1. Institute of Solid State Physics Hefei Institutes of Physical Science Chinese Academy of Sciences Hefei Anhui 230031 P. R. China
2. University of Science and Technology of China Hefei Anhui 230026 P. R. China
3. Henan Key Laboratory of Photovoltaic Materials Henan University Kaifeng Henan 475004 P. R. China
4. Department of Mechanical and Aerospace Engineering Syracuse University Syracuse NY 13244 USA