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 fluctu ations, - that is noise both in the radiance and in the film resistanc e, We demonstrate a correlation between the morphological changes whic h occur during the degradation process and the noise, suggesting that the sampling of noise during LED operation can be used as a very effic ient tool to predict the approaching failure of LEDs in real-life appl ications. 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 t he metal surface. Second, carbonized areas in the form of ''black spot s'' are formed, Accumulation of carbonized areas leads to short and/or open circuits and final LED failure. (C) 1997 American Institute of P hysics.