1. Key Laboratory of Photovoltaic and Energy Conservation Materials, Institute of Applied Technology, Hefei Institutes of Physical Science, Chinese Academy of Sciences, 2221 Changjiangxi Road, Shushan District, Hefei, Anhui 230088, P. R. China
2. University of Science and Technology of China, 96 Jinzhai Road, Baohe District, Hefei 230026, P. R. China