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Zusammenfassung:
Metallization of polycrystalline-silicon/silicon oxide (poly-Si/SiOx) passivating contacts with fire-through silver paste is a crucial process for implementation of passivating contacts in industrial manufacturing of solar cells. For a microscopic understanding of the metallization process, the contact forming interface between the Ag crystallites and the poly-Si layer is investigated with high-resolution transmission electron microscopy. For this purpose, multilayer atmospheric pressure chemical vapor deposition poly-Si samples with a SiOx layer between the individual poly-Si layers are fabricated, screen printed with a lead-free Ag paste, and contact fired. Electron micrographs show that in this process the etching of the paste and the subsequent Ag crystallite formation is stopped by this interface with the SiOx layer. Additionally, energy-dispersive X-Ray mapping reveals the presence of an oxide layer around the Ag crystallites. This finding differs significantly from well-investigated classical cell concepts with contact formation on diffused crystalline silicon. Moreover, an analysis of the Ag crystallite orientation in correlation to the neighboring Si crystallite orientation indicates no direct relationship. Finally, it is shown that the use of this multilayer approach is favorable for integration into a solar cell concept leading to a higher passivation quality at the metallized area and lower contact resistivity.
