Dark spot formation mechanism in organic light emitting diodes

Abstract

The authors report electroluminescence degradation in organic light emitting diode operated at a high current density of 500 mA/cm(2) under nitrogen ambient. The turn-on voltage increased from 6 to 10 V, and a number of dark spots were produced when the device was operated for 420 min. Microscope image showed that dark spots are related to many protrusions and hollows formed on Al electrode. X-ray diffraction and scanning photoemission microscope spectra indicate that the Joule heat from high electric field induced the crystallization of 4'-bis[N-(1-naphtyl)-N-phenyl-amino]biphenyl, forming dark spots via peeling off of the Al cathode. (c) 2006 American Institute of Physics.