1. Key Laboratory for Renewable EnergyChinese Academy of SciencesBeijing Key Laboratory for New Energy Materials and DevicesInstitute of PhysicsChinese Academy of Sciences Beijing 100190 P. R. China
2. Department of Physics ScienceUniversity of Chinese Academy of Sciences Beijing 100049 P. R. China
3. ARC Centre of Excellence in Exciton ScienceDepartment of Materials Science and EngineeringMonash University Clayton Victoria 3800 Australia
4. Center of Materials Science and Optoelectronics EngineeringUniversity of Chinese Academy of Sciences Beijing 100049 China