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Center for Organic Photonics and Electronics
Center for Organic Photonics and Electronics
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ItemLow-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, BernardHigh 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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ItemArea-scaling of organic solar cells(Georgia Institute of Technology, 2009-09) Choi, Seungkeun ; Potscavage, William J., Jr. ; Kippelen, BernardWe 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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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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ItemLow-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, BernardWe 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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ItemOrigin 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, BernardFrom 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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ItemEquivalent circuit model for organic single-layer diodes(Georgia Institute of Technology, 2008-09) Haldi, Andreas ; Sharma, Asha ; Potscavage, William J., Jr. ; Kippelen, BernardA 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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ItemEncapsulation 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, BernardOrganic 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.
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ItemIntegrated organic photovoltaic modules with a scalable voltage output(Georgia Institute of Technology, 2006-12) Yoo, SeungHyup ; Potscavage, William J., Jr. ; Domercq, Benoit ; Kim, Jungbae ; Holt, J. ; Kippelen, BernardEfficient organic photovoltaic modules and cells have been fabricated based on a blend of poly(3-hexylthiophene) (P3HT) and a soluble C₇ ₀ derivative, [6,6]-phenyl C₇ ₁ butyric acid methyl ester (PCBM-70). Power conversion efficiency of P3HT/PCBM-70 cells is estimated at 4.1% for the AM1.5 G condition from the spectra of external quantum efficiency. Integrated photovoltaic modules, in which N- individual P3HT/PCBM-70 cells (N = 1–4) are connected in series, exhibit a short-circuit current /[subscript SC] of 1.4±0.1 mA and an open-circuit voltage V[subscript OC ]of N×0.621 V with a maximum VOC of 2.48 V for N = 4 under simulated solar illumination (AM1.5 G, 85 mW/cm2).