Computational insights into phthalimide-core non-fullerene acceptors for next-generation organic solar cells

Traditional fullerene-based acceptors in organic solar cells (OSCs) suffer from limitations such as poor tunability, narrow absorption spectra, and limited morphological stability, restricting further improvements in device efficiency. To address these challenges, non-fullerene acceptors (NFAs) with tunable energy levels and broad optical absorption have gained increasing attention. In this study, seven novel phthalimide core-based donor-acceptor molecules (BPDM1-BPDM7) are computationally designed by modifying the terminal units of a reference molecule (BPDF). Using density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations at the B3LYP/6-31G(d,p) level, photovoltaic parameters such as bandgap, frontier orbital energies, absorption maxima, light harvesting efficiency (LHE), reorganization energies (λ _e, λ _h), dipole moments, open-circuit voltages (V _oc), and fill factors (FF) are systematically evaluated. The designed derivatives exhibit narrow bandgap (1.84-2.18 eV) and red-shifted absorption (681-829 nm). BPDM3 shows the lowest bandgap (1.84 eV). BPDM7 achieves the highest dipole moment (3.88 D), V _oc (1.93 V), and FF (92.96%), while BPDM4 displays the lowest λ _e and λ _h values, indicating superior charge mobility. The reduced exciton binding energies across all derivatives suggest enhanced charge separation capabilities. These results not only provide a pathway for rational NFA design but also serve as a theoretical foundation for future experimental development of high-efficiency OSCs.