In order to improve the efficiency of top-and bottom-emitting devices, metallic electrodes have been used to create microcavity effects within the OLED structure. Semi-transparent Ag is used as the anode in bottom-emitting microcavity structures, whereas various reflective opaque metallic anodes are used for the top emitters. The cathode used in both configurations is MgAg -thick and opaque in the case of the bottom emitter and thin and semi-transparent in the case of the top emitter.Modeling and experiments show that for the top-emitting structures, the device efficiency is roughly proportional to the reflectivity of the anode in the low reflectivity range and increases significantly more than predicted by reflectivity alone in the high-reflectivity range. An ultrathin CF x or MoO x hole-injecting layer allows for the use of many metals as anodes and is an important feature of the device structure. With an Ag anode, both the top-and bottom-emitting microcavity devices are about twice as efficient (on axis) as the analogous nonmicrocavity bottom-emitting device. Microcavity devices employing a C545T-doped Alq emitter exhibit efficiencies of 21 cd/A at 6.4 V and 20 mA/cm 2 , with operational stability equivalent to conventional bottom-emitting structures.
Deep-level transient spectroscopy (DLTS) was used to study electron traps in n-CdTe crystals. High-purity undoped and indium-doped samples were examined with Au Schottky barriers. Five levels were observed in CdTe:In, three of which are in good agreement with previously observed defect levels. The capture cross sections of these levels were measured. The energy levels relative to the conduction band were measured by isothermal DLTS. A single very deep level was observed in high-purity undoped CdTe. Modified CdTe surfaces were produced by brief (∼60 s) low-temperature (400 °C) annealing in air or argon. Schottky barriers made on these surfaces indicate a decrease in the near-surface carrier concentration. Changes in deep level concentrations were also observed. A very deep level (labeled IR5 in our study) (Ec =0.82 eV) is most changed by this process; annealing increases its concentration. It is suggested that IR5 is a native defect.
Au/CdTe photovoltaic cells have been prepared on modified surfaces of single-crystal substrates with AM2 efficiencies of about 15%. The modification effected by heat treatment in air is found to decrease the reverse saturation current and increase the width of the depletion layer. As a result, the open-circuit voltage, the short-circuit current, and the fill factor are all significantly enhanced. The maximum values of cell parameters obtained in different cells suggest that significantly higher efficiency is feasible.
An analytical technique for the quantitative analysis of oxygen and tellurium in tellurium suboxide optical recording media is proposed. This involves the use of x-ray photoelectron spectroscopy but avoids the customary semiquantitative approach. The bulk concentration of oxygen is determined from the relative abundance of various tellurium species beneath the surface, taking into account the reduction rate of oxide due to ion beam sputtering.
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