1. Collaborative Innovation Center of Sustainable Energy MaterialsGuangxi University Nanning 530004 P. R. China
2. State Key Laboratory of Optoelectronic Materials and TechnologiesSchool of Physics and EngineeringSun Yat‐sen University Guangzhou 510275 P. R. China
3. Department of Applied PhysicsStanford University Stanford CA 94305 USA
4. Department of Materials Science and EngineeringStanford University Stanford CA 94305 USA
5. Stanford Institute for Materials and Energy SciencesSLAC National Accelerator Laboratory Menlo Park CA 94025 USA