Front contact metallization of Si solar cells: Insights from in-situ X-ray diffraction

The front contact metallization of Si solar cells begins with printing a mixture of an Ag powder, glass frit (mixture of metal oxides such as PbO, SiO2, B2O3, and Bi2O3) and an organic binder over the antireflection coating that is subsequently fired up to about 825 °C. It is known that the frit allows the paste to react with and burn through the anti-reflective coating such that the metal can react with the underlying c-Si during firing. However, the precise phase transformations between Ag, Si, SiNx, and the frit constituents, which happen within a few seconds during rapid thermal processing (RTP), giving rise to an Ag-Si contact, are not well understood in the absence of in-situ characterization under the actual processing conditions. We have carried out in-situ x-ray diffraction studies on sample mixtures of different component powders (Ag, sinks, PbO-fruit and Si) under realistic processing conditions using an in-situ rapid thermal processing setup. We track the phase progression and reaction pathways at a time resolution of 100 milliseconds. We show the direct evidence of SiNx oxidation by PbO between 600–750 °C. On subsequent heating to higher temperature, up to 825 °C, Ag dissolves into the frit etches the c-Si surface and is deposited on etch pits forming intimate electrical contacts.