1. Department of Materials Science and Engineering, Solid State Physics, Ångström Laboratory, Uppsala University, Box 34, Uppsala 75121, Sweden
2. Department of Chemical and Biological Engineering, School of Engineering and Applied Science, Princeton University, Princeton 08544, New Jersey, USA
3. Institute for Photovoltaics, Stuttgart University, Stuttgart 70569, Germany
4. Department of Physics, Shiraz University, Shiraz 71454, Iran