1. Key Lab of Organic Optoelectronics and Molecular Engineering of Ministry of Education Department of Chemistry Tsinghua University Beijing 100084 P. R. China
2. College of Chemistry Sichuan University Chengdu 610064 P. R. China
3. Department of Information Display Kyung Hee University Seoul 02447 Republic of Korea
4. Center for Flexible Electronics Technology Tsinghua University Beijing 100084 P. R. China