1. Institute of Photovoltaics/Department of Materials Science and Engineering, Nanchang University, Nanchang 330031, China
2. Department of Applied Physics, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China
3. Department of Materials Science & Engineering, City University of Hong Kong, Kowloon, Hong Kong, China
4. College of Polymer Science & Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, China
5. Key Lab of Advanced Transducers and Intelligent Control System of Ministry of Education, College of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan 030024, China
6. Department of Materials Science and Engineering, University of Washington, Seattle, Washington 98195, United States