Molecular tuning of non-fullerene electron acceptors in organic photovoltaics: a theoretical study

Modifying the backbone, side chains and end groups of non-fullerene acceptors (NFAs) in organic photovoltaics (OPVs) can tune their properties and impact performance. Hence, understanding how different functional groups influence properties and performances is critical for developing novel NFAs for OPVs. Herein, to investigate the effects of introducing methyl groups and substituting dicyano with O, based upon extensive quantum chemistry calculations, we selected a PM6:IT-4F heterojunction as the reference system and studied the geometries, electronic structures, excitation properties, and electrostatic potentials (ESPs) of PM6, IT-4F, IM-4F, and IO-4F as well as PM6:IT-4F, PM6:IM-4F, and PM6:IO-4F complexes as interface models. The rate constants of the charge transfer (CT), exciton dissociation (ED), and charge recombination processes at the heterojunction interface were also calculated. The results indicated that the superior performance of PM6:IM-4F and PM6:IT-4F systems could be ascribed to the redshift of optical absorptions that assure more photon harvest, larger ESP difference between PM6 and NFAs, and larger excitation energy difference between NFA's local excitation and CT that is favorable for efficient ED. However, the larger open-circuit voltage of PM6:IO-4F OPV results from the substitution of dicyano with O, causing a significant increase in the lowest unoccupied molecular orbital and CT excitation energies of PM6:NFA complexes, whereas the smallest short-circuit current density is mainly related to less CT excitations, blue-shift optical absorption that is not complementary with that of PM6, inefficient ED, and the smaller ESP difference between PM6 and IO-4F.