Near-IR Femtosecond Transient Absorption Spectroscopy of Ultrafast Polaron and Triplet Exciton Formation in Polythiophene Films with Different Regioregularities

The formation dynamics of polaron pairs, polarons, and triplet excitons in regiorandom and regioregular poly(3-hexylthiophene) (RRa-P3HT and RR-P3HT) films was comprehensively studied by transient absorption spectroscopy over the wide wavelength region from 500 to 1650 nm under various excitation intensities. In both RRa-P3HT and RR-P3HT films, polaron pairs were generated not from relaxed singlet exciton states but from hot excitons on a time scale of <100 fs and decayed monomolecularly by geminate recombination. In RRa-P3HT films, triplet excitons were rapidly generated on a picosecond time scale from higher exciton states produced by the singlet exciton-exciton annihilation as well as from the lowest singlet exciton states by the normal intersystem crossing. In RR-P3HT films, no triplet excitons were observed; polarons were also generated not from relaxed singlet exciton states but from hot excitons in competition with the formation of polaron pairs. The polarons formed in RR-P3HT can freely migrate and mainly recombine with other polarons bimolecularly in the nanosecond time domain. The ultrafast formation of triplet excitons can be explained by the singlet exciton fission into two triplets, and the ultrafast formation of polaron pairs and polarons can be explained on the basis of the hot-exciton dissociation model where the excess thermal energy of the initially formed hot excitons is necessary to overcome their Coulombic binding energy. The remarkably different formation dynamics in P3HTs with different regioregularities is discussed in terms of the film morphology of conjugated polymers.