1. Helmholtz Centre Berlin for Materials and Energy, Department E-I5 - Materials for Photovoltaics, Hahn-Meitner-Platz 1, 14109 Berlin, Germany
2. Institute for Physics, University of Rostock, Universitätsplatz 3, 18055 Rostock, Germany
3. Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States