Fundamental microscopic studies on the etching behavior of silver pastes on poly-Si/SiOx passivating contacts

Screen printing of Ag paste is a key process for implementing polycrystalline-silicon/silicon oxide (poly-Si/SiOx) passivating contacts in industrial solar cells. Thereby, the formation of Ag crystallites on the Si surface is crucial for a low-ohmic contact. Recently, it has been observed that Ag crystallites are predominantly formed within the poly-Si layer and stop at the thermal SiOx, thermal interface, causing a cuboidal shape. To avoid a direct contact of the Ag crystallites with the underlying SiOx, thermal/crystalline-Si (c-Si) interface, a multilayer approach with interlayer SiOx, inter between several poly-Si layers is presented. A stack of three 50 nm poly-Si layers achieves highest cell potential. Further, the metallization process parameters are scanned to check when this cuboidal Ag crystallite shape occurs. While different amorphous-silicon (a-Si) deposition techniques show no changes, there are strong indications that the glass frit etching is responsible for a different etching mechanism causing the breakdown of the SiOx stop.