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 NiO_x (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 NiO_x 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 NiO_x (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 NiO_x and the perovskite interface.