1. Key Laboratory for Renewable EnergyChinese Academy of Sciences (CAS) Beijing 100190 P. R. China
2. Beijing Key Laboratory for New Energy Materials and DevicesBeijing National Laboratory for Condensed Matter PhysicsInstitute of Physics Beijing 100190 P. R. China
3. School of Physical SciencesUniversity of Chinese Academy of Sciences Beijing 100049 P. R. China
4. Songshan Lake Materials Laboratory Dongguan Guangdong 523808 P. R. China
5. Center of Materials Science and Optoelectronics EngineeringUniversity of Chinese Academy of Sciences Beijing 100049 P. R. China