Bidentate Phosphate Molecular Bridge Heals Buried Interface Defects for Stable NiO _<b>x</b> -Based Perovskite Solar Cells

Buried interface defects in NiOx-based perovskite solar cells (PSCs) severely limit the device performance and stability. While phosphonic acid self-assembled monolayers (SAMs) improve interfacial properties, their monodentate anchoring leads to molecular heterogeneity and nanovoids which induce nonradiative recombination. Here, we introduce 2,2′-bipyridine-4,4′-diylbis(phosphonic acid) (BPyPA) as a bidentate molecular bridge to homogenize the NiOx/SAMs interface. The dual phosphonic acid groups of BPyPA provide strong bidentate anchoring to the NiOx surface, eliminating interfacial nanovoids and significantly enhancing wettability to perovskite precursors. This interfacial healing optimizes the energy alignment of the NiOx substrate, passivates the undercoordinated Ni and Pb sites, and suppresses interfacial nonradiative recombination. Consequently, BPyPA-modified devices achieved a power conversion efficiency (PCE) of 24.56% with a high open-circuit voltage of 1.193 V. Unencapsulated devices with BPyPA modification retained >90% of their initial PCE after 2500 h of dark storage in N2 and 84.6% of their initial PCE after 1200 h under ambient conditions (ISOS-D-1 protocols). This bidentate anchoring strategy presents a universal approach for mitigating interfacial heterogeneities in highly efficient and stable PSCs.