Mechanism for the formation of Ag crystallites in the Ag thick-film contacts of crystalline Si solar cells

This study examined the interfacial reactions between various Ag pastes and (100) Si wafers with and without an SiN_x layer during firing at 800 °C for extended times in order to better understand the mechanism for the formation of Ag crystallites in Ag thick-film contacts of crystalline Si solar cells. The results showed that, in contrast to the model proposed previously, the Ag crystallites were formed by a direct reaction between Ag⁺ ions dissolved in the glass frit and the Si wafer without an aid of liquid-Pb formation. The redox reactions between PbO and Si as well as between PbO and SiN_x were suppressed by adding a few weight percent of Ag powder into the glass frit. The size and distribution of the inverted pyramidal Ag crystallites formed at the glass/Si interface were found to depend critically on the PbO content in the glass frit. The role of PbO in Ag crystallite formation was discussed in terms of the solubility of Ag and the viscosity of the glass frit at the firing temperature.