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

ABSTRACT 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.