1. Department of Physics and Astronomy, Iowa State University and Ames National Laboratory, Ames, Iowa 50011, United States
2. Institute of Inorganic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zurich, CH-8093 Zurich, Switzerland
3. Laboratory for Thin Films and Photovoltaics, Empa-Swiss Federal Laboratories for Materials Science and Technology, CH-8600 Dubendorf, Switzerland