1. School of Materials and Energy University of Electronic Science and Technology of China Chengdu 611731 P. R. China
2. Center for Applied Chemistry University of Electronic Science and Technology of China Chengdu 611731 P. R. China
3. Key Laboratory for Physical Electronics and Devices of the Ministry of Education & Shaanxi Key Lab of Information Photonic Technique School of Electronics & Information Engineering Xi'an Jiaotong University Xi'an 710049 P. R. China
4. Institute of Photovoltaics Southwest Petroleum University Chengdu 610500 P. R. China