Surface roughness effects and their influence on the degradation of organic light emitting devices

Organic light emitting devices typically consist of one or several organic layers which are sandwiched between two electrodes, one of which has to be transparent. In most cases indium tin oxide (ITO) is employed as the transp arent, hole-injecting anode material. Usually, the functional organic layer s possess a thickness of about 100 nm. For such thin films the homogeneity and the surface roughness are especially important factors for the device p erformance. Therefore, the surface roughness of all those layers which are the basis for subsequent deposition processes were systematically studied b y atomic force microscopy (AFM). For these investigations both the ITO subs trate and the layers consisting of different organic materials deposited on to the ITO substrate were analyzed. In addition, the two different basic de position methods for the organic materials, namely the deposition from solu tion by spin coating and the deposition by thermal evaporation, were compar ed to one another with respect to their resulting surface roughness. It was found that the large surface roughness of the ITO substrate induces layer inhomogeneities, especially for the vapor deposited organic layers. They ca n be reduced by the incorporation of a polymeric smoothing layer.