Mitigating Phosphonic Acid–Perovskite Interfacial Degradation via Molecular Engineering for Ultra-Stable Solar Cells

• Weakly bound phosphonic acid self-assembled monolayers (PA-SAMs) on ITO undergo desorption under photothermal stress, triggering three destructive reactions with the perovskite: iodide oxidation, formamidinium decomposition, and lead reduction, which severely degrade device performance. • A novel triphenylamine-based phosphonic acid (1PA-TPD) with a single anchoring group forms robust covalent In–O–P bonds with ITO, reducing molecular detachment from ~30% to ~11% compared to conventional SAMs. • The optimized mixed SAM system (60% 1PA-TPD + 40% EtCz3EPA) simultaneously enhances substrate binding, inhibits interfacial reactions, improves crystallinity, and passivates defects, achieving a PCE of 25.0% and a T90 lifetime of ~3000 hours in small-area PSCs. • This work identifies a previously overlooked degradation pathway and establishes a universal molecular design principle for stable interfacial layers, advancing the commercialization of perovskite solar cells.