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

ABSTRACT 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.