Understanding the Formation and Temperature Dependence of Thick-Film Ag Contacts on High-Sheet-Resistance Si Emitters for Solar Cells

Physical and electrical properties of screen-printed Ag thick-film contacts were studied and correlated to understand and achieve good-quality ohmic contacts to high-sheet-resistance emitters for solar cells. Analytical microscopy and surface analysis techniques were used to study the Ag-Si contact interface of three different screen-printed Ag pastes (A, B, and PV168) subjected to high (∼835°C) and conventional (740-750°C) temperature firing conditions. At ∼750°C firing, all three pastes failed on a 100 Ω/□ emitter because of incomplete etching of the silicon nitride film (PV168), an irregular small distribution of regrown Ag crystallites (paste A), or an excessive diffusion of Ag into the p-n junction (paste B). At a firing temperature of ∼835°C, paste A gave a lower open-circuit voltage because of the diffusion of Al from the glass frit into the emitter region. Paste B failed because of the formation of very large (0.3-1 μm) Ag crystallites that shunted the p-n junction. Of the three pastes, the PV168 paste from DuPont gave the best contact quality on a 100 Ω/□ emitter with a solar cell fill factor of 0.782 only after annealing in a hydrogen atmosphere.