In this work, we develop SiO_x/poly-Si carrier-selective contacts grown by low-pressure chemical vapor deposition and boron or phosphorus doped by ion implantation. We investigate their passivation properties on symmetric structures while varying the thickness of poly-Si in a wide range (20-250 nm). Dose and energy of implantation as well as temperature and time of annealing were optimized, achieving implied open-circuit voltage well above 700 mV for electron-selective contacts regardless the poly-Si layer thickness. In case of hole-selective contacts, the passivation quality decreases by thinning the poly-Si layer. For both poly-Si doping types, forming gas annealing helps to augment the passivation quality. The optimized doped poly-Si layers are then implemented in c-Si solar cells featuring SiO₂/poly-Si contacts with different polarities on both front and rear sides in a lean manufacturing process free from transparent conductive oxide (TCO). At cell level, open-circuit voltage degrades when thinner p-type poly-Si layer is employed, while a consistent gain in short circuit current is measured when front poly-Si thickness is thinned down from 250 to 35 nm (up to +4 mA/cm²). We circumvent this limitation by decoupling front and rear layer thickness obtaining, on one hand, reasonably high current (J_SC-EQE = 38.2 mA/cm²) and, on the other hand, relatively high V_OC of approximately 690 mV. The best TCO-free device using Ti-seeded Cu-plated front contact exhibits a fill factor of 75.2% and conversion efficiency of 19.6%.