The Binding Energy of Triplet Excitons in Non-Fullerene Acceptors: The Effects of Fluorination and Chlorination

One strategy to improve the photovoltaic properties of non-fullerene acceptors (NFAs) is the rational fluorination or chlorination of those molecules. Although this modification improves important acceptor properties, little is known about the effects on the triplet states. Here, we combine the polarizable continuum model with optimally tuned range-separated hybrid functional to investigate this issue. We find that fluorination or chlorination of NFAs decreases the degree of HOMO-LUMO overlap along these molecules. Consequently, the energy gap between $T_1$ and $S_1$ states, $\mathrm{\Delta }{E}_{ST}=E_{S_1}-E_{T_1}$, also decreases. This effect simultaneously enhances the generation of triplet excitons and reduce the binding energy of the triplet excitons ($E_{b,T}$) which favor their dissociation into free charges. Interestingly, although Cl has a lower electronegativity than F, the chlorination is more effective to reduce $\mathrm{\Delta }{E}_{ST}$. Since chlorination of NFAs is easier than fluorination, Cl substitution can be a useful approach to enhance solar energy harvesting using triplet excitons.