina Gerken

Appl. Phys. Lett. 96, 243302 (2010); http://dx.doi.org/10.1063/1.3455098 (3 pages) | Cited 10 times

Online Publication Date: 15 June 2010

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We fabricated organic light-emitting diodes with one-dimensional Bragg gratings as light extraction elements for substrate and waveguide modes. A Ta2O5 layer was introduced to obtain a high refractive index contrast to the subsequent anode layer. As anode we employed a highly conductive polymer. Laser interference lithography and physical plasma etching were used to pattern gratings into the Ta2O5 layer with a lattice constant of 370 nm and various grating depths. Mainly attributed to the outcoupling of the substrate modes, the structured devices exhibit a luminous flux which is up to four times higher compared to the unstructured reference devices.
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85.60.Jb Light-emitting devices
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Optoelectronic conversion by polarization current, triggered by space charges at organic-based interfaces

Laigui Hu, Yukiko Noda, Hiroshi Ito, Hideo Kishida, Arao Nakamura, and Kunio Awaga

Appl. Phys. Lett. 96, 243303 (2010); http://dx.doi.org/10.1063/1.3454915 (3 pages) | Cited 3 times

Online Publication Date: 16 June 2010

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We report that a highly efficient optoelectronic conversion can be achieved by photogenerated space charges, which usually damp photocurrent. Theoretical analysis of metal-organic insulator-organic semiconductor-metal photocells indicates the generation of a large transient current that is triggered by photogenerated space charges and governed by the dielectric properties of the insulator layer. We experimentally demonstrated this mechanism with model photocells, revealing that the quantum efficiency can be dramatically increased by increasing the dielectric constant of the insulator.
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84.60.Jt Photoelectric conversion
79.60.-i Photoemission and photoelectron spectra
77.22.Ej Polarization and depolarization
52.80.-s Electric discharges
77.22.Jp Dielectric breakdown and space-charge effects
77.22.Ch Permittivity (dielectric function)
78.56.-a Photoconduction and photovoltaic effects
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Poly(3-hexylthiophene) crystalline nanoribbon network for organic field effect transistors

M. Arif, Jianhua Liu, Lei Zhai, and Saiful I. Khondaker

Appl. Phys. Lett. 96, 243304 (2010); http://dx.doi.org/10.1063/1.3455097 (3 pages) | Cited 8 times

Online Publication Date: 16 June 2010

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We report on the fabrication of crystalline nanoribbon network field effect transistors (FETs) using low molecular weight (MW) poly(3-hexylthiophene) (P3HT) with different surface treatments and compare with thin film FETs cast from the same MW regioregular P3HT. Nanoribbon FET shows improved performance with a maximum mobility of 0.012 cm2/V s and current on/off ratios of 6.5×104 due to unique crystalline structure and morphology. With various surface treatments, the nanoribbon FETs show less variation in device mobilities, while thin film FETs show more than ten times variation in device mobilities and up to 100 times change in current on/off ratios.
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85.35.-p Nanoelectronic devices
85.30.Tv Field effect devices
81.65.-b Surface treatments
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Transient characteristics of inverted polymer solar cells using titaniumoxide interlayers

H. Schmidt, K. Zilberberg, S. Schmale, H. Flügge, T. Riedl, and W. Kowalsky

Appl. Phys. Lett. 96, 243305 (2010); http://dx.doi.org/10.1063/1.3455108 (3 pages) | Cited 19 times

Online Publication Date: 16 June 2010

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Organic bulk heterojunction solar cells using titania interlayers as electron selective layers prepared by atomic layer deposition or wet processing are reported. Pristine devices show low filling factors (FFs) and consequently low efficiencies. Upon illumination with ultraviolet (UV) light, a significant increase in the FF is found. We study the impact of various ambient conditions (air, vacuum, and oxygen) on the dynamics of the decay of the FF after UV illumination. The interaction of oxygen and titania is evidenced as the dominant mechanism for the transient behavior of the polymer solar cells.
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88.40.H- Solar cells (photovoltaics)
78.40.-q Absorption and reflection spectra: visible and ultraviolet
68.43.Nr Desorption kinetics
68.43.Mn Adsorption kinetics
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Efficient hybrid organic-inorganic light emitting diodes with self-assembled dipole molecule deposited metal oxides

Ji Sun Park, Bo Ram Lee, Ju Min Lee, Ji-Seon Kim, Sang Ouk Kim, and Myoung Hoon Song

Appl. Phys. Lett. 96, 243306 (2010); http://dx.doi.org/10.1063/1.3453759 (3 pages) | Cited 18 times

Online Publication Date: 16 June 2010

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We investigate the effect of self-assembled dipole molecules (SADMs) on ZnO surface in hybrid organic-inorganic polymeric light-emitting diodes (HyPLEDs). Despite the SADM being extremely thin, the magnitude and orientation of SADM dipole moment effectively influenced the work function of the ZnO. As a consequence, the charge injection barrier between the conduction band of the ZnO and the lowest unoccupied molecular orbital of poly(9,9′-dioctylfluorene)-co-benzothiadiazole could be efficiently controlled resulting that electron injection efficiency is remarkably enhanced. The HyPLEDs modified with a negative dipolar SADM exhibited enhanced device performances, which correspond to approximately a fourfold compared to those of unmodified HyPLEDs.
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85.60.Jb Light-emitting devices
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Energy level evolution of air and oxygen exposed molybdenum trioxide films

Irfan, Huanjun Ding, Yongli Gao, Cephas Small, Do Young Kim, Jegadesan Subbiah, and Franky So

Appl. Phys. Lett. 96, 243307 (2010); http://dx.doi.org/10.1063/1.3454779 (3 pages) | Cited 32 times

Online Publication Date: 16 June 2010

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The evolution of electronic energy levels of controlled air and oxygen exposed molybdenum trioxide (MoO3) films has been investigated with ultraviolet photoemission spectroscopy, inverse photoemission spectroscopy, and x-ray photoemission spectroscopy. We found that while most of the electronic levels of as deposited MoO3 films remained largely intact, the reduction in the work function (WF) was substantial. The gradual surface WF change from 6.8 to 5.3 eV was observed for air exposed film, while oxygen exposed film the surface WF saturated at ∼ 5.7 eV. Two distinct stages of exposure are observed, the first dominated by oxygen adsorption for <1013 Langmuir (L) exposure and at the final step moisture absorption >1013 L.
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73.20.Hb Impurity and defect levels; energy states of adsorbed species
65.40.gh Work functions
68.43.Mn Adsorption kinetics
79.60.Dp Adsorbed layers and thin films
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The origin of low water vapor transmission rates through Al2O3/ZrO2 nanolaminate gas-diffusion barriers grown by atomic layer deposition

J. Meyer, H. Schmidt, W. Kowalsky, T. Riedl, and A. Kahn

Appl. Phys. Lett. 96, 243308 (2010); http://dx.doi.org/10.1063/1.3455324 (3 pages) | Cited 8 times

Online Publication Date: 17 June 2010

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This paper reports on thin film gas-diffusion barriers consisting of Al2O3/ZrO2 nanolaminates (NL) grown by low-temperature (80 °C) atomic layer deposition. We show that reliable barriers with water vapor transmission rates of 3.2×10−4 g/(m2 day), measured at 80 °C and 80% relative humidity, can be realized with very thin layers down to 40 nm. We determine that ZrO2 acts as anticorrosion element in our NL. Furthermore, we demonstrate by x-ray photoemission spectroscopy that an aluminate phase is formed at the interfaces between Al2O3 and ZrO2 sublayers, which additionally improves the gas-diffusion barrier due to a densification of the layer system. These Al2O3/ZrO2 NLs prepared at low temperatures hold considerable promises for application in organic electronics and beyond.
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68.35.Fx Diffusion; interface formation
68.55.at Other materials
81.20.Ev Powder processing: powder metallurgy, compaction, sintering, mechanical alloying, and granulation
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
79.60.Jv Interfaces; heterostructures; nanostructures
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Carbon nanotube network electrodes enabling efficient organic solar cells without a hole transport layer

Teresa M. Barnes, Jeremy D. Bergeson, Robert C. Tenent, Brian A. Larsen, Glenn Teeter, Kim M. Jones, Jeffrey L. Blackburn, and Jao van de Lagemaat

Appl. Phys. Lett. 96, 243309 (2010); http://dx.doi.org/10.1063/1.3453445 (3 pages) | Cited 24 times

Online Publication Date: 17 June 2010

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We report on the effects of replacing both In2O3:Sn (ITO) and the hole transport layer (HTL) in organic photovoltaic (OPV) cells with single-walled carbon nanotube (SWNT) network transparent electrodes. We have produced an OPV device without an HTL exhibiting an NREL-certified efficiency of 2.65% and a short-circuit current density of 11.2 mA/cm2. Our results demonstrate that SWNT networks can be used to replace both ITO and the HTL in efficient OPV devices and that the HTL serves distinctly different roles in ITO- and SWNT-based devices.
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85.35.Kt Nanotube devices
88.40.H- Solar cells (photovoltaics)
84.60.Jt Photoelectric conversion
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Energy barrier, charge carrier balance, and performance improvement in organic light-emitting diodes

Amare Benor, Shin-ya Takizawa, C. Pérez-Bolivar, and Pavel Anzenbacher, Jr.

Appl. Phys. Lett. 96, 243310 (2010); http://dx.doi.org/10.1063/1.3452344 (3 pages) | Cited 5 times

Online Publication Date: 18 June 2010

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The charge injection properties of poly(3,4-ethylenedioxythiophene):polystyrene sulfonate anodes are crucial for performance of organic photovoltaics and organic light-emitting diodes (OLEDs). A simple method for tuning hole injection efficiency using UV-ozone is shown to change anode work-function and optimized carriers balance in the devices and improved efficiency in OLEDs. The optimum time of treatment and work-function differs with device architecture.
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85.60.Jb Light-emitting devices
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Highly efficient electro-optic polymers through improved poling using a thin TiO2-modified transparent electrode

Su Huang, Tae-Dong Kim, Jingdong Luo, Steven K. Hau, Zhengwei Shi, Xing-Hua Zhou, Hin-Lap Yip, and Alex K.-Y. Jen

Appl. Phys. Lett. 96, 243311 (2010); http://dx.doi.org/10.1063/1.3453659 (3 pages) | Cited 7 times

Online Publication Date: 18 June 2010

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A sol-gel derived thin titanium dioxide (TiO2) layer was spin-coated onto indium-tin-oxide substrate to improve poling efficiency of recently developed electro-optic (E-O) polymers. The thin TiO2 layer significantly blocks excessive charge injection and reduces the leakage current during high field poling. Ultralarge E-O coefficients, up to 160–350 pm/V at 1310 nm, have been achieved. These results show higher poling efficiency (enhancement of 26%–40%) compared to the results of poled films without the TiO2 layer. This enhancement can be explained by field distribution flattening effect at high injection barrier with the insertion of TiO2 barrier layer.
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78.66.Qn Polymers; organic compounds
77.22.Ej Polarization and depolarization
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
42.70.Jk Polymers and organics
77.84.Bw Elements, oxides, nitrides, borides, carbides, chalcogenides, etc.
78.20.Jq Electro-optical effects
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