1. Institute of Polymer Optoelectronic Materials & Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, P. R. China
2. School of Materials Science and Engineering, Tianjin University, Tianjin 300350, P. R. China
3. Department of Physics, Organic and Carbon Electronics Lab (ORaCEL), North Carolina State University, Raleigh, North Carolina 27695, United States
4. Department of Materials Science and Engineering, University of Washington, Seattle, Washington 98195, United States