The stoichiometric nickel oxide is an insulator while it becomes a p-type semiconductor with the presence of divalent and trivalent states. Here nonstoichiometric nickel oxide nanoparticles (NiOx NPs) were synthesized via a chemical route using two different starter salts; nickel chloride hexahydrate and nickel nitrate hexahydrate. The nanoparticles were investigated by X-ray diffraction and transmission electron microscopy, scanning electron microscopy, to determine the crystal structure, morphology, and particle size. The effect of different nickel salts in the reaction and the formation of NiOx nanoparticles were observed for the first time in the literature. Because of the high uniformity and stability, nitrate-based inks were employed as additives and interface layers for planar perovskite solar cells, using a two-step deposition for regular (n-i-p) and one-step antisolvent washing method for inverted (p-i-n) configurations. Photovoltaic performances were improved in the case of NiOx applied as an interface layer between P3HT and Ag for regular solar cells and both as an additive in sol-gel based NiOx and interface layer between HTL and perovskite in inverted solar cells.