1. Heeger Beijing Research and Development Center; School of Chemistry and Environment; Beihang University; Beijing 100191 P. R. China
2. Beijing National Laboratory for Molecular Science; Key Laboratory of Organic Solids; Institute of Chemistry; Chinese Academy of Sciences; Beijing 100190 P. R. China
3. Department of Physics and Astronomy; Shanghai Jiaotong University; Shanghai 200240 P. R. China
4. Polymer Science and Engineering Department; University of Massachusetts; Amherst MA 01003 USA