1. Department of Physics Chemistry and Biology (IFM) Linköping University SE‐58183 Linköping Sweden
2. Wuhan National Laboratory for Optoelectronics School of Optical and Electronic Information Huazhong University of Science and Technology 430074 Wuhan China
3. Department of Science and Technology Organic Electronics Linkoping University SE‐601 74 Norrkoping Sweden
4. Department of Physics and Astronomy University of Kentucky Lexington KY 40506‐0055 USA
5. Department of Physics The Chinese University of Hong Kong New Territories Hong Kong SAR P. R. China
6. State Key Laboratory of Luminescent Materials and Devices South China University of Technology 510640 Guangzhou China