1. Frontiers Science Center for Flexible Electronics Institute of Flexible Electronics Northwestern Polytechnical University Xi'an 710072 P. R. China
2. Department of Materials Science and Engineering City University of Hong Kong Kowloon Hong Kong 999077 P. R. China
3. Department of Applied Physics and Research Institute for Smart Energy The Hong Kong Polytechnic University Kowloon Hong Kong 999077 P. R. China
4. Songshan Lake Materials Laboratory Dongguan Guangdong 523808 P. R. China
5. Guangdong Provincial Engineering Technology Research Center for Low Carbon and Advanced Energy Materials Institute of Semiconductor Science and Technology South China Normal University Guangzhou 510631 P. R. China
6. Hong Kong Institute for Clean Energy City University of Hong Kong Kowloon Hong Kong 999077 P. R. China