Combined XPS, AFM, TEM and ellipsometric studies on nanoscale layers in organic light emitting diodes

We report on combined investigations on indium–tin oxide (ITO) coated glass substrates in order to reveal the influence of chemical and physical treatments on the morphology and the elemental distribution at the ITO surface. The latter was studied by X-ray photoelectron spectroscopy while a combination of atomic force microscopy (AFM) and transmission electron microscopy was used to study the morphologic aspects. We find that etching ITO with chromosulphuric acid significantly reduces the amount of carbon contamination present at the surface of as received ITO substrates whereas the morphology of ITO remains almost the same. Additionally, we present ellipsometric studies of an organic semiconducting oligomer, para-hexaphenyl (PHP), grown on different substrates by physical vapour deposition. This method enables the determination of the optical parameters and the film thickness. In order to understand the growth mechanism of PHP in more details, its growth on silicon (Si) was also studied. We present AFM images of the first two as well as of several monolayers of PHP on Si giving evidence for the formation of PHP islands on the surface.