A Fused-Ring Electron Acceptor with Phthalimide-Based Ending Groups for Efficient Ternary Organic Solar Cells

The ternary strategy has been widely applied and recognized to be a valid strategy to enhance the organic photovoltaics' (OPVs) performance. Here, a new fused-ring electron acceptor, BTP-PIO, is designed and synthesized, whose ending groups were replaced by a phthalimide-based group (2-butylcyclopenta[f]isoindole-1,3,5,7(2H,6H)-tetraone) from traditional 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile. The phthalimide-based ending groups endow BTP-PIO with the highest lowest unoccupied molecular orbital (LUMO) level and wider band gap than those of Y6. The ternary device based on PM6:Y6 with BTP-PIO as a guest electron acceptor achieved an elevated open-circuit voltage (V_OC) of 0.848 V, a short-circuit current density (J_SC) of 27.31 mA cm^-2, and a fill factor (FF) of 73.9%, generating a remarkable power conversion efficiency (PCE) of 17.10%, which is superior to the PM6:Y6 binary device of 16.08%. The ternary device exhibited improved charge transfer, suppressed carrier recombination, and lower energy loss. BTP-PIO exhibited a good miscibility with Y6, and an alloy phase between BTP-PIO and Y6 was formed in the ternary bulk heterojunction, leading to better phase separation and molecular packing. This research reveals that ending group modification of Y6 derivatives is a feasible way to produce highly efficient ternary devices.