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 transparent, hole-injecting anode material. Usually, the functional organic layers possess a thickness of about 100 nm. For such thin films the homogeneity and the surface roughness are especially important factors for the device performance. Therefore, the surface roughness of all those layers which are the basis for subsequent deposition processes were systematically studied by atomic force microscopy (AFM). For these investigations both the ITO substrate and the layers consisting of different organic materials deposited onto the ITO substrate were analyzed. In addition, the two different basic deposition methods for the organic materials, namely the deposition from solution by spin coating and the deposition by thermal evaporation, were compared 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 can be reduced by the incorporation of a polymeric smoothing layer.