Influence of nickel precursor solutions on nickel oxide thin film formation and photovoltaic properties of air-processed inverted perovskite solar cells

Optically transparent nickel oxide (NiO_X) thin films have been used as hole transport layers in inverted perovskite solar cells (PSCs). However, possible chemical reactions between hydroxide or NiOOH species at the surface of NiO_X and upcoming perovskite precursor solution can deteriorate the photovoltaic performance of NiO_X based PSCs. Although different nickel precursor solutions have been reported for NiO_X deposition, the effect of the precursor solution composition on physicochemical properties of NiO_X is not clear. In this work, five nickel precursor solutions have been prepared with different nickel salts, solvents and stabilizers. It is found that the type of nickel salts influences the most bulk and surface properties of NiO_X thin films. Compared to a basic salt such as nickel acetate, the use of a Lewis base like nickel nitrate results in NiO_X thin films with a higher concentration of the stoichiometric NiO phase, smaller grain sizes, more compact morphology and a lower percentage of hydroxide or NiOOH species at the surface of NiO_X thin films, leading to more efficient and stable air-processed PSCs. It is concluded that the chemical characteristics of the nickel precursor solutions have a significant impact on bulk and surface properties of NiO_X thin films.