1. Department of Materials Science and Engineering; University of Washington; Seattle WA 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 P. R. China
3. Department of Physics and Organic and Carbon Electronics Laboratory; North Carolina State University; Raleigh NC 27695-8202 USA
4. Department of Polymer Science and Engineering; Zhejiang University; Hangzhou 310027 China