NiO is extensively studied as a hole transport layer in perovskite solar cells but syntheses of NiO precursor involves toxic chemicals and time-consuming processes. Moreover, the synthesized NiO contains surface defects acting as trapping sites at the NiO/perovskite interfaces, resulting in poor charge extraction, hysteresis and light soaking. In this manuscript, we developed a non-toxic methodology for NiO precursor solution by using a simple mixture of NiO powder and HCl in an air environment. In addition, a new hot-casting technique was developed to successfully fabricate densely-packed, less defective NiO films. Interestingly, the hot-casting temperature was found to significantly affect morphology, film coverage and surface defects of NiO films. When a hot-casting temperature was below 100 degrees C, non-uniform NiO films were sparsely formed on the FTO surface and were characterized by defects in the form of hydroxyl groups and water on the surface. Such defective NiO films resulted in severe hysteresis and light soaking effect due to the trapped charges at the defective NiO/perovskite interface of perovskite solar cells. In contrast, when the hot-casting temperature was 120 degrees C, the NiO film formed densely-packed morphologies, covering the FTO surface. Furthermore, this film exhibited an ordered chemistry with strong Ni-O octahedral bonding and facilitated charge extraction at NiO/perovskite interface, resulting in a negligible hysteresis and light soaking. Finally, this non-toxic and simple method of fabricating NiO film will assist further development of perovskite solar cells.