Nickel oxide (NiOₓ) has emerged as a leading inorganic hole-transport layer (HTL) for inverted perovskite solar cells because it combines a wide optical band gap (3.6–4.0 eV) and high transparency with a high work function enabling favorable valence-band alignment and strong electron-blocking selectivity that suppresses interfacial recombination [1]. Its chemically and structurally stable oxide lattice supports improved thermal stability and photostability compared to common organic HTLs, while offering low-cost materials and industrially relevant processing routes. Nevertheless, the key properties for device performance, such as hole conductivity, interfacial recombination and energetic alignment, are highly sensitive to the defect chemistry and surface termination of NiOₓ. Since NiOₓ relies on nickel-vacancy driven self-doping and associated Ni³⁺ formation to generate holes, the attainable hole density and conductivity in sputtered films often remain insufficient for low-loss charge extraction [2]. Relating to those challenges, surface engineering via plasma offers a direct route to tune the defect chemistry and surface termination of NiOₓ, enabling improved hole extraction and reduced interfacial recombination.

We introduce a set of plasma-enabled process routes that turn sputter-deposited NiOₓ into a tunable and scalable HTL for perovskite solar cells, targeting improved reproducibility and stability. Plasma is leveraged as a controllable source of radicals, ions and UV photons that can modify oxides without introducing wet-chemical residues, while allowing independent control over chemical reactivity and ion energy. For example, a dielectric-barrier-discharge activation prepares the substrate by removing adventitious contamination and increasing surface energy, which stabilizes NiOₓ growth and improves film continuity and adhesion at technologically relevant thicknesses. After deposition, localized atmospheric plasma-jet processing provides spatially resolved control over near-surface stoichiometry and the Ni³⁺/Ni²⁺ balance, allowing for adjustment of band gap and interfacial energetics while reducing recombination-active sites at the NiOₓ/perovskite junction. To address bulk transport limitations without compromising optical transmission, HiPIMS-driven plasma ion implantation is discussed as a route to introduce controlled transition-metal dopants with tunable ion energy and pulse conditions, enabling deliberate steering of dopant depth profiles, charge state, and defect formation across 5 ‑ 40 nm NiO_x.

Ultimately, our work aims to identify which plasma species and regimes most effectively suppress interfacial losses while enhancing hole transport, providing a mechanistic blueprint for NiOₓ contacts that remain effective through perovskite deposition and device operation.