Ultrafast relaxation dynamics of photoexcitations in poly(3-hexylthiophene) for the determination of the defect concentration

Ultrafast pump–probe spectroscopy of poly(3-hexylthiophene) (P3HT) films was performed using a 9-fs laser and a broadband lock-in detection system. The fast geometrical relaxation (GR) time was attributed to the transition from a free exciton (FE) to form a bound polaron pair (BPP), and the time constant was estimated to be τGR = 90 ± 2 fs. The relaxation time constant of BPP was determined as τBPP = 710 ± 40 fs. The measurement of pump-power dependence enabled us to distinguish the defect trapping process of BPPs from other parallel decay processes and to determine the defect concentration of a P3HT thin film.