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Kippelen, Bernard

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End date: 2011 × Author: Kippelen, Bernard × Has files: Yes ×

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Enhanced carrier mobility and electrical stability of n-channel polymer thin film transistors by use of low-k dielectric buffer layer

2011-10 , Kim, Felix Sunjoo , Hwang, Do-Kyung , Kippelen, Bernard , Jenekhe, Samson A.

Insertion of a low-k polymer dielectric layer between the SiO₂ gate dielectric and poly(benzobisimidazobenzophenanthroline) (BBL) semiconductor of n-channel transistors is found to increase the field-effect mobility of electrons from 3.6 × 10⁻⁴ cm²/Vs to as high as 0.028 cm²/Vs. The enhanced carrier mobility was accompanied by improved multicycling stability and durability in ambient air. Studies of a series of eight polymer dielectrics showed that the electron mobility increased exponentially with decreasing dielectric constant, which can be explained to result from the reduced energetic expense of charge-carrier/dipole interaction.

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Metamaterial metal-dielectric photonic bandgap filters with high in-band transmittance and strong out-of-band rejection

2011-05-24 , Kippelen, Bernard

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Focus issue: Thin-film photovoltaic materials and devices introduction

2010-09 , Kippelen, Bernard

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A comprehensive analysis of the contributions to the nonlinear optical properties of thin Ag films

2010-06 , Owens, Daniel T. , Fuentes-Hernandez, Canek , Hales, Joel M. , Perry, Joseph W. , Kippelen, Bernard

The nonlinear optical properties of 20 nm thick Ag films are investigated by time-resolved white- light continuum pump-probe experiments in both transmission and reflection mode. The dynamics of changes in permittivity Δε are measured at wavelengths between 500 to 700 nm. The data is fitted to a modified Drude model in the frequency domain and to a two-temperature model in the time domain. Changes in the individual Drude parameters are calculated as a function of time. A single, coherent model is proposed based on these fittings that describes the dynamics of the nonlinear optical properties of Ag, which could be used to model the nonlinear responses of multilayer structures containing thin films of Ag. The physical origins of the observed responses are discussed.

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Metal-oxide complementary inverters with a vertical geometry fabricated on flexible substrates

2011-10 , Dindar, Amir , Kim, Jungbae , Fuentes-Hernandez, Canek , Kippelen, Bernard

We report on the fabrication of p-channel thin film transistors (TFTs) and vertically stacked complementary inverters comprised of a p-channel copper oxide TFT on top of an n-channel indium gallium zinc oxide TFT fabricated on a flexible polyethersulfone substrate. The p- and n-channel TFTs showed saturation mobility values of 0.0022 and 1.58 cm²/Vs, respectively, yielding inverters with a gain of 120 V/V. This level of performance was achieved by reducing the copper oxide channel thickness, allowing oxygen diffusion into the copper oxide layer at medium processing temperature (150 °C).

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Inverted polymer solar cells with amorphous indium zinc oxide as the electron-collecting electrode

2010-11 , Cheun, Hyeunseok , Kim, Jungbae , Zhou, Yinhua , Fang, Yunnan , Dindar, Amir , Shim, Jaewon , Fuentes-Hernandez, Canek , Sandhage, Kenneth H. , Kippelen, Bernard

We report on the fabrication and performance of polymer-based inverted solar cells utilizing amorphous indium zinc oxide (a-IZO) as the electron-collecting electrode. Amorphous IZO films of 200 nm thickness were deposited by room temperature sputtering in a high-purity argon atmosphere. The films possessed a high optical transmittance in the visible region (≥ 80%), a low resistivity (3.3 × 10⁻⁴ Ωcm), a low surface roughness (root mean square = 0.68 nm), and a low work function (4.46 ± 0.02 eV). Inverted solar cells with the structure a-IZO/P3HT: PCBM/PEDOT:PSS/Ag exhibited a power conversion efficiency of 3% estimated for AM 1.5G, 100 mW/cm² illumination.

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ITO-free large-area organic solar cells

2010-09 , Choi, Seungkeun , Potscavage, William J., Jr. , Kippelen, Bernard

We report on large-area pentacene / C60 organic solar cells in which indium-tin-oxide (ITO) is replaced with a conductive polymer electrode and a 5 μm-thick metal grid is used to reduce resistive power losses. The performance of cells with the polymer electrode was compared with that of pentacene / C₆₀ devices using ITO as the transparent electrode. For large-area devices (7.3 cm²) on glass substrates with an integrated metal grid, the performance of a device with the polymer electrode is comparable to that of a device with an ITO electrode combined with a grid.

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ITO-free large-area organic light-emitting diodes with an integrated metal grid

2011-07 , Choi, Seungkeun , Kim, Sung-Jin , Fuentes-Hernandez, Canek , Kippelen, Bernard

We report on ITO-free large-area organic light-emitting diodes (OLEDs) fabricated on glass substrates comprising α-NPD as a hole transport layer (HTL) and coevaporated CBP:Ir(ppy)3 as the emission layer. Indium-tin-oxide (ITO) was replaced with a conductive polymer electrode and an electroplated thick metal grid was used to improve the homogeneity of the potential distribution over the transparent polymer electrode. An electrical model of a metal grid integrated OLED shows the benefits of the use of metal grids in terms of improving the uniformity of the light emitted as the area of the OLED increases as well as the conductivity of the transparent electrode decreases. By integrating metal grids with polymer electrodes, the luminance increases more than 24% at 6 V and 45% at 7 V compared to the polymer electrode devices without a metal grid. This implies that a lower voltage can be applied to achieve the same luminance, hence lowering the power consumption. Furthermore, metal grid integrated OLEDs exhibited less variation in light emission compared to devices without a metal grid.

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Indium tin oxide-free and metal-free semitransparent organic solar cells

2010-10 , Zhou, Yinhua , Cheun, Hyeunseok , Choi, Seungkeun , Potscavage, William J., Jr. , Fuentes-Hernandez, Canek , Kippelen, Bernard

We report on indium tin oxide (ITO)-free and metal-free semitransparent organic solar cells with a high-conductivity poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (PH1000) as both the bottom and the top electrodes. The PH1000 film showed a conductivity of 680±50 S/cm. A ZnO layer was used as an interlayer to produce an electron-selective electrode. The semitransparent devices with a structure of glass/PH1000/ZnO/poly(3-hexylthiophene):phenyl-C ₆₁-butyric acid methyl ester/PEDOT:PSS (CPP 105 D)/PH1000 exhibited an average power conversion efficiency of 1.8% estimated for 100 mW/cm² air mass 1.5 global illumination. This geometry alleviates the need of vacuum deposition of a top electrode.

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Nonlinear optical properties of induced transmission filters

2010-08 , Owens, Daniel T. , Fuentes-Hernandez, Canek , Hales, Joel M. , Perry, Joseph W. , Kippelen, Bernard

The nonlinear optical (NLO) properties of induced transmission filters (ITFs) based on Ag are experimentally determined using white light continuum pump-probe measurements. The experimental results are supported using simulations based on the matrix transfer method. The magnitude of the NLO response is shown to be 30 times that of an isolated Ag film of comparable thickness. The impacts of design variations on the linear and NLO response are simulated. It is shown that the design can be modified to enhance the NLO response of an ITF by a factor of 2 or more over a perfectly matched ITF structure.

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