1. Henan Key Laboratory of Photovoltaic Materials Henan University No. 1 Jinming Road Kaifeng Henan 475004 China
2. State Key Laboratory on Integrated Optoelectronics and College of Electronic Science and Engineering Jilin University No. 2699 Qianjin Street Changchun Jilin 130012 China
3. Department of Electrical & Computer Engineering University of Pittsburgh 1238 Benedum Hall Pittsburgh PA 15261 USA