1. Key Laboratory of Organic Optoelectronics and Molecular Engineering of Ministry of Education Department of Chemistry Tsinghua University Beijing 100084 P. R. China
2. Laboratory of Flexible Electronics Technology Tsinghua University Beijing 100084 P. R. China
3. School of Materials Science and Engineering Tianjin Key Laboratory of Molecular Optoelectronic Sciences Tianjin University Tianjin 300072 P. R. China