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Center for Organic Photonics and Electronics

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Highly efficient inverted top-emitting green phosphorescent organic light-lightemitting diodes on glass and flexible substrates

(Georgia Institute of Technology, 2012-07) Najafabadi, E. ; Knauer, K. A. ; Haske, Wojciech ; Fuentes-Hernandez, Canek ; Kippelen, Bernard

Green phosphorescent inverted top-emitting organic light-emitting diodes with high current efficacy and luminance are demonstrated on glass and polyethersulfone (PES) substrates coated with polyethylene dioxythiophene-polystyrene sulfonate (PEDOT:PSS). The bottom cathode is an aluminum/lithium fluoride bilayer that injects electrons efficiently into an electron transport layer of 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TpPyPB). The cathode is found to be highly sensitive to the exposure of trace amounts of O₂ and H₂O. A high current efficacy of 96.3 cd/A is achieved at a luminance of 1387 cd/m² when an optical outcoupling layer of N,N′-Di-[(1-naphthyl)-N,N′-diphenyl]-(1,1′-biphenyl)-4,4′-diamine (α-NPD) is deposited on the anode.
Linear and nonlinear optical properties of Ag/Au bilayer thin films

(Georgia Institute of Technology, 2012-04) Hsu, James ; Fuentes-Hernandez, Canek ; Ernst, Alfred R. ; Hales, Joel M. ; Perry, Joseph W. ; Kippelen, Bernard

The linear and nonlinear optical properties of Ag/Au bilayer metallic thin films with a total thickness of around 20 nm and with different Ag/Au mass-thickness ratios were studied. This study shows that the spectral dispersion of the effective refractive index of bilayer films can be tuned by controlling the mass-thickness ratio between Au and Ag. Improvement of the figure-of-merit for potential plasmonic applications and linear optical filters in the visible spectral range are reported and discussed. The nonlinear optical properties of bilayer metal films studied using femtosecond white-light continuum pump-probe experiments are also shown to be tunable with this ratio. The nonlinear change of optical path length is extracted from the pump-probe data and agrees with simulated values derived from a combination of the two-temperature model, describing the ultrafast electron heating dynamics, and a physical model that describes the dielectric permittivity of Au as a function of electron and lattice temperature
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.
Enhanced carrier mobility and electrical stability of n-channel polymer thin film transistors by use of low-k dielectric buffer layer

(Georgia Institute of Technology, 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.
Metal-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, 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).
ITO-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, 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.
Inverted 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, 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.
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.
Focus issue: Thin-film photovoltaic materials and devices introduction

(Georgia Institute of Technology, 2010-09) Kippelen, Bernard
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.