Defect engineering and functional modulation of nickel oxide for electronic and energy devices

Nickel oxide (NiO) is a prototypical correlated transition-metal oxide in which Ni 3d–O 2p hybridization, charge-transfer interactions, and intrinsic defect chemistry govern its electronic structure and device-relevant functionality. In NiO thin films, p-type conduction, optical absorption, work-function alignment, and interfacial stability are strongly coupled to Ni vacancies, oxygen-related defects, mixed Ni 2+ /Ni 3+ valence states, localized polaron transport, and surface adsorbates. This review examines NiO thin films through three connected axes: defect chemistry, polaron-mediated charge transport, and interface energetics. The crystal structure, electronic band characteristics, defect-mediated p-type conduction mechanisms, and intrinsic conductivity–transparency–stability trade-offs of NiO are summarized, and physical-vapor, chemical-vapor, atomic-layer, and solution-based processing routes are compared. Emphasis is placed on how the oxygen partial pressure, substrate temperature, dopant incorporation, post-annealing, nanostructuring, and interface passivation control the V Ni /V O populations, Ni 3+ /Ni 2+ ratios, microstructure, work-function alignment, and interfacial band offsets. Across photovoltaic, electrochromic, gas-sensing, catalytic, and electrochemical energy-storage applications, this review argues that NiO optimization should be understood not as the maximization of the dopant concentration or defect density, but as the stabilization of an application-specific defect/redox window. By reorganizing reported advances into cross-cutting design rules and device-specific descriptors, this review provides a materials-chemistry framework for linking NiO defect populations, Ni 2+ /Ni 3+ redox chemistry, interfacial energetics, and coupled ion/electron transport to energy conversion, electrochemical modulation, surface-redox reactions, and storage functions.