1. Department of Materials Science and Engineering, College of Engineering, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, Peking University, Beijing 100871, China
2. School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States
3. Department of Physics, Chinese University of Hong Kong, New Territories, Hong Kong, China
4. Frontier Institute of Science and Technology, Xi’an Jiaotong University, Xi’an 710054, China