Novel Fused-Ring Electron Acceptors Based on Double Electron-Deficient Central Core for High-Performance Organic Solar Cells

Rapid advances in A-DA'D-A fused-ring electron acceptors have significantly boosted organic solar cells (OSCs), yet non-radiative energy loss (ΔE₃) remains a persistent bottleneck, and reliance on single electron-deficient cores limits further molecular diversification. Herein, we report CBT-4F, a novel acceptor featuring a double electron-deficient bis([1,2,5]thiadiazolo)[3,4-c:3',4'-g]carbazole (CBT) core constructed via stepwise intramolecular Cadogan reductive cyclization. This synthetic strategy circumvents the challenges of carrying multiple nitro groups through key cross-coupling steps and offers an extensible route to multi-electron-deficient-core acceptors. Compared with the benchmark acceptor Y6, CBT-4F exhibits extended π-conjugation, increased backbone curvature, a lower average molecular electrostatic potential, and a distinct packing motif. These coupled molecular and solid-state characteristics, together with optimized blend morphology and recombination behavior, are associated with significantly suppressed ΔE₃ in PM6:CBT-4F devices (0.220 vs. 0.262 eV for PM6:Y6). Benefiting from a wider photovoltaic bandgap and reduced ΔE₃, PM6:CBT-4F achieves a power conversion efficiency (PCE) of 19.02% with a high open-circuit voltage (V_OC) of 0.923 V, substantially outperforming the PM6:Y6 counterpart (17.63%, 0.834 V). Furthermore, incorporating CBT-4F as a third component into the PM6:BTP-eC9 system boosts the ternary PCE to 20.57%. This work establishes CBT-4F as a high-performance molecular platform and expands synthetic accessibility to multi-electron-deficient-core acceptors.