Dark spot formation mechanism in organic light emitting diodes

The authors report electroluminescence degradation in organic light emitting diode operated at a high current density of 500mA∕cm2 under nitrogen ambient. The turn-on voltage increased from 6to10V, and a number of dark spots were produced when the device was operated for 420min. 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.