1. Key Laboratory of Photovoltaic Materials of Henan Province and School of Physics & Electronics, Henan University, Kaifeng 475004, P. R. China
2. School of Materials, Sun Yat-sen University, Guangzhou 510275, P. R. China
3. Institute of Physics, Chinese Academy of Sciences, Beijing 100190, P. R. China
4. Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States