A Top‐Down Strategy for Reforming the Characteristics of NiO Hole Transport Layer in Inverted Perovskite Solar Cells

The hole transport layer (HTL) plays a key role in inverted perovskite solar cells (PSCs), and nickel oxide has been widely adopted for HTL. However, a conventional solution‐processed bottom‐up approach for NiOx (S‐NiO) HTL fabrication shows several drawbacks, such as poor coverage, irregular film thickness, numerous defect sites, and inefficient hole extraction from the perovskite layer. To address these issues, herein, a novel NiOx HTL top‐down synthesis route via electrochemical anodization is developed. The basicity of the electrolyte used in anodization considerably influences electrochemical reactions and results in the structure of the anodized NiOx (A‐NiO). The optimized A‐NiO provides outstanding optoelectrical properties, including uniform film thickness, enhanced transmittance, deep‐lying valance band, low trap density, and better hole extraction ability from the perovskite. Owing to these advantages, the A‐NiO‐based inverted PSC exhibits an improved power conversion efficiency of 21.9% compared with 19.1% for the S‐NiO‐based device. In addition, the A‐NiO device shows a higher inlet and long‐term ambient stability than the S‐NiO device due to the superior hole transfer ability of A‐NiO, which suppresses charge accumulation between NiOx and the perovskite interface. A novel nickel oxide hole transport layer top‐down synthesis route via electrochemical anodization is developed. A‐NiO shows outstanding optoelectrical properties such as uniform film thickness, enhanced transmittance, low trap density, and high hole extraction ability. The A‐NiO‐based inverted perovskite solar cell shows improved power conversion efficiency of 21.9% and long‐term stability.