Direct observation of structural changes in organic light emitting devicesduring degradation

A method for studying the degradation of organic light emitting devices (OL EDs) in real time is described. Transparent OLEDs allow for the spatial cor relation of cathode topographic images with optical images (transmission, p hotoluminescence, and electroluminescence) of the devices throughout the de gradation process. In this study we focused on the evolution of nonemissive , "dark" spots during device operation. We conclude that the electrolumines cent dark spots originate as nonconductive regions at the cathode/organic i nterface and expand or grow as a result of exposure to atmosphere. We propo se a mechanism of dark spot growth involving aerobic oxidation of the catho de/organic interfacial region, leading to a highly resistive, carrier block ing interface at the dark spot locations. No initial defects on the cathode surface, which might be responsible for the formation of dark spots, were detected by atomic force microscopy. Structural changes, such as degradatio n of organic materials and the cathode surface, occur well after the format ion and growth of the dark spots. (C) 2001 American Institute of Physics.