1. Key Lab of Organic Optoelectronics and Molecular Engineering of Ministry of EducationDepartment of ChemistryTsinghua University Beijing 100084 P. R. China
2. Center for Flexible Electronics TechnologyTsinghua University Beijing 100084 P. R. China
3. Key Laboratory of Organic SolidsInstitute of ChemistryChinese Academy of Sciences Beijing 100190 P. R. China