Effect of oxygen partial pressure on Ag crystallite formation at screen-printed Pb-free Ag contacts of Si solar cells

In order to understand the mechanism underlying the formation of Ag thick-film contacts to the emitter Si of crystalline Si solar cells, the reactions between Pb-free Ag pastes containing Bi₂O₃-based glass frit and an n-type (1 0 0) Si wafer during firing at 800 °C were examined by varying the ambient oxygen partial pressure (PO₂ ). When the Bi₂O₃-based glass frit alone was reacted with the Si wafer, the redox reaction leading to the formation of liquid Bi was insensitive to PO₂ in the firing ambient. When a mixture of glass frit with Ag powder was reacted with the Si wafer, however, the firing reaction was significantly influenced by PO₂ in the ambient gas. With increasing PO₂ , the reaction leading to the formation of liquid Bi was gradually suppressed, whereas the reaction producing Ag crystallites became increasingly active, resulting in more Ag crystallites at the contact interface. The present study results strongly support the hypothesis that the Ag crystallites are formed by the reaction between the dissolved A^g+ and O^2- ions in the molten glass and Si wafer without the aid of liquid Bi formation.