Nickel Oxide Hole Transport Layers for Efficient and Stable Perovskite Solar Cells: Recent Progress and Prospects

Among the various perovskite solar cell (PSC) architectures, inverted (p‐i‐n) structured PSCs are particularly promising for commercialization owing to their minimal hysteresis, excellent operational stability, and compatibility with low‐temperature solution processing on flexible substrates. Nickel oxide (NiO X ), a hole transport layer used in such devices, has been widely studied because of its high hole mobility, good chemical stability, low cost, and suitable energy‐level alignment with perovskite absorbers. However, its performance is limited by inherent issues such as the high defect density, low intrinsic conductivity, and Fermi‐level misalignment. This review systematically summarizes three major strategies for addressing these challenges: ionic doping, interfacial molecular modification, and energy‐field‐assisted surface treatment. In addition, recent advances in these methods are discussed, focusing on their mechanisms, advantages, and limitations, and an outlook toward performance improvement and the future industrial application of NiO X ‐based PSCs is provided.