1. Experimentelle Physik 2, Technische Universität Dortmund, 44221 Dortmund, Germany
2. Institute of Solid State Theory, University of Münster, 48149 Münster, Germany
3. Laboratory of Inorganic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zürich, CH-8093 Zürich, Switzerland
4. Laboratory for Thin Films and Photovoltaics, Empa-Swiss Federal Laboratories for Materials Science and Technology, CH-8600 Dübendorf, Switzerland