1. School of Materials Science and Engineering; Nanyang Technological University; 50 Nanyang Avenue Singapore 639798 Singapore
2. School of Advanced Materials and Nanotechnology; Key Laboratory of Wide Band Gap; Semiconductor Technology; Xidian Univeristy; Xi'an 710071 China
3. Solar and Photovoltaics Engineering Research Center; King Abdullah University of Science and Technology; Thuwal 23955-6900 Kingdom of Saudi Arabia
4. School of Physical Science and Technology; Lanzhou University; Lanzhou 730000 China
5. Materials Centre of Innovation; Institute of Materials Research and Engineering; 3 Research Link Singapore 117602 Singapore