1. Institute of Polymer Optoelectronic Materials and Devices State Key Laboratory of Luminescent Materials and Devices South China University of Technology Guangzhou 510640 P. R. China
2. Innovation Center for Printed Photovoltaics South China Institute of Collaborative Innovation Dongguan 523808 P. R. China
3. Department of Materials Science and Engineering City University of Hong Kong Kowloon Hong Kong
4. School of Energy and Environmental Science City University of Hong Kong Kowloon Hong Kong