1. Department of Physics, Chemistry and Biology (IFM) Linköping University Linköping 58183 Sweden
2. Guangzhou Key Laboratory of Vacuum Coating Technologies and New Energy Materials Siyuan Laboratory Department of Physics Jinan University Guangzhou 510632 P. R. China
3. Beijing Key Laboratory of Microstructure and Properties of Solids Faculty of Materials and Manufacturing Beijing University of Technology Beijing 100124 P. R. China
4. Laboratory of Organic Electronics Department of Science and Technology Linköping University Norrköping 60174 Sweden