Resolving the Atomistic Morphology of Domains and Interfaces in PM6:Y6 with Molecular Dynamics

Modeling charge generation and transport dynamics in organic semiconductors benefits greatly from an atomistic description of the microstructure. In this work, virtual site coarse-grained molecular dynamics offers a nanoscale view of a PM6:Y6 donor-acceptor blend. In pure systems cooled from an equilibrated melt, Y6 developed substantial order, which occurred on time scales much faster than for PM6. For PM6:Y6 interfaces, we can achieve an equilibrated melt interface starting from either a completely mixed or a completely demixed initial state. After several microseconds of simulation, the resulting interface between Y6 and PM6 rich domains spans a few nanometers. Upon cooling the blend to room temperature, π-stacked aggregates become prevalent in the Y6 domain, while PM6 chains begin to align parallel to the interface. Because exciton separation depends on the conformations of the donor and acceptor molecules at the interface, we report the probabilities for Y6 and PM6 moieties to come in to close contact with one another. The most common contacts occur between electron-poor end groups on Y6 and electron-rich moieties on PM6, which are relatively free of steric hindrances from solubilizing side groups. By tracking close contacts between acceptor molecules, we also extract the connectivity of Y6 throughout the blend. Y6 molecules are well connected at the interface and within the Y6 domain, but some Y6 is also found within the PM6 domain in the form of disconnected clusters. A lack of percolated pathways from these isolated Y6 molecules to acceptor domains would increase recombination in these regions.