stacks. This important application of ultra-thin SiO_x layers is as tunnel oxides for the rear side of the Tunnel Oxide Passivated Contact (TOPCon) cell concept. The structural properties of theSiO_X layers that were analyzed by X-ray photoelectron spectroscopy (XPS) and compared to the iV_OC data. It indicate that a minimum oxide layer thickness of approx. 1.4 nm and a high amount of oxygen rich sub oxides species are required to obtain a good interface passivation stable up to annealing temperatures of 900℃