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

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 T80 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.