NiO_x is widely used as a hole transport material in perovskite solar cells (PSCs). This wide band gap p-type material is conveniently deposited via high throughput RF-sputtering, making it suitable for the industrialization of PSCs. Nonetheless, for the cells to pass accelerated degradation tests such as the IEC 61215 damp heat (DH) test, the chemistry of the NiO_x film should remain constant at elevated temperaturs to preserve its optoelectronic properties. This study emphasizes that structural defects resulting from Ni vacancies in NiO_x lead to significant degradation of the PSCs after just a few hours of exposure to elevated temperatures (85 °C). We introduce here an approach to fine-tune the chemistry of the NiO_x film by adjusting the gas flow during sputtering deposition and by incorporating Cs. Through this control on the chemistry of the layer, the optimized NiO_x-based PSCs exhibit remarkable stability, with devices passing 5 times the IEC 61215 norm (<5% rel after 5000 h of DH testing) and also showing better stability under light soaking. XPS analysis reveals that the concentration of Ni³⁺ in the bulk of the standard NiO_x film is twice that in the optimized NiO_x. This suggests that the Ni³⁺ concentration, typically equal to the Ni vacancy concentration and beneficial for charge transport in NiO_x, may actually compromise the stability of the PSCs. Additionally, the film density of the optimized NiO_x film was significantly higher than that of the standard film.