Competitive Ligand Modulation Reduces Buried Interfacial Voids and Defects Enabling 26.17% Efficient Perovskite Solar Cells

The buried interface between perovskite and SnO 2 is plagued by defects and voids, limiting n‐i‐p solar cell performance. It is challenged for interfacial modification such as potassium salts to simultaneously passivate defects and modulate the buried PbI 2 ·DMSO adduct. Here, we introduce a heterocyclic potassium salt, acesulfame potassium (Ace‐K), with dual C═O and ‐SO 2 ‐ groups for defect passivation and competitive ligand modulation. Ace‐K anchors uncoordinated Sn 4 + and oxygen vacancies on SnO 2 via bidentate chelation. During PbI 2 deposition, Ace‐K competes with DMSO for PbI 2 , reducing PbI 2 ·DMSO at the SnO 2 /perovskite interface. This reduction leads to a void‐free perovskite bottom interface. Concurrently, it promotes DMSO escape to create a porous PbI 2 structure, which facilitates organic salt penetration and yields high‐quality perovskite films with released residual stress. Ace‐K remaining at the interface enhances charge transfer kinetics. Consequently, the champion device (0.0729 cm 2 ) achieves a lab‐measured PCE of 26.17% with an open‐circuit voltage of 1.19 V. The heterocyclic structure also imparts UV resistance, and the devices retain 92.4% of their initial efficiency after 1000 h of maximum power point tracking under continuous illumination. This work demonstrates a competitive ligand modulation strategy, offering a microstructural pathway toward efficient and stable perovskite photovoltaics.