1. Michael Grätzel Center for Mesoscopic Solar Cells, Wuhan National Laboratory for Optoelectronics, Key Laboratory of Materials Chemistry for Energy Conversion and Storage of Ministry of Education Huazhong University of Science and Technology Wuhan Hubei 430074 P. R. China
2. Key Laboratory of Flexible Electronics (KLoFE) & Institute of Advanced Materials (IAM), School of Flexible Electronics (Future Technologies) Nanjing Tech University (NanjingTech) 30 South Puzhu Road Nanjing Jiangsu 211816 P. R. China
3. Department of Physics The Chinese University of Hong Kong New Territories Hong Kong Hong Kong SAR 999077 P. R. China