Orientation and electronic properties of pentacene molecules on SiO2 and GeS(0001) studied using x-ray absorption spectroscopy

We present an investigation on the morphology of pentacene molecules, a candidate for organic semiconductors in organic electronic devices. In this case the molecules are deposited on the technologically relevant substrate SiO2. In order to observe possible changes in the morphology of a monolayer of pentacene compared to a much thicker pentacene layer, we have performed x-ray absorption experiments on both films. We did not find significant differences in the orientation of the pentacene molecules as a function of film thickness. The same is true for the orientation of pentacene on the model substrate GeS(0001), however, the molecular orientation is opposite to that of pentacene on SiO2. Whereas the pentacene molecules on GeS(0001) lie flat on the substrate surface, on SiO2 they are standing. These results have important consequences for our understanding of the functionality of organic devices based on pentacene.