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.