Understanding and controlling interface chemistry is key to enhancing the performance and stability of halide perovskite (MHP) semiconductor devices. However, analyzing buried interfaces remains challenging due to their complex chemical reactivity and sensitivity to external stimuli. In this work, we employ a combination of hard X-ray photoelectron spectroscopy (HAXPES) and operando X-ray photoelectron spectroscopy (opXPS) to investigate the chemical and electronic structure of MHP interfaces with adjacent functional layers.[1]

Focusing on atomic layer deposited (ALD) SnO₂ and NiO layers on mixed-cation mixed-halide perovskite films, we demonstrate how advanced photoemission techniques enable the detection of buried chemical species and subtle changes in energy level alignment that are critical to device operation.[2] In particular, our HAXPES measurements reveal the formation of new interfacial species that can detrimentally affect charge transport, while opXPS provides insights into the dynamic chemical evolution of these interfaces under light and bias. Furthermore, we explore the impact of introducing protective organic interlayers between the perovskite and oxide layers, which can mitigate interfacial reactions and preserve favorable band alignment.

In this framework, we further investigate nickel oxide (NiOₓ), a widely used inorganic hole transport layer (HTL) in inverted MHP solar cells. Despite its advantageous optical and electronic properties, NiOₓ suffers from high surface reactivity and defect formation at the NiOₓ/MHP interface, limiting device performance. We show that ultraviolet-ozone (UVO) treatments, commonly employed to improve wettability and surface activation, can in fact enhance NiOₓ surface reactivity and introduce additional defect states. By systematically comparing pristine and UVO-treated NiOₓ interfaces, we identify the chemical nature of these defects and demonstrate that the insertion of a MeO-2PACz organic interlayer effectively mitigates interfacial reactions. Importantly, neither the UVO treatment nor the grafting of the organic interlayer significantly alters the bulk properties of NiOₓ, emphasizing the interfacial specificity of these modifications.

This methodological approach highlights the strength of combining synchrotron-based and lab-based XPS to gain a comprehensive understanding of interface stability and defect formation in MHP-based optoelectronics.