This study reports on electronic properties and cationic defects in the thin films of non-stoichiometric nickel oxide (NiOx) synthesized with plasma-enhanced atomic layer deposition (PEALD). Starting from nickel acetylacetonate [Ni(acac)2] as a metal-organic precursor, we employed water (P-W), ozone (P-Oz), and oxygen plasma (P-Ox) as oxidizing agents for direct plasma-enhanced ALD (DPALD) to evaluate their impact on the electronic structure of NiOx. The primary objective is to identify the most suitable oxidizing agent for producing NiOx films with optimal performance for hole transport layers (HTLs) in optoelectronic devices. Angle-resolved X-ray photoelectron spectroscopy (AR-XPS) confirmed the non-stoichiometric nature of our films. Measurements made at the 45° takeoff angle revealed significant variations in the Ni2+/Ni3+ ratio depending on the oxidizing agent chosen, increasing from 0.80 (P-W) to 0.89 (P-Ox). This trend indicates improved defect formation control, permitting to achieve valence band shift of 0.98 eV for P-Ox sample relative to EV of MAPbI3. Additionally, reflection electron energy loss spectroscopy (REELS) revealed band gap narrowing from 3.53 eV (P-W) to 3.38 eV (P-Ox), suggesting enhancement of electronic properties. Band alignment analysis based on XPS and REELS data demonstrates that NiOx films synthesized with oxygen plasma exhibit favorable band alignment with MAPbI3 perovskite. Moreover, contact angle measurements reveal that subtle changes in defect chemistry affect the wettability of NiOx surfaces with respect to perovskite precursor solutions. Put together, these findings identify oxygen plasma-assisted synthesis as the most promising strategy for fabricating NiOx-based HTLs with superior electronic properties and advantageous interface characteristics, which are promising for the next-generation optoelectronic devices. This work emphasizes the importance of PEALD oxidazing agent choice for achieving a precise tailoring of NiOx electronic structure and enhancing the resulting device performance.
