Controlled functionalization of 2D black phosphorus (BP) nanosheets provides unique opportunities to tune their chemical, physical, and electronic properties. Herein, the preparation of single-atom nickel-doped BP (Ni–BP) sheets using a simple solution-based strategy is reported. Using the Ni–BP sheets as a passivation layer on top of a perovskite film leads to standard perovskite solar cells (PSCs) with improved performance. The standard n–i–p PSCs with the Ni–BP interlayer achieve maximum power conversion efficiencies of over 22%, with negligible hysteresis and a modest improvement in stability when subjected to different testing conditions. The perovskite films prepared with Ni–BP sheets-based passivation are found to have reduced defect densities as well as improved charge-transfer properties and carrier lifetimes. Density-functional theory calculations support the experimental results through showing that the atomic Ni-doping increases the work function of the BP interlayer, enabling better hole extraction, as well as increasing the surface hydrophobicity of the BP layer, hence reducing water sorption into the perovskite film.