1. Songshan Lake Materials Laboratory Dongguan Guangdong 523808 P. R. China
2. Institute of Physics Chinese Academy of Science Beijing 100190 P. R. China
3. School of Science Xi'an Polytechnic University Xi'an 710048 P. R. China
4. Henan Key Laboratory of Photovoltaic Materials Henan University Kaifeng 475004 P. R. China