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_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 ΔE_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 ΔE_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.