1. Institute of Polymer Optoelectronic Materials and Devices State Key Laboratory of Luminescent Materials and Devices South China University of Technology Guangzhou 510640 China
2. Laboratory of Organic Electronics Department of Science and Technology Linköping University Norrköping SE‐60174 Sweden
3. Department of Physics Chemistry and Biology Linköping University Linköping SE‐58183 Sweden
4. Institute of Nanostructure and Solid State Physics University Hamburg Hamburg 20355 Germany