Degradation of top electrodes is one of the most important factors to determine the lifetimes of organic electroluminescence (EL) devices. An organic EL device [indium thin oxide (ITO)/N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4, 4′-diamine (TPD)/tris(8-hydroxy- quinoline)aluminum (Alq3)/Al] was prepared and a morphological change of the Al top electrode was observed during and/or after applying voltage by atomic force microscopy and scanning electron microscopy (SEM). The change in the electrode surface, i.e., the increase in surface roughness was observed during the current flow. The degradation process started from faint dark core parts and propagated into disks with different rates depending on the magnitude of applied voltage. Degraded sites of the Al electrode, which were analyzed as aluminum oxide by Auger electron spectroscopy, protruded into the air on the organic layers. In SEM images of a life-end electrode, discontinuities due to crevasse formation in the organic layers sandwiched by the ITO base and the metal top electrodes were observed in many places. These results confirm that one of the most crucial factors of the degradation process was deformation of metal and organic layers due to heat, gas evolution, and oxidation caused by applied voltage.

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Thermally stimulated currents (TSC) from polyethylene terephthalate (PET) electrets have been investigated in the temperature range from -190 to +120°C. PET electrets were formed by applying a high field in a short time at various temperatures. Both dependencies of TSC on electrode metals (Au, Al) and the polarity of an applied field were remarkable in the temperature range above -40°C. The results indicate that electrons injected from the Al cathode greatly contribute to the TSC above -40°C and that the TSCs below -40°C for Al electrodes and at all temperatures for Au electrodes are mainly induced by the depolarization of dipoles.

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