Nickel Oxide for Perovskite Photovoltaic Cells

Organic–inorganic perovskite solar cells (PSCs) have shown tremendous progress from 3.8% power conversion efficiency (PCE) in 2003 to 25.2% in 2020 because of their wide range of light absorption, fast charge separation, long carrier diffusion length, and long carrier lifetime. The optoelectronic characteristics of hole transport material (HTM) and electron transport material (ETM) strongly affect photovoltaic (PV) performance and stability of PSCs. Recently, various inorganic HTMs with high efficiency, stability, and cost‐effectiveness have been investigated. Among them, nickel oxide (NiO x ) is one of the most studied inorganic HTMs in terms of device performance and stability because it has high hole mobility, electrical conductivity, transmittance, and energetically favorable band alignment, along with environmental stability. This article overviews the recent progress on NiO x ‐based planar PSCs. The main focus is on the structural, electrical and optical properties of the NiO x thin film, which mainly depends on the synthesis methods and post‐treatments. Firstly, a variety of methods are investigated to fabricate dense, compact and high crystallized NiO x thin film. Moreover, multifarious doping strategies and surface functionalization using organic materials are summarized as approaches to improve their properties for realizing high performance p–i–n planar PSC devices. Herein, the approaches to prepare high‐performance NiO x thin films for p–i–n‐structured pervoskite solar cell (PSC) devices are reviewed, of which contents are mainly about their synthetic methods including both solution‐ and vacuum‐assisted processes and modification of their properties with dopants and functional materials.