Low parasitic absorption and high conductivity enable (n)-type hydrogenated nanocrystalline silicon [(n)nc-Si:H], eventually alloyed with oxygen [(n)nc-SiOx:H], to be deployed as window layers for front/back contacted (FBC) silicon heterojunction (SHJ) solar cells. In this study, we investigated and compared the optoelectrical properties of (n)a-Si:H, (n)nc- SiOx:H and (n)nc-Si:H layers for applications in rear junction FBC-SHJ solar cells. Aiming at minimizing the (n)-type window layer thickness, we demonstrated the successful use of a 3-nm-thick (n)nc-Si:H that delivers a certified of 22.20%. This cell shows 0.59 mA/cm2 JSC gain over the (n)a-Si:H counterpart owing to the higher transparency of (n)nc-Si:H, while maintaining comparable VOC > 714 mV and FF > 80%. Further with MgF2/ITO dual-antireflection-coating (DARC), cell with 3 nm (n)nc-Si:H exhibits a JSC,EQE up to 40.0 mA/cm2. Interestingly, we also observed that a 2 nm (n)a-Si:H capping layer on the 3-nm-thick (n)nc-Si:H can improve the FF by 1.4%abs.. Results analyses are supported by high-resolution transmission electron microscopy (HRTEM) and energy dispersive X-Ray (EDX) elemental mapping.