1. Shanghai Key Laboratory of Magnetic Resonance School of Physics and Electronic Science East China Normal University No. 500 Dongchuan Road Shanghai 200241 P. R. China
2. International Joint Laboratory of Renewable Energy Materials and Devices of Henan Province and School of Physics & Electronics Henan University Kaifeng 475004 P. R. China
3. Ningxia Key Laboratory of Photovoltaic Materials Ningxia University Yinchuan Ningxia 750021 P. R. China