Comparative study on the effect of annealing temperature on sol–gel-derived nickel oxide thin film as hole transport layers for inverted perovskite solar cells

Sol–gel-processed nickel oxide (NiO x ) is a promising hole transport layer (HTL) for inverted perovskite solar cells (PSCs) because of its excellent carrier transport properties, low costs, and high stability. Annealing temperature plays a vital role in the sol–gel process as different temperatures can alter the physicochemical properties of the materials. In the present study, we investigated the effect of annealing temperature on NiO x thin films prepared through sol–gel processing from predominantly used precursors: (1) nickel acetate tetrahydrate (Ni-ac) and (2) nickel acetylacetonate (Ni-acac). X-ray diffraction and X-ray photoelectron spectroscopy results revealed that the crystallinity and stoichiometry of both NiO x thin films improve with an increase in annealing temperature, indicating the formation of a standard Ni–O octahedral in a cubic rock NiO structure. Higher annealing temperatures have been shown to improve film optical transparency and electrical conductivity due to the complete decomposition of precursors and improved crystallinity. The device characterization has shown both films can work as an effective HTL for PSCs and showed the best device performance for films annealed at 400 °C. It is believed that this study can provide a basic understanding and strategy for developing highly efficient NiO x HTL in PSCs.