Single-junction perovskite solar cells (PSCs) have achieved certified powerconversion efficiencies (PCEs) of 26.1%, which approaches their practicalperformance limit. Multi-junction tandem solar cells can unlock even higherPCEs, where narrow-bandgap lead-tin (Pb-Sn) perovskites, with a bandgapof 1.21–1.25 eV, are well-suited as the bottom photo absorber in all-perovskitetandems. Bulk engineering and surface treatments of Pb-Sn perovskites usingLewis base molecules have been shown to reduce the defect density withinthe bulk and at the electron transport layer interface, thereby improving deviceperformance. Nevertheless, the buried interface between Pb-Sn perovskiteand the commonly used hole transport layer PEDOT:PSS remains problematicdue to the reactivity of polystyrene sulfonate (PSS) with Sn 2+ ions,which negatively impacts device performance. To overcome this issue, a novelcarbazole-based self-assembled monolayer, BrNH3 -4PACz is synthesized,that provides a suitable dipole moment at the indium-tin oxide interfacefor efficient hole extraction and features an ionic ammonium head groupthat passivates the perovskite at the buried interface. This dual functionalityenabled the fabrication of a p-i-n architecture Pb-Sn PSC with a bandgapof 1.24 eV, achieving a champion PCE of 23% and an open-circuit voltageof 0.88 V, which ranks among the highest reported values in the literature.