1. Department of Materials Science and Engineering; University of Washington; Box 352120 Seattle Washington 98195 USA
2. State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Optoelectronic Information; University of Electronic Science and Technology of China (UESTC); Chengdu 610054 PR China
3. State Key Laboratory of Organic Solids; Beijing National Laboratory for Molecular Sciences; Institute of Chemistry; Chinese Academy of Sciences; Beijing 100190 PR China