1. School of Photovoltaic and Renewable Energy Engineering University of New South Wales Sydney NSW 2052 Australia
2. Wuhan National Laboratory for Optoelectronics (WNLO) and School of Optical and Electronic Information Huazhong University of Science and Technology Wuhan Hubei 430074 P. R. China
3. Department of Physics and Astronomy University of Florence I-50019 Sesto Fiorentino (FI) Italy
4. College of Physics and Information Engineering, and Institute of Micro-Nano Devices and Solar Cells Fuzhou University Fuzhou 350108 P. R. China
5. School of Materials Science & Engineering UNSW Sydney Sydney NSW 2052 Australia