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 (similar to 835 degrees C) and conventional (740-750 degrees C) temperature firing conditions. At similar to 750 degrees C firing, all three pastes failed on a 100 Omega/square 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 similar to 835 degrees 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 mu m) 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 Omega/square emitter with a solar cell fill factor of 0.782 only after annealing in a hydrogen atmosphere. (c) 2005 The Electrochemical Society.