Ultra-thin a-Si:H p-i-n solar cell has been proposed as advance to form three-dimensional structures on nano structured substrates for achieving optically thick and electrically thin solar cells. However, over the years, although extensive studies have been carried out, no high efficiency was achieved yet. We found that not only the short circuit current density (J(sc)) decreases, but also the open circuit voltage (V-oc) and fill factor (FF) decreases with the reduction of i-layer thickness, which is opposite to the expectation. We investigated the possible root-causes for this unusual phenomenon and speculated the direct recombination of the electrons in the n-layer and the holes in the p-layer by tunneling through the thin i-layer is the main reason for the reduced V-on and FF in ultra-thin solar cells. Furthermore, the absorption in the doped layers is the most critical limitation for ultra-thin silicon solar cell efficiency because the doped layer thickness becomes comparable to the intrinsic layer. To resolve this issue, we used nanocrystalline silicon oxide (nc-SiOx:H) doped layers with a wide bandgap to reduce the parasitic absorption in the doped layers and the highly asymmetric conductivity improves the carrier collection and made a significant improvement in the cell efficiency. We achieved 8.79%, 7.65%, and 5.32% efficiencies with the i-layer thickness of only 70 nm, 50 nm and 20 nm, respectively, which are the highest ones in ultra-thin silicon solar cells.