Dual Suppression of Triplet Exciton and Energetic Disorder via Organic Room‐Temperature Phosphorescent Additive to Reduce Energy Loss in Organic Solar Cells

Non‐radiative energy loss originating from triplet exciton generation and intrinsic energetic disorder severely restricts the efficiency breakthrough of organic solar cells (OSCs). To address this dual bottleneck, herein, two benzodiimide‐based solid additives with distinct photophysical properties, namely fluorescent HPmDI and room‐temperature phosphorescent (RTP) BrPmDI, are developed to simultaneously suppress triplet exciton dissipation and mitigate energetic disorder for high‐efficiency OSCs. Both additives construct favorable intermolecular interactions with the D18 donor and L8‐BO acceptor, optimizing active layer molecular packing, enhancing film crystallinity, and reducing Urbach energy to alleviate intrinsic energetic disorder. Benefiting from the unique long‐lived triplet exciton characteristics of organic RTP BrPmDI, an efficient Dexter energy transfer (DET) pathway is additionally enabled, which effectively inhibits non‐radiative decay channels, including intersystem crossing and charge recombination‐induced triplet exciton formation. Consequently, the optimized D18:L8‐BO‐based OSCs modified with BrPmDI achieve a champion power conversion efficiency of 20.52% with a low non‐radiative energy loss of 0.217 eV, far outperforming the pristine device (19.01% and 0.226 eV) and HPmDI‐modified device (19.70% and 0.219 eV). This work reveals the dual regulatory mechanism of organic RTP additives on triplet exciton dynamics and energetic disorder, providing a facile strategy for the design of high‐efficiency additive materials toward low‐energy‐loss and high‐performance OSCs.