NOVELTY - Producing at least one silicon layer on a substrate, comprises (a) providing a substrate, (b) providing a formulation comprising at least one silicon compound, (c) applying the formulation on the substrate, (d) irradiating and/or thermally treating the coated substrate, (e) treating the layer obtained in step (d) with oxygen present in the form of elemental oxygen, carbon dioxide, at least one oxygen containing compound or their mixtures, and (f) irradiating and/or thermally treating the coated substrate obtained in step (e) to form an at least partially polymorphous layer comprising silicon.    USE - The oxygen in the form of elemental oxygen, carbon dioxide, at least one oxygen containing compound or their mixture present in pure form or in the form of liquid or gaseous mixture, is useful for silicon suboxide structures in the production of at least partially polymorphic layer comprising silicon, which exhibits the suboxide structures on its surface or completely comprises the suboxide structures. The method is useful in producing photovoltaic devices, preferably solar cells or combination of solar cells (all claimed).    ADVANTAGE - The method: eliminates or reduces the defects of the surfaces or interfaces to be modified; and produces the solar cells with high efficiency.    DETAILED DESCRIPTION - Producing at least one silicon layer on a substrate, where at least one of the silicon layers exhibits suboxide structures on its surface or completely comprises suboxide structures, comprises: either (a) providing a substrate, (b) providing a formulation comprising at least one silicon compound, (c) applying the formulation on the substrate, (d) irradiating and/or thermally treating the coated substrate, (e) treating the layer obtained in step (d) with oxygen present in the form of elemental oxygen, carbon dioxide, at least one oxygen containing compound or their mixtures present in pure form or in the form of a liquid or a gaseous mixture, and (f) irradiating and/or thermally treating the coated substrate obtained in step (e) to form an at least partially polymorphous layer comprising silicon, which exhibits suboxide structures at its surface; or (a1) providing a substrate, (b1) providing a formulation comprising at least one silicon compound and oxygen in the form of elemental oxygen, carbon dioxide, at least one oxygen containing compound or their mixtures present in pure form or in the form of a liquid or a gaseous mixture, (c1) applying the formulation on the substrate, and (d1) irradiating and/or thermally treating the coated substrate obtained in step (c1) to form an at least partially polymorphous layer comprising silicon, which completely comprises suboxide structures. INDEPENDENT CLAIMS are also included for:    (1) a coated substrate comprising layer sequences, containing (substrate)-(an at least partially polymorphic, undoped intrinsic layer comprising silicon, which exhibits suboxide structures on its surface or completely comprises suboxide structures (i-Si(suboxic))), (substrate)-(an undoped layer comprising silicon (i-Si))-(i-Si(suboxic)), (substrate)-(an n-type doped layer comprising silicon (n-Si)-(i-Si(suboxic))-(i-Si)-(i-Si(suboxic))-(a p-type doped layer comprising silicon (p-Si), (substrate)-(p-Si)-(i-Si(suboxic))-(i-Si)-(i-Si(suboxic))-(n-Si), (substrate)-(i-Si(suboxic))-(n-Si)-(i-Si(suboxic))-(i-Si)-(i-Si(Suboxic))-(p-Si)-(i-Si(suboxic)) or (substrate)-(i-Si(suboxic))-(p-Si)-(i-Si(suboxic))-(i-Si)-(i-Si(suboxic))-(n-Si)-(i-Si(suboxic)), where the layer sequences does not comprise other intermediate layers within the respective layer sequences; and    (2) a photovoltaic device, preferably a solar cell or combination of solar cells, produced by the above method.