We present and discuss advances in the development of p-type Si heterojunction solar cells with Al-doped ZnO (AZO) on both front and rear side, by means of mixed-phase Si- and SiOx-based doped layers. We demonstrate that while rear AZO can be deleterious when using p-type amorphous silicon (a-Si:H), p-type microcrystalline silicon (pc-Si:H) allows for: (1) suppression of the p-layer/AZO Schottky-like barrier, (2) relevant V-oc, increase, and (3) improved transport and extraction of carriers. In contrast, V-oc reduction and increased low-injection recombination is observed with n-type pc-Si:H in conjunction with AZO, likely due to the low defect density of the material which entails severe depletion effects. These issues are avoided with the more defective mixed phase n-SiOx, which additionally provides for reduced parasitic absorption losses and is thus established as the optimal emitter choice.