Rationalizing NiO x Processing through XPS Chemical Analysis: A Case Study on p-i-n Perovskite Solar Cells

Nonstoichiometric nickel oxide (NiOx) is a promising inorganic hole transport layer (HTL) for perovskite solar cells (PSCs), yet its surface chemistry lacks definitive modeling. Here, we investigate the evolution of chemical species and optoelectronic properties in ultrathin NiOx films deposited via ion beam sputter deposition (IBSD) under various thermal and oxidative treatments. Using in situ and ex situ X-ray photoelectron spectroscopy (XPS), we resolve up to seven distinct oxygen species in the core level O 1s spectra, including oxygen bound Nivac3+, Ni(OH)2, NiOOH, and interstitial oxygen (O–). Our results support that p-type conductivity, transparency, and band alignment in NiOx are governed directly by the concentration of Ni3+ related to nickel vacancies. Additionally, UV/Ozone exposure is shown to introduce NiOOH, enhancing surface wettability and increasing the work function by the creation of a surface dipole, improving charge extraction but reducing the long-term stability of NiOx films due to hygroscopic decomposition. High temperature deposition at 300 °C yields the best balance between optoelectronic properties, recombination, and most stable surface chemistry, yielding the best photovoltaic performance among tested conditions. This work establishes a robust spectroscopic framework for studying and engineering vacancy-controlled NiOx interfaces, with implications for improving the stability and efficiency of inorganic HTLs in p-i-n PSCs and other applications.