Photoexcitation dynamics in regioregular and regiorandom polythiophene films

Using a variety of optical probes techniques we studied the photoexcitation dynamics in thin films of poly-3-hexyl thiophene with regioregular order that forms lamellae structures with increased interchain interaction, as well as regiorandom order that keeps a chainlike morphology. In regiorandom films we found that intrachain excitons with correlated induced absorption and stimulated emission bands are the primary excitations; they give rise to a moderately strong photoluminescence band. In regioregular films, on the contrary, we found that the primary excitations are excitons with a much larger interchain component; this results in lack of stimulated emission, vanishing intersystem crossing, and very weak photoluminescence band. In regioregular films we also measured photogenerated geminate polaron pairs with ultrafast dynamics that are precursor to long-lived polaron excitations.