1. Department of Electronic Science and Engineering, Key Laboratory of Photo-Electronic Thin Film Device and Technology of Tianjin, Engineering Research Center of Thin Film Optoelectronics Technology (Ministry of Education), Nankai University, Tianjin 300350, P. R. China
2. Shanxi Key Laboratory of Metal Forming Theory and Technology, School of Material Science and Engineering, Taiyuan University of Science and Technology, Taiyuan 030024, Shanxi, P. R. China
3. Research and Development Department, NICE Solar Energy Company Limited, Beijing 102209, P. R. China