1. School of Materials Shenzhen Campus of Sun Yat-sen University No. 66, Gongchang Road Shenzhen Guangdong 518107 P. R. China
2. Institute for Solar Energy Systems Guangdong Provincial Key Laboratory of Photovoltaic Technology State Key Laboratory of Optoelectronic Materials and Technologies Sun Yat-Sen University Guangzhou 510275 P. R. China