Modulating Interfacial Energetics Through Stoichiometry and Defect Engineering in Solution-Processed Vanadium Oxide Hole Transport Layers for Perovskite Solar Cells

Low-temperature, solution-processed vanadium oxide (VOx) emerges as a chemically versatile p-type transition metal oxide, offering tunable stoichiometry, variable oxidation states, and adaptable defect landscapes, all central to its function as a hole transport layer (HTL) in inverted perovskite solar cells (PSCs). However, its practical deployment is limited by high-temperature processing conditions and insufficient understanding of the influence of processing conditions on its electronic and interface quality. In this study, using a low-temperature solution process, we present for the first time a systematic investigation of VOx films annealed under oxygen-rich and nitrogen-rich environments, highlighting how annealing atmosphere and temperature (varied from 150 to 300 °C) critically influence the stoichiometry, energy levels, film morphology, etc. Comprehensive characterisations, including UV–vis spectroscopy, X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), work function analysis (WF), surface photovoltage (SPV), and conductivity studies, were employed to establish direct correlations between processing conditions and HTL performance. Crucially, the 150 °C O2-annealed VOx yields a chemically uniform, electronically favorable HTL, enabling coherent growth of FA0.9Cs0.1PbI3 absorbers with reduced Urbach energies and enhanced interfacial energetics. We further demonstrate, for the first time, the integration of VOx with this double-cation composition in inverted PSCs, achieving strong device performance and confirming its suitability for advanced applications. These results establish direct processing-chemistry-property-performance correlations, offering a chemical blueprint for engineering stoichiometry, oxidation states, and defect chemistry in VOx for next-generation inorganic interlayers.