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 (NiOx) has been highlighted by its recent application in organometallic halide PSCs, due to the favorable band alignment formed between the halide perovskite absorber and NiOx HTL. This comprehensive review summarizes the recent progress in the fabrication of NiOx films and their application in PSCs. Special attention is paid to the optoelectronic properties of NiOx 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 NiOx films and the resulting device performance are discussed with emphasis. Finally, a short perspective of NiOx‐based PSCs is also provided.

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Flexible solar cells have been considered as a promising photovoltaic (PV) technology due to their intrinsic advantages such as lightweight and bendability, which make them very convenient for transportation, installation, and integration with architectures and wearable electricity‐generating devices. As a novel PV technology being compatible with roll‐to‐roll fabrication, flexible perovskite solar cells (FPSCs) have achieved a great progress within the past few years, partially due to the superior photoelectronic properties of the halide perovskite absorber and the easy fabrication of the device. As a result, the champion power conversion efficiency of the FPSCs has exceeded 18% recently. In this review, the recent developments of FPSCs are summarized and discussed in the following four aspects: perovskite absorbers, flexible substrates, transparent conductive electrodes, and carrier transport layers, which are the main components for a FPSC device. The flexibility, stability, and other properties are also discussed in terms of the specialized FPSC devices. Finally, a rough prediction for the future development of FPSCs is provided at the end of this review.

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