Phthalimide-based unfused-ring non-fullerene acceptors for constructing efficient organic solar cells with high open-circuit voltage

Unfused-ring non-fullerene acceptors (UF-NFAs) have garnered substantial attention because of their simple chemical structure and easy synthesis, making them promising candidates for advancing the application of organic solar cells (OSCs). The design and synthesis of UF-NFAs with upshifted lowest unoccupied molecular orbitals (LUMOs) can improve the open-circuit voltage (VOC) and thus the photovoltaic performance of devices. Herein, we developed three UF-NFAs, namely, BPD-H, BPD-F, and BPD-Cl, using the phthalimide electron-deficient unit as the building block. Because of the stronger electron-withdrawing effect of chlorine atoms, BPD-Cl exhibited higher molar extinction coefficients and red-shifted absorption than BPD-H or BPD-F. Additionally, the PM6:BPD-Cl-based blended film displayed highly efficient exciton separation, better charge transportation, and an appropriate active layer morphology. Thus, PM6:BPD-Cl-based OSCs achieved a higher PCE of 10.72%, outperforming PM6:BPD-H-based (6.44%) and PM6:BPD-F-based (7.98%) devices. Our findings indicate that choosing the phthalimide electron-deficient unit as the building block and modifying terminal groups are promising strategies for fabricating efficient UF-NFAs.