1. Department of Microelectronic Science and Engineering, Ningbo University, Zhejiang 315211, China
2. School of Materials Science and Engineering, Ningbo University of Technology, Zhejiang 315016, China
3. Department of Applied Physics, School of Physics and Electronics, Hunan University, Hunan 410082, China
4. Photovoltaics Materials Unit, National Institute for Materials Science, Tsukuba Ibaraki, 305-0047, Japan