1. College of Physics; Optoelectronics and Energy; Center for Energy Conversion Materials and Physics (CECMP); Soochow University; Suzhou 215006 P. R. China
2. Key Laboratory of Materials for Energy Conversion; Chinese Academy of Sciences; Department of Materials Science and Engineering; University of Science and Technology of China; Hefei 230026 P. R. China
3. Max Planck Institute for Solid State Research; Heisenbergstr. 1 Stuttgart 70569 Germany