Dimeric Acceptor with Small Singlet‐Triplet Energy Gap Enables Suppressed Triplet Loss and 20.85% Efficiency of Organic Solar Cells

Abstract The relatively high non‐radiative energy loss has become a major limiting factor for improving the performance of organic solar cells (OSCs), with triplet exciton formation being a primary source. Narrowing the energy gap between the first singlet and triplet excited states (Δ E ST ) in low‐bandgap acceptors is considered an effective strategy to mitigate this issue. In this work, we design and synthesize a dimeric acceptor, DY‐TXT, utilizing a thermally activated delayed fluorescence (TADF) molecule as the bridging unit. This novel structure exhibits a higher photoluminescence quantum yield and a significantly reduced Δ E ST (∼0.1 eV) compared to conventional nonfullerene acceptors. When incorporated into the D18:L8‐BO host system, DY‐TXT enhances the electroluminescence quantum efficiency and markedly suppresses triplet exciton generation, thereby reducing energy loss via triplet states. The small Δ E ST also facilitates reverse intersystem crossing process, enabling recycling of triplet excitons. Consequently, the resulting ternary device achieves a low non‐radiative energy loss of 0.194 eV and an outstanding power conversion efficiency of 20.85%. This work demonstrates an effective strategy for suppressing triplet‐mediated energy losses and provides a promising avenue for advancing the performance of OSCs.