ELECTRICAL-FIELD INDUCED AGING OF POLYMER LIGHT-EMITTING-DIODES IN ANOXYGEN-RICH ATMOSPHERE STUDIED BY EMISSION MICROSCOPY, SCANNING ELECTRON-MICROSCOPY AND SECONDARY-ION MASS-SPECTROSCOPY

Polymer light-emitting diodes (PLEDs) made with poly(p-phenylene vinyl ene) (PPV) using a non-ionic precursor route with indium-tin oxide (IT O) as anode and Al as cathode have been examined during continuous ele ctrical stress in an oxygen-rich atmosphere. Three distinct regions in the time evolution of the equivalent electrical resistance and the li ght output of PLEDs are identified. Various electrical and analytical measurement results are presented to explain the main failure mechanis ms. The most severe degradation mode can be identified as dielectric b reakdown, resulting in 'hot spots' and ohmic leakage paths. The inhibi tion of the ohmic path formation by oxidation under ambient conditions results in a local delamination of the electrode, shrinking the activ e area of the device. This loss of active area caused by these oxidati ve burn-outs can clearly be observed by scanning electron microscopy ( SEM) and is consistent with secondary ion mass spectroscopy (SIMS) res ults, Emission microscopy (EMMI) inspection provides evidence for elec tric held induced ageing at defects present in the device. These defec ts are already present in the as-produced samples, e.g. particle impur ities, interface roughness and structural weakness (edges of the Al el ectrodes). (C) 1998 Elsevier Science S.A. All rights reserved.