Direct observation of structural changes in organic light emitting devices during degradation

Abstract

A method for studying the degradation of organic light emitting devices(OLEDs) in real time is described. Transparent OLEDs allow for the spatial correlation of cathode topographic images with optical images (transmission, photoluminescence, and electroluminescence) of the devices throughout the degradation process. In this study we focused on the evolution of nonemissive, “dark” spots during device operation. We conclude that the electroluminescent dark spots originate as nonconductive regions at the cathode/organic interface and expand or grow as a result of exposure to atmosphere. We propose a mechanism of dark spot growth involving aerobic oxidation of the cathode/organic interfacial region, leading to a highly resistive, carrier blocking interface at the dark spot locations. No initial defects on the cathodesurface, which might be responsible for the formation of dark spots, were detected by atomic force microscopy. Structural changes, such as degradation of organic materials and the cathodesurface, occur well after the formation and growth of the dark spots.