1. College of Physical Science and Technology/Microelectronics Industry Research Institute, Yangzhou University, 225002 Yangzhou, P. R. China
2. School of Electronics Science and Engineering/National Laboratory of Solid State Microstructures, Nanjing University, 210023 Nanjing, P. R. China
3. Cell R&D Center, JA Solar Holdings Co. Ltd, Yangzhou, 225000, P. R. China