1. Key Laboratory of Photovoltaic Materials of Henan Province, School of Physics and Electronics, Henan University, Kaifeng 475004, P. R. China
2. School of Information Engineering, Nanyang Institute of Technology, Nanyang 473004, P. R. China
3. National Demonstration Center for Experimental Physics and Electronics Education, School of Physics and Electronics, Henan University, Kaifeng 475004, P. R. China