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Kippelen,
Bernard
Kippelen,
Bernard
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ItemMetal-oxide complementary inverters with a vertical geometry fabricated on flexible substrates(Georgia Institute of Technology, 2011-10) Dindar, Amir ; Kim, Jungbae ; Fuentes-Hernandez, Canek ; Kippelen, BernardWe 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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ItemITO-free large-area organic light-emitting diodes with an integrated metal grid(Georgia Institute of Technology, 2011-07) Choi, Seungkeun ; Kim, Sung-Jin ; Fuentes-Hernandez, Canek ; Kippelen, BernardWe 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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ItemInverted polymer solar cells with amorphous indium zinc oxide as the electron-collecting electrode(Georgia Institute of Technology, 2010-11) Cheun, Hyeunseok ; Kim, Jungbae ; Zhou, Yinhua ; Fang, Yunnan ; Dindar, Amir ; Shim, Jaewon ; Fuentes-Hernandez, Canek ; Sandhage, Kenneth H. ; Kippelen, BernardWe 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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ItemIndium tin oxide-free and metal-free semitransparent organic solar cells(Georgia Institute of Technology, 2010-10) Zhou, Yinhua ; Cheun, Hyeunseok ; Choi, Seungkeun ; Potscavage, William J., Jr. ; Fuentes-Hernandez, Canek ; Kippelen, BernardWe 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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ItemNonlinear optical properties of induced transmission filters(Georgia Institute of Technology, 2010-08) Owens, Daniel T. ; Fuentes-Hernandez, Canek ; Hales, Joel M. ; Perry, Joseph W. ; Kippelen, BernardThe 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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ItemA comprehensive analysis of the contributions to the nonlinear optical properties of thin Ag films(Georgia Institute of Technology, 2010-06) Owens, Daniel T. ; Fuentes-Hernandez, Canek ; Hales, Joel M. ; Perry, Joseph W. ; Kippelen, BernardThe 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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ItemLow-voltage InGaZnO thin-film transistors with Al₂O₃ gate grown by atomic layer deposition(Georgia Institute of Technology, 2009-06) Kim, Jungbae ; Fuentes-Hernandez, Canek ; Potscavage, William J., Jr. ; Zhang, Xiaohong ; Kippelen, BernardWe report on low-voltage, high-performance amorphous indium gallium zinc oxide n-channel thin-film transistors fabricated using 100-nm-thick Al₂O₃ grown by atomic layer deposition as the gate dielectric layer. The Al₂O₃ gate dielectric shows very small current densities and has a capacitance density of 81±1 nFcm². Due to a very small contact resistance, transistors with channel lengths ranging from 100 μm down to 5 μm yield a channel-independent, field-effect mobility of 8±1 cm² V s, subthreshold slopes of 0.1±0.01 Vdecade, low threshold voltages of 0.4±0.1 V, and high on-off current ratios up to 6 x10⁷ (WL=4005 μm) at 5 V.
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ItemHigh-performance InGaZnO thin-film transistors with high-k amorphous Ba₀.₅Sr₀.TiO₃gate insulator(Georgia Institute of Technology, 2008-12) Kim, Jungbae ; Fuentes-Hernandez, Canek ; Kippelen, BernardWe report on high-performance n-channel thin-film transistors (TFTs) fabricated using amorphous indium gallium zinc oxide (a-IGZO) and amorphous Ba₀.₅Sr₀.₅TiO₃ (α-BST) as the channel and gate dielectric layers, respectively. a-BST/a-IGZO TFTs achieve low-voltage operation with a high saturation mobility value of 10±1 cm²/V s, excellent subthreshold slopes of 0.06±0.01 V/decade, a low threshold voltage of 0.5±0.1 V, and a high on-off current ratio up to 8×10⁷ (W/L = 1000 μm/5 μm) at 3 V. The high capacitance density of a-BST (145±2 nF/cm²) and the small contact resistance, smaller than the channel resistance, are responsible for the high performance of these TFTs.
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ItemUltrafast optical image processing based on third-harmonic generation in organic thin films(Georgia Institute of Technology, 2007-09-24) Fuentes-Hernandez, Canek ; Tseng, Shuo-Yen ; Owens, Daniel T. ; Kippelen, BernardWe report on the use of the noncollinear third-harmonic generation in an amorphous polymer film operating in the eye safe and telecommunication compatible near-infrared range to perform ultrafast all-optical two-dimensional (2D) image processing at 1550 nm using 100 fs pulses. The background-free and nondegenerate outputs at 517 nm are easily spatially filtered and detected with low cost electronic components. We describe this Fourier transform based technique and demonstrate its application to the classical problem of 2D image recognition.
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ItemCompact and self-aligned all-optical image correlator based on third-harmonic generation(Georgia Institute of Technology, 2007-09) Tseng, Shuo-Yen ; Fuentes-Hernandez, Canek ; Kippelen, BernardWe demonstrate a compact optical correlator using a diffractive optical element (DOE) beam splitter for 2D optical image processing. Image frequency conversion and correlation are demonstrated using third-harmonic generation (THG) in an organic film with a 1550nm femtosecond laser. Spatial and temporal alignment of the femtosecond pulses are obtained by imaging the DOE onto the organic film.