1. Institute of Photoelectronic Thin Film Devices and Technology, Nankai University, Tianjin 300071, People's Republic of China
2. Key Laboratory of Photoelectronic Thin Film Devices and Technology of Tianjin, Nankai University, Tianjin 300071, People's Republic of China
3. Institute of Photovoltaics, College of Physics Science and Technology, Hebei University, Baoding 071002, China