1. School of Materials and Energy, University of Electronic Science and Technology of China, Chengdu 611731, China
2. School of Metallurgy and Environment, Central South University, Changsha 410083, China
3. Yitai Technology Ltd., Hunan 410083, China
4. Australian Centre for Advanced Photovoltaics, School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney, NSW 2052, Australia