P-contacts with low activation energy (Ea) and wide bandgap (Eg) are critical for achieving effective positive-charge collection in silicon heterojunction (SHJ) solar cells. In this work, we firstly investigate and optimize PECVD p-type hydrogenated nanocrystalline silicon oxide, (p)nc-SiOx:H, and then combine it with (p)nc-Si:H as p-contact stack in front/back-contacted (FBC) SHJ solar cells. We optimize interface treatments to accelerate the nucleation of thin nanocrystalline film to obtain a lower Ea within a limited thickness. At cell level, we observe higher FF when using (p)nc-SiOx:H with an increased difference between the optical bandgap (E04) and Ea, which enhances the c-Si band bending. Further thickness optimizations of p-contact are conducted based on observed thickness-dependent Ea of the p-contact. With the guidance of electrical simulation studies , we obtain front and rear junction cells with FF above 80% and independently certified efficiency of 22.47%.