1. Guangdong Provincial Key Laboratory of Optical Fiber Sensing and Communications Guangzhou Key Laboratory of Vacuum Coating Technologies and New Energy Materials Siyuan Laboratory Physics Department Jinan University Guangzhou 510632 P. R. China
2. Biomolecular and Organic Electronics Department of Physics Chemistry and Biology (IFM) Linköping University SE‐581 83 Linköping Sweden
3. 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
4. Department of Physics and Astronomy Shanghai Jiao Tong University Shanghai 200240 P. R. China
5. Materials Sciences Division Lawrence Berkeley National Laboratory Berkeley CA 94720 USA