1. Department
of Physics, Chalmers University of Technology, 41296 Gothenburg, Sweden
2. Laboratory
of Organic Electronics, Linköping University, SE-601 74 Norrköping, Sweden
3. Faculty of
Electrical Engineering, K. N. Toosi University of Technology, Tehran 1631714191, Iran
4. NILT Sweden Filial, Stena
Center 1B, 41292 Gothenburg, Sweden
5. Institute
of Physics, Chinese Academy of Sciences, 100190 Beijing, P. R. China
6. Department
of Physics, University of Gothenburg, Gothenburg 412 96, Sweden
7. Geballe
Laboratory for Advanced Materials, Stanford University, Stanford, California 94305-4045, United States