1. Shenzhen Key Laboratory of Advanced Thin Films and ApplicationsCollege of Physics and Optoelectronic EngineeringShenzhen University Shenzhen 518060 P. R. China
2. School of Photovoltaic and Renewable Energy EngineeringUNSW Sydney Sydney NSW 2052 Australia
3. School of Metallurgy and EnvironmentCentral South University Changsha 410083 P. R. China