1. CAS Key Laboratory of Materials for Energy ConversionShanghai Institute of CeramicsChinese Academy of Sciences Shanghai 200050 P. R. China
2. Center of Materials Science and Optoelectronics EngineeringUniversity of Chinese Academy of Sciences Beijing 100049 P. R. China
3. CAS Key Laboratory of Materials for Energy ConversionUniversity of Science and Technology of China Hefei Anhui 230026 P. R. China