The donor-acceptor interface in organic photovoltaics plays a crucial role in exciton dissociation and charge recombination. Despite its importance, its structure has eluded experimental characterization. To address this, our group has recently developed tools for efficient atomistic simulations using virtual site coarse-graining, which we use to equilibrate the donor-acceptor interface of a high-performing blend (PM6:Y6). Our simulations show PM6 donor chains and Y6 acceptor molecules separate into a pure acceptor region and a donor-rich region with some dissolved acceptors. From the interfacial width, we infer the Flory Huggin's interaction parameter for these materials, which we compare to previous blends (P3HT:O-IDTBR). By tracking close contacts between acceptor molecules, we estimate the ability to form connecting pathways for charge extraction. While Y6 exhibits improved connectivity near the interface compared with O-IDTBR, both systems contain disconnected clusters within the donor-rich domain. Isolated acceptor molecules in the donor phase would increase recombination and decrease device efficiency. Because exciton separation depends on the conformations of donor and acceptor molecules at the interface, we report the probabilities for close contacts between donor and acceptor moieties. The most common contacts occur between moieties that are free of steric hindrances, such as solubilizing alkyl groups. We discuss the implications of our results on strategies to control the interfacial structure.
