1. State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, Fudan University, Shanghai 200438, China
2. Laboratory of Organic Electronics, Department of Science and Technology, Linköping University, Norrköping 60174, Sweden
3. State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai, 200241, P. R. China
4. Department of Physics and Astronomy, Uppsala University, Box 516, Uppsala, SE-751 20, Sweden