Inner-Sphere Nitrate Enables Highly Stable Aqueous NiOx Nanoparticle Inks and Promotes Phosphonic-Acid SAM Anchoring for Efficient Inverted Perovskite Solar Cells

Aqueous processing of NiO x nanoparticle (NP) hole-selective layers suffers from a narrow precipitation window, poor colloidal stability, and poorly controlled surface chemistry that hampers the adsorption of phosphonic-acid self-assembled monolayers (SAMs). Here we introduce an inner-sphere nitrate surface-engineering strategy that converts trace Ni(NO 3 ) 2 , intentionally retained within Ni(OH) 2 precipitates, into strongly coordinated NO 3 − ligands during the Ni(OH) 2 -to-NiO x calcination. The resulting NiO x NPs form highly stable aqueous inks up to 40 mg mL −1 for >30 days and pass a 0.22 µm filter with ∼95% efficiency. By controlling base-addition kinetics during precipitation, we decouple particle size from nitrate coverage, tuning the surface NO 3 − /lattice-O ratio from 0.05 to 0.16 while maintaining an ∼7 nm NP size. The coordinated nitrate provides efficient anchoring sites for carbazole-based phosphonic-acid SAMs (e.g., MeO-2PACz), which increase effective SAM loading, enhance hole extraction, and suppress non-radiative recombination. Devices with the structure FTO/NiOx/SAM/perovskite/C 60 /BCP/Ag deliver a champion PCE of 26.1% (vs 24.4% for nitrate-poor NiO x ) and retain 80% of the initial efficiency after ∼1100 h MPP tracking at 65°C under encapsulation. This inner-sphere anion engineering establishes a simple, aqueous-compatible route to programmable NiO x surface chemistry, enabling reproducible ink processing and robust NiO x /SAM interfaces for high-efficiency inverted perovskite solar cells.