Exciton Binding Energies of Nonfullerene Small Molecule Acceptors: Implication for Exciton Dissociation Driving Forces in Organic Solar Cells

Reducing the driving force of exciton dissociation into charge-transfer states is one effective solution to minimize energy loss and thus to improve power conversion efficiencies for organic solar cells. Traditionally, the driving force should be larger than 0.3 eV to achieve efficient exciton dissociation. Recent experiments have shown that excitons can be effectively dissociated, whereas the energy offsets between donor and acceptor are extremely small, but the mechanisms are not understood yet. Here, we use system-optimized long-range corrected functional with solid-state electronic polarization to investigate exciton binding energies of 14 typical nonfullerene small molecule acceptors in organic solar cells. The results point to that the driving forces for dissociation of the acceptor excitons into charge-transfer states are linearly correlated to the exciton binding energies. The smaller the exciton binding energy, the lower driving force required. Moreover, primarily owing to the largest dielectric constants, IDT- or IDTT-based fused-ring acceptors have the smallest exciton binding energies with respect to other acceptors, i.e., DPP-, PDI-, and BFI-based systems. The influence of conjugation lengths, strengths of electron-donating and withdrawing units, and molecular volumes on the dielectric constants are analyzed in detail. Our work rationalizes the experimental observations and would be helpful for designing active materials to reduce energy loss for organic solar cells.