​Nickel oxide (NiO_x) is a promising hole transport layer (HTL) for perovskite solar cells (PSCs), as it combines good chemical stability, high broadband optical transparency and a high work function. Excellent power conversion efficiencies (PCE) have already been reported using solution-processed NiO_x. However, solution-based techniques usually require high-temperature post-annealing to achieve the required HTL properties of NiO_x, which jeopardizes its use for many applications, such as monolithic tandem solar cells. To resolve this issue, we developed room-temperature sputtered NiO_x and demonstrated p−i−n PSCs with 17.6% PCE (with negligible hysteresis), which is comparable to the best PSCs using sputtered and annealed NiO_x without heteroatom doping. Through detailed characterization and density functional theory (DFT) analysis, we explored the electrical and optical properties of the obtained NiO_x films and find that they are strongly linked with the specific defect chemistry of this material. Finally, in view of its use in perovskite/silicon tandem solar cells, we find that direct sputtering on random-pyramid textured silicon wafers results in highly conformal NiOx films.