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.