Based on its band alignment, p-type nickel oxide (NiO_x) is an excellent candidate material for hole transport layers in crystalline silicon heterojunction solar cells, as it has a small ΔE_V and large ΔE_C with crystalline silicon. Herein, to overcome the poor hole selectivity of stoichiometric NiO_x due to its low carrier concentration and conductivity, silver-doped nickel oxide (NiO_x:Ag) hole transport layers with high carrier concentrations were prepared by co-sputtering high-purity silver sheets and pure NiO_x targets. The improved electrical conductivity of NiO_x was attributed to the holes generated by the Ag⁺ substituents for Ni²⁺, and moreover, the introduction of Ag⁺ also increased the amount of Ni³⁺ present, both of which increased the carrier concentration in NiO_x. Ag⁺ doping also reduced the c-Si/NiO_x contact resistivity and improved the hole-selective contact with NiO_x. Furthermore, the problems of particle clusters and interfacial defects on the surfaces of NiO_x:Ag films were solved by UV-ozone oxidation and high-temperature annealing, which facilitated separation and transport of carriers at the c-Si/NiO_x interface. The constructed c-Si/NiO_x:Ag solar cell exhibited an increase in open-circuit voltage from 490 to 596 mV and achieved a conversion efficiency of 14.4%.