1. Royal Institute of Technology (KTH), Experimental Biomolecular Physics, Department Applied Physics, Albanova Univ Center, 106 91 Stockholm, Sweden
2. Laboratory of Organic Electronics, Department of Science and Technology, Linköping University, SE-60174 Norrköping, Sweden
3. Department of Physics and Astronomy, Uppsala University, P.O. Box 516, SE-751 20 Uppsala, Sweden