Systematically Controlling Acceptor Fluorination Optimizes Hierarchical Morphology, Vertical Phase Separation, and Efficiency in Non-Fullerene Organic Solar Cells

Non-fullerene acceptor (NFA) end group (EG) functionalization, especially by fluorination, affects not only the energetics but also the morphology of bulk-heterojunction (BHJ) organic solar cell (OSC) active layers, thereby influencing the power conversion efficiency (PCE) and other metrics of NFA-based OSCs. However, a quantitative understanding of how varying the degrees of NFA fluorination influence the blend morphological and photovoltaic properties remains elusive. Here a series of three A-DAD-A type NFAs (D = π-donor group and A = π-acceptor EG) which systematically increase the degree of EG fluorination and comprehensively investigate the resulting blends with the polymer donor PM6 in terms of optical properties, electronic structure, film crystallinity, charge carrier transport, and OSC performance is reported. The results indicate that the most highly fluorinated NFA, BT-BO-L4F, achieves an optimal BHJ hierarchical morphology where enhanced NFA molecule intermolecular π–π stacking and optimal vertical phase gradation are achieved in the BHJ blend. These factors also promote optimum NFA-cathode contact, more balanced electron and hole mobility, and suppress both monomolecular and bimolecular recombination. As a result, both the short-circuit current density and fill factor in this OSC series progressively increase with increasing EG fluorine density, and the resulting PCEs increase from 9 to 16.8%.