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 [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%. Self-assembled monolayers are common hole-selective contacts in inverted perovskite solar cells, but acidic head groups reduce interface quality. Yang et al. neutralize these groups with alkali metal phosphonate salts and mixed layers, improving charge extraction, stability, and efficiency.