1. National-Local Joint Engineering Laboratory of New Energy Photovoltaic Devices, Machine Vision Engineering Technology Center of Hebei Province, Key Laboratory of Digital Medical Engineering of Hebei Province, College of Electron and Information Engineering, Hebei University, Baoding 071002, P. R. China
2. Department of Materials Science and Engineering, National University of Singapore, Singapore 117576, Singapore
3. Department of Electrical and Computer Engineering, Southern Illinois University Carbondale, Carbondale, Illinois 62901, United States