1. Institute of Applied Technology, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, P. R. China
2. University of Science and Technology of China, Hefei 230026, P. R. China
3. Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, P. R. China
4. Center for Advanced Photovoltaics and Sustainable Energy, Department of Electrical Engineering and Computer Sciences, South Dakota State University, Brookings, South Dakota 57007, United States