1. Materials Science Center, National Renewable Energy Laboratory Golden, Colorado 80401, United States
2. Applied Science and Technology Graduate Group, University of California, Berkeley, California 94720, United States
3. Energy Technologies Area, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
4. School of Physics and Microelectronics, Zhengzhou University, Zhengzhou 450052, China
5. Department of Applied Science and Technology, University of California, Berkeley, California 94720, United States