Ionic self-assembled monolayers enable neutral interfaces and synergistic charge extraction in high-efficiency...

Self-assembled monolayers (SAMs) have become indispensable hole-selective contacts for high-efficiency inverted perovskite solar cells (PSCs). However, the intrinsically acidic head groups of conventional SAMs lead to interfacial inhomogeneity, limited charge transfer, and poor operational stability. Here, we introduce a family of alkali metal-based phosphonate salts (2PACz-M) through targeted head-group functionalization of the benchmark [2-(9H-carbazol-9-yl)ethyl]phosphonic acid (2PACz) SAM, achieving a chemically neutralized and electronically delocalized interface. The ionic phosphonate moiety enhances π-electron conjugation, improves energy-level alignment, and strengthens chemical coordination with metal oxide electrodes, resulting in homogeneous and stable surface coverage. Moreover, when combined with [4-(3,6-dimethyl-9H-carbazol-9-yl)butyl]phosphonic acid (Me-4PACz), the mixed-SAM interface exhibits a synergistic effect that facilitates efficient hole extraction, suppresses non-radiative recombination, and reinforces environmental robustness. This interfacial engineering enables 1.55 eV PSCs to achieve a champion power conversion efficiency (PCE) of 26.88% with a fill factor (FF) of 86.57%, alongside a 23.32% PCE for a 29.7 cm² module. The SAM synergy proves universal across perovskites of varied bandgaps, yielding two-terminal (2T) all-perovskite tandem solar cells with an enhanced PCE of 29.05%.