1. Organic Optoelectronics Laboratory, Sincrotrone Trieste SCpA, SS 14, km 163.5, 34149 Basovizza
(TS), Italy
2. Department
of Physics and Astronomy, University of Bologna, viale Berti Pichat 6/2, 40127
Bologna, Italy
3. Eurotech SpA, V. Fratelli Solari 3/a, 33020 Amaro (UD),
Italy
4. CNR-IOM and Department
of Physics, Università di Roma “La Sapienza”, Piazzale Aldo Moro 2, 00185 Roma, Italy