1. Information Materials and Intelligent Sensing Laboratory of Anhui Province Institutes of Physical Science and Information Technology Anhui University Hefei 230601 P. R. China
2. Photoelectric Conversion Energy Materials and Devices Key Laboratory of Anhui Province Anhui University Hefei 230601 P. R. China
3. Department of Applied Physics The Hong Kong Polytechnic University Kowloon Hong Kong 999077 P. R. China