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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.

In solution-processed organic and inorganic halide perovskite solar cells (PSCs), managing the interfacial characteristics between multi-layers has been a crucial factor in determining their performance and stability. Here, we newly design an indolocarbazole-based organic molecule that can enhance the interfacial properties between a solution-processed NiOx hole transport layer and perovskite in p-i-n planar PSC. Especially, to be beneficial to this architecture, its visible transparency and energy level are optimized, and its chemical structure is crosslinked for better solution processability as a hole-selective interlayer. More importantly, it can passivate the defects in the perovskite film, consequently enhancing the quality of the film and suppressing non-radiative recombination in the devices. With this interface modification, we confirm that the performance and stability of PSC devices are largely enhanced.

Defects in solution-processed organic and inorganic halide perovskite thin films deteriorate their photovoltaic performance and stability. Here, a bisimidazole-based organic small molecular hole transport material (HTM) that can suppress interfacial defects of the perovskite with its functional groups working as Lewis bases is newly introduced. Surface passivation of perovskite film leads to the reduction in non-radiative recombination trap density as well as the improvement of its crystallinity, consequently enhancing the efficiency of perovskite solar cell (PSC) from 16.3% to 18.3% with superior stability, preserving 92% of its initial efficiency for 500 h in an atmosphere.