Carrier‐selective contact‐based silicon heterojunction solar cells are fabricated using nickel oxide (NiO_x) as a hole‐selective layer by thermal evaporation. The highest power conversion efficiency of ≈15.20% with a chemically grown SiO_x interlayer is achieved from a Ag/ITO/NiO_x/n‐Si/LiF_x/Al cell structure in comparison with ≈12.43% without SiO_x. The cells without and with the SiO_x layer are analyzed by considering crucial parameters for conversion efficiency, such as minority carriers' diffusion lengths, lifetimes, recombination resistance, and density of interface defect states at the NiO_x/n‐Si junction, by studying the dark/light current density–voltage, quantum efficiency, impedance, and parallel conductance characteristics. Device analysis provides evidence for the cell's open‐circuit voltage and short‐circuit current enhancement with the SiO_x interlayer. This is due to an improvement in minority carrier lifetimes from ≈8.6 to ≈48.27 μs (photo‐conductance decay analysis), which is also estimated from ≈7.45 to ≈49.20 μs by impedance spectra analysis, increased minority carrier diffusion length from ≈171 to ≈952 μm, and decreased rear surface recombination velocity from ≈1106 to ≈170 cm s⁻¹ (quantum efficiency analysis). These investigations reveal that engineering the n‐Si/LiF_x interface by the SiO_x interlayer is more important than the NiO_x/n‐Si interface because of a thin unintentionally grown SiO_x layer during NiO_x evaporation simultaneously mediating silicon surface passivation.