1. Institute of Photovoltaics, Southwest Petroleum University, Chengdu 610500, P. R. China
2. School of Materials Science and Engineering and Tianjin Key Laboratory of Composites and Functional Materials, Tianjin University, Tianjin 300350, P. R. China
3. Australian Centre for Advanced Photovoltaics, School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney, New South Wales 2052, Australia