Anisotropic exciton diffusion in O-IDTBR from first-principles molecular dynamics in the first excited state

We studied the exciton diffusion in the O-IDTBR crystal, an acceptor material for organic photovoltaics (PV), using first-principles molecular dynamics simulations in the first excited state S₁. Our methodology was formerly benchmarked for another PV material, P3HT [Diarra et al., Phys. Chem. Chem. Phys., 2023, 25, 15539], to assess its reliability. Here, we show that the acceptor-donor-acceptor character of the molecule is lost upon transition from the ground to the excited state. We then monitor the trajectories of the associated Wannier centres of the excited electron and hole within the crystal. This enables to observe that the transfer occurs via the BR terminal groups of neighbouring molecules realising a stacking via π-π interactions. The exciton diffusion turns out to be remarkably anisotropic, and the intermolecular diffusion along the π stacking direction is characterised by a coefficient equal to D = (12.2 ± 0.3) × 10^-3 cm² s^-1, nearly twice the value found in P3HT.