Managing the Double-Edged Sword of Ni³⁺ in Sputter-Deposited NiO_x by Interfacial Redox Reactions for Efficient Perovskite Solar Cells

Nickel oxide (NiO x ) is widely used as a promising hole transport material for perovskite solar cells (PSCs). A high concentration of Ni 3+ in the NiO x film is generally beneficial for charge transport of the PSCs; however, chemical redox reactions between surface Ni 3+ and perovskite materials result in decomposition of perovskite materials, which causes carrier recombination and impedes charge transport at the perovskite–NiO x interface. Herein, we employ magnetron sputtering to fabricate NiO x thin films with adjustable Ni 3+ concentrations to optimize the hole-transporting properties. A thin layer of phenylethylamine iodide (PEAI) is further introduced to reduce the detrimental Ni 3+ at the surface of NiO x, which eliminates the formation of undesirable defects and chemical species when in contact with the perovskite layer, leading to a dramatic increase in the power conversion efficiency (PCE) from 16.37 to 20.01%, which is one of the best performance using sputtered charge transport layers. The unencapsulated devices retain 88% of their initial PCE after storage in a nitrogen atmosphere for 1000 h under light. We further perform solar cell capacitance simulator (SCAPS) simulation to understand the effects of bulk and interfacial charge transport on the performance of PSCs, which agree with our experimental results. This work not only highlights the double-edged sword effect of the Ni 3+ content on the performance and stability of PSCs but also demonstrates a simple yet effective strategy to avoid the undesirable reaction between Ni 3+ and perovskite materials in the fabrication of PSCs with sputter-deposited NiO x .