1. Experimentelle Physik 2, Technische Universität Dortmund, Dortmund 44221, Germany
2. Institute of Physics, University of Oldenburg, Carl von Ossietzky Strasse 9-11, Oldenburg 26129, Germany
3. Laboratory of Inorganic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zürich, Zürich CH-8093, Switzerland
4. Laboratory for Thin Films and Photovoltaics, Empa-Swiss Federal Laboratories for Materials Science and Technology, Dübendorf CH-8600, Switzerland