Photoconductivity of organic semiconductor polymers studied by time-resolved THz-TDS

This study employs optical-pump terahertz- probe time-domain spectroscopy to measure relative carrier mobilities of semiconductor organic polymer films of poly-3-hexylthiophene (P3HT) and poly(2,5-bis(3- tetradecylthiophen-2-yI)thieno[3,2-b]thiophene) (PBTTT) polymers and these results directly correlate to recently reported electrical device mobility measurements.. An approximately four-fold increase in mobility of P3HT is observed with molecular weight increase from c.a. 18 kD to 150 kD where the intrinsic film mobility saturates for molecular weight > 70 kD. The decay dynamics studies of these polymers showed almost no dependence on the molecular weight of the polymer. The studies also showed an order of magnitude increase in mobility between as cast P3HT films and annealed PBTTT films supporting a strong conjugation length effect. We also measured the mobility dependence on the film quality that is affected by regularity of the polymer, dispersing solvent, electron acceptor traps, and other film growth related parameters relevant to device manufacturing.