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One strategy to improve the photovoltaic properties of non-fullereneacceptors (NFAs) is the rational fluorination or chlorination of thosemolecules. Although this modification improves important acceptor properties,little is known about the effects on the triplet states. Here, we combine thepolarizable continuum model with optimally tuned range-separated hybridfunctional to investigate this issue. We find that fluorination or chlorinationof NFAs decreases the degree of HOMO-LUMO overlap along these molecules.Consequently, the energy gap between $T_{1}$ and $S_{1}$ states, $\Delta E_{ST}= E_{S_{1}} - E_{T_{1}}$, also decreases. This effect simultaneously enhancesthe generation of triplet excitons and reduce the binding energy of the tripletexcitons ($E_{b,T}$) which favor their dissociation into free charges.Interestingly, although Cl has a lower electronegativity than F, thechlorination is more effective to reduce $\Delta E_{ST}$. Since chlorination ofNFAs is easier than fluorination, Cl substitution can be a useful approach toenhance solar energy harvesting using triplet excitons.

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