Fully Reduced Core-Shell Organotin-Polyoxometalate {Sn₁₂Mo₁₉}: A Synergistic Modulation Material for the Buried Interface of Inverted Perovskite Solar Cells

The commercialization of inverted perovskite solar cells (PSCs) suffers from poor buried interface and interfacial charge loss at the hole transport layer. Therefore, designing and synthesizing materials that can precisely optimize the buried interface is crucial. Herein, we employ a molecular design strategy to synthesize an organotin-functionalized fully reduced polyoxometalate (POM), (NH₄)₂H₁₅[Sn^IV ₃(CH₃)₃(OH)₃O]₄[Mo^V ₁₉O₆₄]·20H₂O ({Sn₁₂Mo₁₉}), which serves as a molecular-level synergistic interfacial layer between NiO_x and a self-assembled monolayer (SAM) in PSCs. {Sn₁₂Mo₁₉} features a fully reduced Mo-oxo cluster core surrounded by four organotin shells, endowing the entire molecule with a T_d-symmetric triply-nested architecture that enables ordered spatial organization of bifunctional moieties. Comprehensive experimental and theoretical results reveal that nanoscale, well-defined {Sn₁₂Mo₁₉} fills voids within the NiO_x layer, where its Mo-oxo core suppresses detrimental Ni⁴⁺ and the organotin shell induces SAM homogenization via hydrogen bonding. Such synergistic effects substantially improve the quality of the buried interface by suppressing interfacial side reactions and SAM spatial mismatch. Consequently, the V_oc of PSCs increases by 3.31%, and PCE rises from 23.81% to 26.11%, setting a new record for POM-modified PSCs and rendering {Sn₁₂Mo₁₉} universal across various SAMs. This work demonstrates that POM molecular engineering can precisely address interfacial challenges, offering new design concepts and technical routes for broader photovoltaic applications.