INVESTIGATION OF DEVICE FAILURE MECHANISMS IN POLYMER LIGHT-EMITTING-DIODES

A polymer light-emitting diode (LED) of configuration - Indium-Tin-Oxi de (ITO)/polymer/metal was used for the study of failure mechanisms us ing optical microscopy, scanning electron microscopy (SEM) and X-ray p hotoelectron spectroscopy (XPS). Better lifetimes were observed for LE Ds that were operated at lower voltages. At constant voltage, poly(oct ylthiophene-3-carboxylate) (POT-3-C) diodes exhibited a sharp decay fo llowed by a gradual decrease with time in total current passing throug h the device and corresponding absolute light intensity. With the appl ication of voltage, metal electrode surface was observed to lose refle ctivity and become dull, leading to increased surface roughness of the film. At high electric fields, bright spots appeared which eventually turned dark and transparent-like. XPS studies of failed devices indic ated that oxidation of both metal electrode and polymer film occurred. SEM micrographs of failed devices revealed discontinuities in the met al electrode and polymer film. Post-failure surface composition maps o f ITO showed destroyed ITO film exposing bare silicon oxide. Similar f ailure patterns were observed in the failure of poly(p-phenylene vinyl ene) (PPV) LEDs under high electric fields.