1. College of Physics and Optoelectronics, Faculty of Science, Beijing University of Technology, Beijing 100124, China
2. Department of Materials and Centre for Plastic Electronics, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom
3. Department of Physics & Regroupement québécois sur les matériaux de pointe, Université de Montréal, C. P. 6128, Succursale centre-ville, Montreal, Quebec H3C 3J7, Canada
4. Department of Chemistry and Centre for Plastic Electronics, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom