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)(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 PO2PO2 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 PO2PO2 in the ambient gas. With increasing PO2PO2, 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 Ag+ and O2− ions in the molten glass and Si wafer without the aid of liquid Bi formation.

► The mechanism for the formation of Ag crystallites is proposed for Ag pastes containing Bi2O3-based glass frit. ► The rate of Ag crystallite formation is greatly enhanced by increasing PO2PO2 in the firing ambient. ► The sensitivity of Ag crystallite formation to PO2PO2 is apparent even in a spike firing. ► The Ag crystallite formation occurs without an aid of liquid Bi formation.