Electrical field induced ageing of polymer light-emitting diodes in an oxygen-rich atmosphere studied by emission microscopy, scanning electron microscopy and secondary ion mass spectroscopy

Abstract

Polymer light-emitting diodes (PLEDs) made with poly(p-phenylene vinylene) (PPV) using a non-ionic precursor route with indium-tin oxide (ITO) as anode and Al as cathode have been examined during continuous electrical stress in an oxygen-rich atmosphere. Three distinct regions in the time evolution of the equivalent electrical resistance and the light output of PLEDs are identified. Various electrical and analytical measurement results are presented to explain the main failure mechanisms. The most severe degradation mode can be identified as dielectric breakdown, resulting in 'hot spots' and ohmic leakage paths. The inhibition of the ohmic path formation by oxidation under ambient conditions results in a local delamination of the electrode, shrinking the active area of the device. This loss of active area caused by these oxidative burn-outs can clearly be observed by scanning electron microscopy (SEM) and is consistent with secondary ion mass spectroscopy (SIMS) results. Emission microscopy (EMMI) inspection provides evidence for electric field induced ageing at defects present in the device. These defects are already present in the as-produced samples, e.g. particle impurities, interface roughness and structural weakness (edges of the Al electrodes). (C) 1998 Elsevier Science S.A. All rights reserved.