1. College of Material Science and Chemical Engineering, Ningbo University of Technology, Ningbo 315211, China
2. State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, Fudan University, Shanghai 200438, China
3. Laboratory of Organic Electronics, Department of Science and Technology, Linköping University, SE-601 74 Norrköping, Sweden
4. Department of Physics and Astronomy, Uppsala University, SE-751 20 Uppsala, Sweden