Nickel Oxide (NiOx) Hole Transport Layers in Perovskite Solar Cells: Doping Strategies, Interface Engineering, and Device Performance

ABSTRACT

Perovskite solar cells (PSCs) have demonstrated remarkable progress, achieving power conversion efficiencies exceeding 25% within a decade. Their further advancements critically depend on the stability and efficiency of the hole transport layer (HTM). Although organic HTMs such as spiro‐OMeTAD have dominated early PSC architectures, their limited durability and high cost have prompted growing interest in inorganic alternatives. Among them, nickel oxide (NiOx) has emerged as a compelling candidate owing to its wide bandgap, high stability, and compatibility with the energy bands of perovskite absorbers. This review traces the development of NiO‐based HTMs, from early breakthroughs to the current state‐of‐the‐art, focusing on doped NiO and NiO composites that enhance stability and charge transport. We critically analyze the influences of various dopants and fabrication routes on PSC performance across n‐i‐p and p‐i‐n architectures and outline key challenges that will shape future research toward high‐efficiency, scalable NiO‐based perovskite solar cells.