Breaking the “Seesaw Effect”: 20.72% Efficiency Organic Solar Cells via Cascade Energy Transfer with a Fluorescent Sensitizer

Though organic solar cells (OSCs) have achieved remarkable progress recently, the intrinsic limitation of the "seesaw effect" between open-circuit voltage (V_oc) and short-circuit current density (J_sc) hinders their further improvements. In this study, a fluorescent small molecule, namely SD-EDOT, is incorporated into the widely used PM6:BTP-eC9 system to overcome this challenge. The addition of SD-EDOT lowers the energetic disorder and electron-phonon coupling within the system. More importantly, it forms an energy cascade between donor and acceptor, enabling a side cascade energy transfer pathway and a more favorable energetic landscape. The high fluorescent property of SD-EDOT improves the utilization rate of high-energy excitons. These effects collectively promote faster and more efficient charge generation (improving J_sc), deeper highest occupied molecular orbit of donors, and suppressed non-radiative energy loss (elevating V_oc). Consequently, the ternary devices achieve simultaneously higher V_oc and J_sc, significantly boosting the power conversion efficiency from 18.75% to 20.72%. This study provides a rational strategy for mitigating the V_oc-J_sc trade-off by incorporating a highly fluorescent third component with tailored energy levels, offering a promising pathway toward higher-performance OSCs.