Dynamics of Short-Lived Polaron Pairs and Polarons in Polythiophene Derivatives Observed via Infrared-Activated Vibrations

Infrared-activated vibrations in the fingerprint region induced by structural rearrangement of the polymer in the presence of a charge are used to identify polaronic species in femtosecond spectroscopy. This is possible without resorting to complex spectral modeling or excitation density dependence of the different charge carriers. Their dynamics are investigated in bulk heterojunctions formed of a polythiophene derivative with TiO₂ nanoparticle acceptors, as well as in the pure polymer. The water-soluble polymer poly[3-(potassium-6-hexanoate)thiophene-2,5-diyl] (P3P6T) is compared with the intensely studied poly(3-hexylthiophene-2,5-diyl) (P3HT). Distinct bands are attributed to polarons and polaron pairs. Pairs with a lifetime of a few picoseconds are generated within the time resolution. In the presence of the acceptor, polaron yield rises with respect to the generated polaron pairs. Diffusion of polarons from 10 ps onward is identified as being either of predominantly intrachain or interchain character, depending on the degree of order present in the polymer.