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12 Apr 2010

Volume 96, Issue 15, Articles (15xxxx)

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Appl. Phys. Lett. 96, 153701 (2010); http://dx.doi.org/10.1063/1.3385388 (3 pages)

Sinan Keten and Markus J. Buehler
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Polarization-independent multiple selective reflections from bichiral liquid crystal films

Na Young Ha, Soon Moon Jeong, Suzushi Nishimura, and Hideo Takezoe

Appl. Phys. Lett. 96, 153301 (2010); http://dx.doi.org/10.1063/1.3393996 (3 pages) | Cited 3 times

Online Publication Date: 12 April 2010

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We have demonstrated polarization-independent multiple reflections covering overall visible region from a bichiral liquid crystal (LC) film with both chiral structures. The bichiral LC film was fabricated by an all-solution process using left-handed and right-handed polymeric cholesteric liquid crystal (CLC) films with a single pitch and an isotropic polymer film. The multiple reflections exceeding 50% reflectance were clearly observed at various polarizations of normally incident light. This characteristic can extend practical applications of CLCs to polarization-independent devices over broad-band regions.
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61.30.Vx Polymer liquid crystals
78.40.Dw Liquids
42.70.Df Liquid crystals
78.66.Qn Polymers; organic compounds
78.15.+e Optical properties of fluid materials, supercritical fluids and liquid crystals
68.15.+e Liquid thin films
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Dual carrier traps related hysteresis in organic inverters with polyimide-modified gate-dielectrics

Wei-Yang Chou and Bo-Liang Yeh

Appl. Phys. Lett. 96, 153302 (2010); http://dx.doi.org/10.1063/1.3395389 (3 pages) | Cited 2 times

Online Publication Date: 13 April 2010

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We integrated pentacene- and NN′-diheptyl-3,4,9,10-perylenebiscarboximide-based transistors into an organic complementary metal oxide semiconductor (O-CMOS) whose gate-dielectric surface was modified by polyimide (PI). The hysteresis behaviors in metal-oxide-semiconductors, field-effect transistors, and O-CMOS were reported clearly. Measurements of hysteresis showed that the PI exhibited high trapping and detrapping speeds for charge carriers, including holes and electrons, to result in high performance transistors and O-CMOSs; moreover, the trapping and detrapping speeds were matched. Finally, a PI-modified organic inverter with little hysteresis, low static power dissipation, high noise margins, and switching voltage near VDD/2 was achieved simultaneously.
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85.30.Tv Field effect devices
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths
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Reversible tuning of photonic crystal cavities using photochromic thin films

Deepak Sridharan, Edo Waks, Glenn Solomon, and John T. Fourkas

Appl. Phys. Lett. 96, 153303 (2010); http://dx.doi.org/10.1063/1.3377910 (3 pages) | Cited 9 times

Online Publication Date: 14 April 2010

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We demonstrate reversible tuning of a photonic crystal cavity resonance using a thin photochromic film composed of spiropyran and polymethylmethacrylate that serves as a photosensitive cladding layer. Exposure of spiropyran to ultraviolet light results in smooth redshift of the cavity resonance that can be reversed by exposure to visible wavelength light. We achieve a reversible resonance shift of up to 2.7 nm, which can be performed locally on individual cavities. The resonance shift over multiple successive UV and visible light exposures is studied to determine the repeatability of the photochromic film.
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42.82.Gw Other integrated-optical elements and systems
42.65.-k Nonlinear optics
82.50.-m Photochemistry
42.79.Wc Optical coatings
42.70.Qs Photonic bandgap materials
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Ultimate quality factor of silica microtoroid resonant cavities

Xiaomin Zhang, Hong Seok Choi, and Andrea M. Armani

Appl. Phys. Lett. 96, 153304 (2010); http://dx.doi.org/10.1063/1.3398008 (3 pages) | Cited 8 times

Online Publication Date: 15 April 2010

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Silica optical microcavities with quality (Q factors above 1×108 have applications throughout science and engineering. While both the microtoroid and microsphere resonant cavity have demonstrated Q>1×108, only the microsphere has surpassed 1×109. Surprisingly, the reason for this performance disparity is directly related to type of silicon substrate used in the fabrication process. In the present work, the theoretical Q of planar toroidal silica resonant cavities is calculated and compared to experimental results from a series of devices fabricated from oxide on doped silicon wafers. As predicted, the Q depends on the substrate dopant concentration.
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42.25.Fx Diffraction and scattering
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Improved light extraction efficiency of white organic light-emitting devices by biomimetic antireflective surfaces

Yunfeng Li, Feng Li, Junhu Zhang, Chunlei Wang, Shoujun Zhu, Huijun Yu, Zhanhua Wang, and Bai Yang

Appl. Phys. Lett. 96, 153305 (2010); http://dx.doi.org/10.1063/1.3396980 (3 pages) | Cited 19 times

Online Publication Date: 16 April 2010

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One simple method to increase the light extraction from white organic light-emitting devices by using biomimetic silica antireflective surfaces is demonstrated. A silica cone array was directly etched on the opposite side of the indium–tin–oxide coated fused silica substrate. The antireflective surfaces can dramatically suppress the reflection loss and increase the transmission of light over a large range of wavelength and a large field of view. Using such surfaces, the luminance efficiency of the device in the normal direction is increased by a factor of 1.4 compared to that of the device using flat silica substrate.
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85.60.Jb Light-emitting devices
42.79.Wc Optical coatings
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Improvement of electron injection in inverted bottom-emission blue phosphorescent organic light emitting diodes using zinc oxide nanoparticles

Hyunkoo Lee, Insun Park, Jeonghun Kwak, Do Y. Yoon, and Changhee Lee

Appl. Phys. Lett. 96, 153306 (2010); http://dx.doi.org/10.1063/1.3400224 (3 pages) | Cited 17 times

Online Publication Date: 16 April 2010

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We fabricated highly efficient iridium(III) bis[(4,6-di-fluorophenyl)-pyridinato-N,C2′] picolinate doped inverted bottom-emission blue phosphorescent organic light-emitting diodes, with an electron injection layer of zinc oxide (ZnO) nanoparticles (NPs). The ZnO NPs layer lowers the turn-on voltage by about 4 V and significantly enhances the efficiency. The device with ZnO NPs shows peak efficiencies of 16.5 cd/A and 8.2%, about three times higher than those of the device without ZnO NPs. Since the ZnO NPs layer has a wide band gap, good electron transporting properties and low work function, it can be utilized as an effective electron injection layer with good transparency.
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85.60.Jb Light-emitting devices
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Embedded indium-tin-oxide nanoelectrodes for efficiency and lifetime enhancement of polymer-based solar cells

Peichen Yu, Chia-Hua Chang, Ming-Shin Su, Min-Hsiang Hsu, and Kung-Hwa Wei

Appl. Phys. Lett. 96, 153307 (2010); http://dx.doi.org/10.1063/1.3395395 (3 pages) | Cited 11 times

Online Publication Date: 16 April 2010

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In this paper, distinctive indium-tin-oxide (ITO) nanorods are employed to serve as buried electrodes for polymer-based solar cells. The embedded nanoelectrodes allow three-dimensional conducting pathways for low-mobility holes, offering a highly scaffolded cell architecture in addition to bulk heterojunctions. As a result, the power conversion efficiency of a polymer cell with ITO nanoelectrodes is increased to about 3.4% and 4.4% under one-sun and five-sun illumination conditions, respectively, representing an enhancement factor of up to ∼ 10% and 36% compared to a conventional counterpart. Also, the corresponding device lifetime is prolonged twice as much to about 110 min under five-sun illumination.
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88.40.jr Organic photovoltaics
82.45.Fk Electrodes
73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
81.16.Be Chemical synthesis methods
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