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ItemOrganic Semiconductors in the Fourth Industrial Revolution(Georgia Institute of Technology, 2019-03-12) Kippelen, BernardIn this talk, we will discuss how printable organic conjugated semiconducting molecules and polymers are creating new disruptive technologies that are impacting all industries. We will present recent advances in various solid-state device platforms including, organic light-emitting diodes (OLEDs), organic photodetectors (OPDs), organic photovoltaic devices (OPVs), and organic thin-film transistors (OTFTs). We will emphasize the importance of interfaces in devices and show examples on how to engineer their electrical properties. We will present a simple processing technique for the electrical doping of organic semiconductors over a limited depth near the surface of the film that is based on immersing the film into a polyoxometalate solution. Such approached can drastically reduce the fabrication cost of such devices, simplify device architecture, and lead to all-organic devices fabricated by all-additive printing techniques. As an illustration of the simplicity and versatility of this process we will discuss how high-performance organic solar cells with simplified architecture can be implemented. Finally, we will present the results of a detailed operational lifetime study of OTFTs showing that organic photonics and electronics can yield a stability level superior to that of amorphous silicon.
ItemStrategies to Control Interfaces in Organic Electronic Designs( 2014-10-16) Kippelen, BernardPrinted organic electronics, a technology based on organic semiconductors that can be processed into thin films using conventional printing and coating techniques, has been the subject of active research and development over the past decades. Due to their ability to be processed at low temperature, over large areas, at low cost, organic semiconductors are experiencing an accelerated development that will lead to a new generation of products with thin and flexible form factors. While the organic semiconductor layer plays a central role, the interfaces that are formed between the organic semiconducting layer and adjacent oxide layers or electrodes are very critical and often determine the overall electrical performance of the device. In this talk, we will discuss the performance of a range of solid-state devices, including organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), sensors, and organic solar cells. We will present strategies to modify and stabilize the electronic properties of interfaces that can yield devices with improved performance and longer lifetime. We will show that these advances are likely to accelerate the deployment of flexible printed electronic technologies
ItemOrganic Semiconductors for Flexible Printed Electronics(Georgia Institute of Technology, 2012-09-11) Kippelen, BernardAdvances during the last 30 years in the synthesis and processing of organic materials with nonlinear optical and semiconducting properties, have fueled the emergence of a new technology that can potentially lead to low cost, flexible, and large area plastic optoelectronic devices and systems. Recent research breakthroughs in light-emitting diodes for displays and lighting, solar cells for portable power, and thin-film transistors are bringing flexible electronic technologies closer to commercialization. However, despite these technological advances, many challenges still remain in understanding the fundamental physical properties of the organic semiconductors used as active layers and the contacts they form at interfaces with adjacent organic layers or other materials used as electrodes. In this talk, we will discuss selected examples of recent advances made in developing new materials and device architectures that lead to organic light-emitting diodes, organic solar cells, and field-effect transistors with superior performance and stability. In particular, we will review several strategies that were employed to control the work function of electrodes and show how these approaches can be used to design organic solar cells with novel geometries. We will also present studies of the operational and environmental stability of flexible organic field-effect transistors that are comprised of a dual-layer gate dielectric. This architecture was found to yield to devices with unprecedented stability. Finally, we will discuss some of the challenges and future directions of this disruptive emerging technology platform.
ItemHighly 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, BernardGreen 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.
ItemLinear 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, BernardThe 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
ItemTop-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, BernardWe 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.
ItemEnhanced 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.
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).
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.
ItemMetamaterial metal-dielectric photonic bandgap filters with high in-band transmittance and strong out-of-band rejection(Georgia Institute of Technology, 2011-05-24) Kippelen, Bernard