Congested Ring‐Fused Perylene Diimide Acceptors With Elevated Triplet Levels Suppressing Non‐Radiative Energy Loss in Organic Solar Cells

ABSTRACT Non‐radiative energy loss via low‐lying triplet excitons remains a critical bottleneck for advancing the performance of non‐fullerene acceptor (NFA)‐based organic solar cells (OSCs). Elevating triplet energy levels of photovoltaic materials is a direct and viable way to suppress this loss. Beyond the established thermally activated delayed fluorescence approach, here we demonstrate a congested ring‐fusion strategy that raises the triplet energy levels by inducing an antibonding character of the highest occupied molecular orbital and a bonding character of the lowest unoccupied molecular orbital between spatially adjacent carbon atoms, thereby hindering the formation of low‐lying triplet excitons and suppressing the non‐radiative loss (Δ E 3 ). Compared with the uncrowded analogue TPA‐PDI, OSCs employing the congested, ring‐fused f‐Ph‐PDI exhibit a shorter π–π stacking distance (3.95 vs. 4.05 Å), ∼2 times faster charge mobility, lower Urbach energy (30.3 vs. 38.5 meV), and an order‐of‐magnitude higher electroluminescence quantum efficiency corresponding to a 64 meV reduction in Δ E 3 . These improvements collectively propel the PCE past 10% — more than double that achieved with the extended, unfused counterpart (4.97%). This work establishes congested ring‐fusion as a potent strategy for designing novel NFAs with high triplet energy levels to mitigate non‐radiative recombination and advances high‐efficiency OSCs.