Investigation of the sites of dark spots in organic light-emitting devices

Poor environmental stability has been a major concern for organic light-emitting devices. Exposure to ambient conditions leads to the formation of nonemissive areas (dark spots) that result in a decrease in device luminescence. Although a number of mechanisms for the formation of dark spots have been proposed, the causes underlying their initiation, and their nucleation sites are still far from being clear. In this study, optical microscopy is used to investigate the sites of dark spots of devices in which the original cathodes are peeled off and replaced by newer cathodes. Results confirm that the growth of dark spots occurs primarily due to cathode delamination. The growth of dark spots is also associated with changes in the organic layers, especially at the organic/cathode interface. Results also suggest that the nucleation of dark spots takes place at the organic/cathode interface and originates during the deposition of the cathode. On the other hand, both the anode and the hole transport layer do not appear to play a role in the formation of dark spots.