Cooperative Dipole and Anchoring Engineering via Bimolecular Self‐Assembled Monolayer for Organic Solar Cells with Efficiency of 20.26%

ABSTRACT Self‐assembled monolayers (SAMs) are attractive hole‐transporting layers for solution‐processed organic solar cells (OSCs). However, the formation of defect‐free and densely packed films is often hindered by poor dispersibility, micelle‐like aggregation, and incomplete surface coverage. Here, we develop a cooperative dipole and anchoring strategy by pairing an asymmetric, conjugation‐extended phosphonic‐acid SAM (3‐BFCz) with a fluorinated quinoline‐4‐carboxylic acid co‐adsorbate (QLCA‐F) to construct a bimolecular co‐adsorbed interface (CA). Owing to its enlarged dipole moment (2.70 D vs. 1.89 D for 2PACz), 3‐BFCz increases the ITO work function (WF) from 4.90 to 5.01 eV, while co‐adsorption with QLCA‐F further raises the WF to 5.07 eV and yields a more uniform, densely packed SAM layer. As a result, the CA interface mitigates interfacial energy/charge losses, suppresses leakage and recombination, and facilitates hole extraction/transport, delivering consistently improved device performance across multiple high‐efficiency photoactive layer systems: the champion PCEs are enhanced from 19.02% to 19.54% (PM6:L8‐BO), from 19.10% to 19.62% (D18:N3), and from 19.69% to 20.26% (D18:L8‐BO‐X). Moreover, CA markedly improves device stability, extending the T ₈₀ lifetime at 60°C by 43%. This work highlights bimolecular co‐adsorption as a transferable route to simultaneously optimize WF tuning, interfacial assembly quality, and device stability for high‐efficiency OSCs.