Organic photovoltaics prepared by solution processing provide state-of-the-art performance. During processing, donor and acceptor molecules precipitate and form bulk heterojunctions. The resulting microstructure is highly dependent on processing conditions and impacts device performance. In this study, we use molecular dynamics to decouple the effects of polymer-solvent interactions and solvent evaporation kinetics. To compare with experiments, simulations must be well equilibrated. Using virtual sites coarse-graining, we can simulate a polymer donor (PM6) and small molecule acceptor (Y6) in various solvents for several microseconds. To investigate thermodynamic interactions, we examined chain conformation and clustering. To explore the effect of solvent drying rate on the bulk structure, we compared morphologies obtained from drying concentrated solutions at two different rates. Y6 exhibits two types of aggregates at low concentration, fibers and globules, depending on solvent choice. For PM6, we identify structural features that may limit its ability to order. PM6 chains tend to adopt helical conformations at low concentration. But as the chains cluster in solution, they exclude solvent and differences in chain conformation and clustering can be observed based on solvent choice and polymer concentration. The results of our work provides insight into how aggregates form in bulk heterojunctions and how we can use solvent engineering to change clustering behavior in solution.
