It was recalled that dark non-emissive defects tended to form on the metal cathodes, of most molecular organic and polymer-based light-emitting devices, and eventually led to failure. These defects were characterized  in situ  by using optical microscopy, and  ex situ  by using atomic force microscopy and Auger electron spectroscopy. On the basis of these observations, an electromigration mechanism was proposed for the formation of dark-spot defects. It was suggested that the high current densities which were required in order to operate polymer-based light-emitting devices led to an electron-induced diffusion of the Al cathode when a short-circuit formed in the emissive polymer layer. This process resulted in a pile-up of metal at the short-circuit (anode), together with a surrounding circular region in which the Al was depleted, thus appearing as a dark spot on the cathode.

B.H.Cumpston, K.F.Jensen: Applied Physics Letters, 1996, 69[25], 3941-3