Solution-Processed Nickel Oxide as Efficient Hole Transport Layers in Inverted Perovskite Solar Cells

Nickel oxide (NiO_x), an efficient inorganic hole transport layer (HTL), has emerged as a key material driving the industrialization of inverted perovskite solar cells (PSCs). Its success stems from favorable energy-level alignment, excellent charge transport features, high stability, and low cost. Among various preparation methods, solution-based syntheses are particularly attractive for NiO_x-based PSCs, due to their simplicity, compatibility, and scalability. This review systematically summarizes four mainstream solution-based techniques for synthesizing NiO_x HTLs, including pre-synthesized nanoparticles, sol-gel, solution combustion, and chemical bath depositions. For each method, we discuss the reaction mechanisms and processing features, the resulting film properties, and strategies for performance optimization in PSCs. We then outline challenges associated with each route and highlight recent advances in material modification, process engineering, and structural design aimed at overcoming these bottlenecks and enhancing device efficiency. Further research should address critical issues such as uniformity control in large-area, rapid synthesis processes and reliable preparation of high-crystallinity, low-defect films at low temperatures. A deeper understanding of the relationships among solution processes, microstructure, and device performance will be essential for fully realizing the potential of solution-based NiO_x HTLs in high-performance, high-stability, and low-cost perovskite photovoltaic technologies.