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

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 (ΔE) 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 ΔE (∼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 ΔE 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.