1. Laboratory for Photovoltaics, Physics and Materials Science Research Unit, University of Luxembourg, 41 rue du Brill, 4422 Belvaux, Luxembourg
2. Helmholtz-Zentrum für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, D-14109 Berlin, Germany
3. Ångström Solar Center, Solid State Electronics, Uppsala University, Box 534, SE-751 Uppsala, Sweden