Origin of the Excited-State Absorption Spectrum of Polythiophene

The excited states of conjugated polymers play a central role in their applications in organic solar photovoltaics. The delocalized excited states of conjugated polymers are short-lived (tau < 40 fs) but are imperative in the photovoltaic properties of these materials. Photoexcitation of poly(3-hexylthiophene) (P3HT) induces an excited-state absorption band, but the transitions that are involved are not well understood. In this work, calculations have been performed on P3HT analogues using nonlinear response time-dependent density functional theory to show that an increase in the oligomer length correlates with the dominance of the S-1 > S-3 transition. Furthermore, the predicted transition energy shows an excellent agreement with experiment. The calculations also yielded results on intramolecular charge transfer in P3HT due to the S-1 -> S-3 transition, providing insight into the mechanism of exciton dissociation to form charge carriers.