Ultrafast internal conversion in a low band gap polymer for photovoltaics: experimental and theoretical study

Ultrafast dynamics upon photoexcitation in a low band gap polymer for photovoltaics is investigated both experimentally and theoretically. Our work sheds light on the excess energy relaxation processes occurring immediately after photon absorption and responsible for dissipation in the photovoltaic process of light harvesting and energy storage. A peculiar non-adiabatic decay path through a conical intersection (CI) between the higher excited state S2 and the first singlet state S1 is identified by ultrafast spectroscopy and theoretical calculations. Ultrafast twisting of the initially flat conformation in S2 drives the system to the CI connecting the two potential energy surfaces, actually eliciting an internal conversion within 60 femtoseconds, followed by planarization along the adiabatic surface in S1. Relaxed potential energy profiles (PEPs) of ground and lowest excited states along a dihedral coordinate, calculated within the time dependent density functional theory (TDDFT) approach, support the S2/S1 CI mechanism. Furthermore a screening of the widely used hybrid and range separated exchange–correlation (XC) DFT functionals has been carried out finding different descriptions of S2/S1 PEPs and good agreement between experimental data and long-range corrected DFT.