Impact of Intermolecular Interactions between Halogenated Volatile Solid Additives and the Nonfullerene Acceptor in Organic Solar Cells

The halogenated volatile solid additives can delicately optimize the active layer morphology of organic solar cells, improving the devices' performance, stability, and reproducibility. However, what type of intermolecular interaction occurs between the solid additives and the active layer and whether the interaction truly impacts the donor or acceptor remains debatable. Herein, the focus is on halogenated volatile solid additives with conjugated benzene rings and their influence on the morphology of the active layer composed of PM6:Y6 as they evaporated. The absorbance spectra exhibit apparent red-shift features in Y6 absorption regions, while the donor part is unaffected. The theoretical calculation results reveal that the additives can stay between two Y6 molecules and form halogen bonds, affecting the π–π aggregation properties of Y6. As a result, the crystalline features of the active layer are altered, leading to increased charge carrier mobilities, extended charge carrier diffusion lengths, reduced bimolecular charge recombination, and thus the device performance. Especially when 1,3,5-tri bromobenzene is used, a champion power conversion efficiency of 17.9% is attained, among the best-performed organic solar cells comprising PM6:Y6. The findings shed light on theoretical and experimental guidelines for designing and developing volatile solid additives for highly efficient nonfullerene organic solar cells.