1. Key Lab of Organic Optoelectronics and Molecular Engineering of Ministry of Education Department of Chemistry Tsinghua University Beijing 100084 P. R. China
2. School of Chemical Sciences University of Chinese Academy of Sciences Beijing 100049 P. R. China
3. Institute of Polymer Optoelectronic Materials DevicesState Key Laboratory of Luminescent Materials DevicesSouth China University of Technology Guangzhou 510640 P. R. China
4. Center for Flexible Electronics Technology Tsinghua University Beijing 100084 P. R. China