Comparison of Symmetric and Asymmetric Non-Fullerene Acceptors Based on a Benzotriazole Heterocycle DA′D Core for High-Efficiency Organic Solar Cells

We have designed symmetric A1-DA′D-A1 (NFA-18S) and asymmetric A1-DA′D-A2 (NFA-17AS) nonfullerene acceptors with the same benzotriazole heterocycle DA′D core and dicyanoindanone (IC) (A1) and fluorinated dicyanoindanone (2F-IC) (A2) terminal units. The absorption spectra and frontier molecular orbitals of these two NFAs are consistent with those of the wide-bandgap polymer donor P154. The DFT simulations showed that, compared to NFA-18S, the dipole moment of asymmetric NFA-17AS is higher, which is beneficial for reducing the exciton binding energy. Employing a wide-bandgap polymer P154, the organic solar cells based on NFA-17AS, processed under ambient conditions, deliver an overall PCE of 15.96%, compared to NFA-18S (12.64%), which is due to the improved values of all photovoltaic parameters. Although NFA-18S has a higher lowest unoccupied molecular orbital level than NFA-17AS, the organic solar cell using NFA-17AS showed a higher open-circuit voltage than that using NFA-18S, which may be associated with a larger dipole moment and lower energy loss (Eloss 0.524 vs 0.566 eV, dipole moment 5.00 vs 1.87 D). The enhanced short-circuit current density and fill factor in organic solar cells based on NFA-17AS result from effective exciton dissociation, better charge transport balance, and suitable nanoscale morphology. This work shows that using different terminal units in nonfullerene acceptors effectively improves the photovoltaic performance of organic solar cells by reducing the exciton binding energy and energy losses through increased dipole moments in the nonfullerene acceptor.