1. College of Materials Science and Engineering Changsha University of Science and Technology Changsha Hunan 410004 P. R. China
2. School of Materials Science and Engineering Nanyang Technological University Block N4.1, 50 Nanyang Avenue Singapore 639798 Singapore
3. Jiangsu Key Lab for Organic Electronics and Information Displays & Institute of Advanced Material School of Materials Science and Engineering Nanjing University of Posts and Telecommunications Nanjing 210023 China
4. Department of Chemistry National University of Singapore 3 Science Drive 3 Singapore 117543 Singapore