1. National-Local Joint Engineering Laboratory of New Energy Photovoltaic Devices, Key Laboratory of Digital Medical Engineering of Hebei Province, College of Electron and Information Engineering, Hebei University, Baoding 071002, People's Republic of China
2. Department of Materials Science and Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117575, Singapore
3. Department of Electrical and Computer Engineering, Southern Illinois University Carbondale, Carbondale, Illinois 62901, USA