Degradation of nonencapsulated polymer-based light-emitting diodes: Noise and morphology

We show that the degradation of a nonencapsulated polymer-based light-emitting diode (LED) is accompanied by the appearance of strong fluctuations, - that is noise both in the radiance and in the film resistance, We demonstrate a correlation between the morphological changes which occur during the degradation process and the noise, suggesting that the sampling of noise during LED operation can be used as a very efficient tool to predict the approaching failure of LEDs in real-life applications. The morphological changes in LED degradation are essentially a two-stage process. First, there is formation of ''bubbles'' at the metal-polymer interface due to delamination of the polymer film from the metal surface. Second, carbonized areas in the form of ''black spots'' are formed, Accumulation of carbonized areas leads to short and/or open circuits and final LED failure. (C) 1997 American Institute of Physics.