Person:
Kippelen, Bernard

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Publication Search Results

Now showing 1 - 10 of 11
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    Top-gate hybrid complementary inverters using pentacene and amorphous InGaZnO thin-film transistors with high operational stability
    (Georgia Institute of Technology, 2012-03) Kim, Jungbae ; Fuentes-Hernandez, Canek ; Hwang, D. K. ; Potscavage, William J., Jr. ; Kippelen, Bernard
    We report on the operational stability of low-voltage hybrid organic-inorganic complementary inverters with a top-gate bottom source-drain geometry. The inverters are comprised of p-channel pentacene and n-channel amorphous InGaZnO thin-film transistors (TFTs) with bi-layer gate dielectrics formed from an amorphous layer of a fluoropolymer (CYTOP) and a high-k layer of Al₂O₃. The p- and n- channel TFTs show saturationmobility values of 0.1±0.01 and 5.0±0.5 cm²/Vs, respectively. The individual transistors show high electrical stability with less than 6% drain-to-source current variations after 1 h direct current (DC) bias stress. Complementary inverters yield hysteresis-free voltage transfer characteristics for forward and reverse input biases with static DC gain values larger than 45 V/V at 8 V before and after being subjected to different conditions of electrical stress. Small and reversible variations of the switching threshold voltage of the inverters during these stress tests are compatible with the observed stability of the individual TFTs.
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    Indium 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, 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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    ITO-free large-area organic solar cells
    (Georgia Institute of Technology, 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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    Low-voltage solution-processed n-channel organic field-effect transistors with high-k HfO₂ gate dielectrics grown by atomic layer deposition
    (Georgia Institute of Technology, 2009-12) Tiwari, Shree Prakash ; Zhang, Xiaohong ; Potscavage, William J., Jr. ; Kippelen, Bernard
    High performance solution-processed n-channel organic field-effect transistors based on [6,6]-phenyl C61 butyric acid methyl ester with low operating voltages (3 V) are demonstrated using a high-k hafnium dioxide gate dielectric grown by atomic layer deposition. Devices exhibit excellent n-channel performance with electron mobility values up to 0.14 cm²/V s, threshold voltages of ∼ 0.3 V, current on/off ratios >10⁵, and very low values of subthreshold slope ( ∼ 140 mV/decade).
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    Area-scaling of organic solar cells
    (Georgia Institute of Technology, 2009-09) Choi, Seungkeun ; Potscavage, William J., Jr. ; Kippelen, Bernard
    We report on the performance of organic solar cells based on pentacene/ C₆₀ heterojunctions as a function of active area. Devices with areas of 0.13 and 7 cm² were fabricated on indium-tin-oxide (ITO) coated glass. Degradation of the performance with increased area is observed and analyzed in terms of the power loss density concept. The various power loss contributions to the total series resistance (RSA) are measured independently and compared to the values of the series resistance extracted from the current-voltage characteristics using a Shockley equivalent circuit model. The limited sheet resistance of ITO is found to be one of the major limiting factors when the area of the cell is increased. To reduce the effects of series resistance, thick, electroplated, metal grid electrodes were integrated with ITO in large-area cells. The metal grids were fabricated directly onto ITO and passivated with an insulator to prevent electrical shorts during the deposition of the top Al electrode. By integrating metal grids onto ITO, the series resistance could be reduced significantly yielding improved performance. Design guidelines for metal grids are described and tradeoffs are discussed.
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    Low-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, Bernard
    We 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 nFcm². 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 Vdecade, low threshold voltages of 0.4±0.1 V, and high on-off current ratios up to 6 x10⁷ (WL=4005 μm) at 5 V.
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    Low-voltage flexible organic complementary inverters with high noise margin and high dc gain
    (Georgia Institute of Technology, 2009-01) Zhang, Xiaohong ; Potscavage, William J., Jr. ; Choi, Seungkeun ; Kippelen, Bernard
    We report on flexible organic complementary inverters using pentacene and C₆₀ as active semiconductors fabricated on a plastic substrate. Individual transistors as well as inverters show good operational stability with negligible hysteresis. The threshold voltages are comparable for p-channel pentacene and n-channel C₆₀ organic field-effect transistors, and noise margins larger than 80% of the maximum theoretical values were obtained at a supply voltage VDD as low as 3 V. A high dc gain of 180 was achieved at VDD=5 V. The inverters demonstrated good mechanical stability when tested after bending under both tensile and compressive stress.
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    Origin of the open-circuit voltage in multilayer heterojunction organic solar cells
    (Georgia Institute of Technology, 2008-11-10) Potscavage, William J., Jr. ; Yoo, SeungHyup ; Kippelen, Bernard
    From temperature dependent studies of pentacene/C₆₀ solar cells in the dark, the reverse saturation current is found to be thermally activated with a barrier height that corresponds to the difference in energy between the highest occupied molecular orbital of the donor and the lowest unoccupied molecular orbital of the acceptor corrected for vacuum level misalignments and the presence of charge-transfer states. From the reverse saturation current in the dark and the short-circuit current under illumination, the open-circuit voltage can be predicted. Examination of several donor materials supports the relationship between reverse saturation current, this barrier height, and open-circuit voltage.
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    Equivalent circuit model for organic single-layer diodes
    (Georgia Institute of Technology, 2008-09) Haldi, Andreas ; Sharma, Asha ; Potscavage, William J., Jr. ; Kippelen, Bernard
    A simple equivalent circuit is proposed to model single-layer organic diodes. The circuit is based on thermionic emission to describe carrier injection from the electrode into the organic semiconductor and on space-charge limited currents across the semiconductor. By fitting the electrical characteristics measured as a function of temperature with the model, intrinsic material and interface parameters such as the mobility and the injection barrier energy are extracted. The resulting parameters agree well with independently measured values in the literature.
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    Encapsulation of pentacene/C₆₀ organic solar cells with Al₂O₃ deposited by atomic layer deposition
    (Georgia Institute of Technology, 2007-06) Potscavage, William J., Jr. ; Yoo, SeungHyup ; Domercq, Benoit ; Kippelen, Bernard
    Organic solar cells based on pentacene/C₆₀ heterojunctions were encapsulated using a 200-nm-thick film of Al₂O₃ deposited by atomic layer deposition (ALD). Encapsulated devices maintained power conversion efficiency after exposure to ambient atmosphere for over 6000 h, while devices with no encapsulation degraded rapidly after only 10 h of air exposure. In addition, thermal annealing associated with the ALD deposition is shown to improve the open-circuit voltage and power conversion efficiency of the solar cells.