Multiscale approach to compute donor-acceptor interchain couplings for realistic interfacial geometries

In an organic photovoltaic (OPV) cell, excitons dissociate in the donor-acceptor interfacial region. Brute-force electronic calculations in this region are prohibitively expensive. Here, we provide a tractable method to evaluate electronic couplings between adjacent donor and acceptor moieties, with realistic local geometries from atomistic molecular dynamics simulations of mixed donor oligomer and acceptor molecules. In these simulations, we exploit aromatic moieties' stiffness by using virtual sites for many atoms, significantly reducing degrees of freedom while retaining the shapes and dipole moments of moieties. Realistic local geometries of nearby donor-acceptor pairs serve as input to electronic structure calculations. As an example, we have examined the Poly(3-hexylthiophene) P3HT (donor) and O-IDTBR (acceptor) blend. We efficiently obtain the donor-acceptor hopping matrix element for nearly 10,000 nearby pairs of moieties from the splitting between HOMO and HOMO-1 for each pair. In this way, we obtain the histogram of interchain couplings, which exhibit a long tail of rare but strong couplings above 100 meV. By referring to the input geometries, we can identify criteria for strongly coupled pairs.