Nickel Oxide as Efficient Hole Transport Materials for Perovskite Solar Cells

Organic–inorganic halide perovskite solar cells (PSCs) have achieved great success in recent years with a demonstrated power conversion efficiency (PCE) increasing rapidly from 3.8% to 22.3% for single junction devices. Most high-performance PSCs consist of a perovskite absorber sandwiched between an electron transport layer (ETL) and a hole transport layer (HTL), which extracts electrons (holes) and blocks holes (electrons) from the absorber efficiently. Inorganic hole transport materials have extracted extensive attention due to their higher mobility and better stability. Particularly, the excellent hole selective transport property of nickel oxide (NiO_x) has been highlighted by its recent application in organometallic halide PSCs, due to the favorable band alignment formed between the halide perovskite absorber and NiO_x HTL. This comprehensive review summarizes the recent progress in the fabrication of NiO_x films and their application in PSCs. Special attention is paid to the optoelectronic properties of NiO_x films, which strongly depend on the synthesis methods and post-treatment conditions, as well as the resulting photovoltaic device performance. Surface modification and doping strategies that are used to improve the optoelectronic properties of NiO_x films and the resulting device performance are discussed with emphasis. Finally, a short perspective of NiO_x-based PSCs is also provided.