1. School of Electrical Engineering and Automation, Hefei University of Technology, Hefei 230009, P. R. China
2. Special Display and Imaging Technology Innovation Center of Anhui Province, Academy of Opto-Electric Technology, Hefei University of Technology, Hefei 230009, P. R. China
3. Research Center for Photovoltaic System Engineering of Ministry of Education, Hefei 230009, P. R. China