Ionic Self-Assembled Monolayers Enable Neutral Interfaces and Synergistic Charge Extraction in High-Efficiency Perovskite Solar Cells

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 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 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 perovskite–perovskite tandem solar cells with an enhanced PCE of 29.05%.