1. School of Physics and Engineering, Institute for Solar Energy Systems, State Key Laboratory of Optoelectronic Materials and Technologies, Guangdong Provincial Key Laboratory of Photovoltaics Technologies, Sun Yat-Sen University, Guangzhou 510006, P. R. China
2. School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, P. R. China
3. Department of Mechanical Engineering, The University of Hong Kong, Hong Kong, P. R. China