Amphiphilic interactions drive clustering in organic semiconductors

High-performance organic photovoltaics are prepared through solution processing. Despite the importance of solution processing conditions, the driving factors of aggregation in solution during the preparation of bulk heterojunctions remain a subject of debate. Enabled by advances in simulation techniques for molecules consisting of stiff rings, this study focuses on the clustering behavior of push-pull organic semiconductors in explicit solvent. Semidilute solutions of polymer donor (PM6) or small molecule acceptor (Y6) were simulated to observe the aggregation process over several microseconds. Differences in clustering behavior in chloroform and chlorobenzene solvents become apparent at the nanoscale. Trends in the chain conformations and clustering dynamics indicate that solvent-solute interactions guide the assembly of domains in bulk heterojunctions. Analogous to micelles, Y6 moieties within the core avoid solvent, while end groups are more likely to interact with solvent molecules. PM6 hexamers do not form tight clusters like Y6, though fused-ring moieties will briefly interact, attracted by similar quadrupoles. These results provide insight into designing donor-acceptor-solvent systems, and highlight the potential of controlling solution aggregation through donor or acceptor architectures.