Two-step annealing of NiO enhances the NiO –perovskite interface for high-performance ambient-stable p–i–n perovskite solar cells

In this study, we applied two-step annealing to nickel oxide (NiOx) films that we then used in perovskite solar cells (PSCs). The optimized annealing process resulted in a change in the structure and chemical composition of the NiOx, leading to a change in the work function and improved conductivity for NiOx-coated ITO substrates. X-ray photoelectron spectroscopy suggested that a change in the Ni2+/Ni3+ ratio of NiO (Ni 2p at 854.0 eV) and the presence of Ni3+ species induced by vacancies in Ni2O3 (Ni 2p at 855.6 eV) and NiOOH (Ni 2p at 856.7 eV) were responsible for the enhanced conductivity of the two-step-annealed NiOx films. The modified NiOx served as an efficient hole transporting layer, enhancing the PL quenching behavior at the perovskite–NiOx interface. Time-resolved photoluminescence measurements provided evidence for efficient carrier extraction. These improvements led to increases in the fill factors and power conversion efficiencies (PCEs) of corresponding PSC devices. The champion device displayed a PCE of 19.04%—a value comparable with those of state-of-the-art NiOx-based PSCs. Furthermore, the devices possessed excellent air-stability, retaining 97% of their PCEs after storage in air for over 672 h (at 25 °C, with a humidity of 40%).