Nuclei of dark spots in organic EL devices: detection by DFM and observation of the microstructure by TEM

We report the observation of nuclei that exist in the centers of the dark spots frequently found in organic electroluminescent (EL) devices. The nuclei can be easily and effectively detected by dark field microscopy (DFM). We used scanning electron microscopy (SEM), scanning ion microscopy (SIM), transmission electron microscopy (TEM) and Auger electron spectroscopy (AES) to study the microstructure of nuclei. We found that the most typical nuclear structure, among several types of nuclei we observed, has a small ‘Al grain cluster’ (usually less than 1–2 μm in size) in the Al cathode. They feature chinks and hollows in their surface, that allow water and oxygen to penetrate an EL cell. We assume that this penetration hastens the growth of dark spots even under no-drive conditions. We also report our study of the mechanism of nuclei formation.