1. Solid State Physics, Lund University, Box 118, 22100 Lund, Sweden.
2. Sol Voltaics AB, Ideon Science Park, Scheelevägen 17, 22370 Lund, Sweden.
3. Fraunhofer ISE, Heidenhofstrasse 2, D-79110 Freiburg, Germany.
4. University of Kassel, Wilhelmshoeher Allee 71, 34121, Kassel, Germany.
5. Key Laboratory for the Physics and Chemistry of Nanodevices and Department of Electronics, Peking University, Beijing 100871, China.