Ultra‐High Dielectric Acceptor Enables 21% Efficiency and Thickness‐Insensitive Organic Solar Cells

Dielectric constant (ε r ) of non‐fullerene acceptors (NFAs) is a crucial parameter in organic solar cells (OSCs), significantly influencing exciton dissociation efficiency and charge recombination dynamics. However, the state‐of‐the‐art NFAs typically exhibit relatively low dielectric constants (ε r ≈ 3–5), which inherently limit device performance by promoting substantial recombination losses. Herein, a new guest acceptor named L8‐BO‐FO is reported by replacing the branched alkyl chains on L8‐BO with dimethyl ether units, which presents a higher ε r value of 7.41 than that of L8‐BO (4.57). Subsequently, the high‐ε r L8‐BO‐FO was introduced into the PM6:L8‐BO system to improve the dielectric property of the active layer, which, encouragingly, increases the ε r value of the active layer, accelerates exciton dynamics, and suppresses the nonradiative charge recombination. As a result, the power conversion efficiency (PCE) of the ternary OSCs is boosted up to 21.0%. Surprisingly, the ternary OSCs maintained an outstanding PCE of 19.1% even when the active‐layer thickness increased to 300 nm. This work brings new insights into the design of high‐performance OSCs with excellent thickness tolerance by dielectric engineering, which is essential for high‐throughput and scale‐up fabrication of OSCs.