As a result from the development of silicon thin-film solar cells that had been conducted at Applied Materials over the last few years, we present a new kind of microcrystalline silicon oxide (mu c-SiOx:H) based p-layer for the application in amorphous/microcrystalline (a-Si:H/mu c-Si:H) tandem solar cells on ZnO substrates. The refractive index of this p-layer can be adjusted in the range 2-3.5 and therefore serves as a refractive index matching layer between ZnO (n similar to 2) and silicon (n similar to 4). By applying such a layer the reflection of solar cells can be reduced to 3%. This results in a significant short circuit current increase in thin-film solar cells. As a follow up to the recently published results of large area thin film silicon modules by us [1,2] we describe in this paper in detail the material properties of this new silicon oxide p-layer, the optimization of electrical and optical properties in solar cells and also the impact on the light induced degradation of a-Si/mu c-Si tandem junction cells. (c) 2012 Elsevier B.V. All rights reserved.