1. Department of Physics, Tsinghua University, Beijing 100084, China.
2. Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305, USA.
3. Michael Grätzel Centre for Mesoscopic Solar Cells, Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China.
4. Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou, Jiangsu 215123, China.
5. SLAC National Accelerator Laboratory, Stanford Institute for Materials and Energy Sciences, 2575 Sand Hill Road, Menlo Park, CA 94025, USA.