Influence of oxygen on Ag ionization in molten lead borosilicate glass during screen-printed Ag contact formation for Si solar cells

In order to gain further insight into the formation mechanism of fire-through Ag contacts of Si solar cells, the ionization of Ag during the dissolution of Ag powder into a lead borosilicate glass melt was electrochemically investigated at 800°C under various ambient conditions with different oxygen partial pressures (PO₂). Voltammetric analyses of the Ag-free and Ag-containing glass melts confirmed that some of the Ag powder dissolved into the molten glass as Ag⁺ ions through interaction of the powder with oxygen in the ambient atmosphere. The concentration of Ag⁺ in the molten glass significantly increased with increasing PO₂. The dependence of the Ag⁺ solubility in the molten glass on PO₂ was estimated from chronoamperometric measurements for a series of glass melts containing different amounts of Ag powder. The chronoamperometry results clearly demonstrated that the solubility limit of Ag⁺ in the molten glass at 800 °C also increased significantly with increasing PO₂. The present results strongly support the mechanism proposed recently for fire-through Ag contact formation in which Ag⁺ ions dissolved in the molten glass play a crucial role. The present study also suggests that the reaction kinetics during the fire-through Ag contact formation is effectively controlled by adjusting PO ₂ in the ambient firing conditions as well as by modifying the glass chemistry to alter the solubility of Ag⁺ ions.