Investigation of device failure mechanisms in polymer light-emitting diodes

A polymer light-emitting diode (LED) of configuration —Indium-Tin-Oxide (ITO)/polymer/metal was used for the study of failure mechanisms using optical microscopy, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Better lifetimes were observed for LEDs that were operated at lower voltages. At constant voltage, poly(octylthiophene-3-carboxylate) (POT-3-C) diodes exhibited a sharp decay followed by a gradual decrease with time in total current passing through the device and corresponding absolute light intensity. With the application of voltage, metal electrode surface was observed to lose reflectivity 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 indicated that oxidation of both metal electrode and polymer film occurred. SEM micrographs of failed devices revealed discontinuities in the metal electrode and polymer film. Post-failure surface composition maps of ITO showed destroyed ITO film exposing bare silicon oxide. Similar failure patterns were observed in the failure of poly(p-phenylene vinylene) (PPV) LEDs under high electric fields.