Examining the Impact of 3D Multi-Arm Small Molecules on PM6 : Y6 Blend Reveals the Key Requirements for Their Electronic Properties

While the emergence of PM6 : Y6 active layer re-energized the organic photovoltaic community, excessive aggregation of Y6 molecules induced by their strong intermolecular interactions has limited the performance of PM6 : Y6-based organic solar cells (OSCs). Adding 3D multi-arm small-molecule acceptors is an effective strategy to inhibit such aggregation. However, to maximize OSC efficiency, these molecules should also contribute to the electronic processes. Here, by taking a benzotriazole-based 3D four-arm small molecule (i.e., SF-BTA1) as representative example, we combine molecular dynamics simulations and density functional theory calculations to examine the molecular-scale impact of 3D multi-arm small molecules on morphological characteristics (especially at the nanoscale) and electronic properties of PM6 : Y6 blends. By considering the intermolecular packing distances, density, and patterns among PM6, Y6, and SF-BTA1 components, exciton transfer rates from SF-BTA1 to Y6 or PM6, charge transfer rates from Y6 or PM6 to SF-BTA1, electron/hole transfer rates among adjacent Y6/PM6 pairs, and radiative and non-radiative recombination processes, we draw a comprehensive picture that describes how 3D multi-arm small molecules improve morphological and electronic properties of PM6 : Y6 blends and thus the OSC efficiency. Furthermore, successful rationalization of these aspects allows us to point out key requirements regarding the electronic properties of 3D multi-arm small molecules.