Efficiency of industrial screen-printed silicon solar cells depends critically on their firing conditions. We investigated the microstructure of the Ag/emitter contact of solar cells fired at temperatures from below to above optimal by electron microscopy. We found that the interfacial microstructure evolved from one richly decorated with nanometer-size Ag colloids into one with many Ag crystallites grown onto the emitter surface. Selective etching revealed a dearth of Ag crystallites in optimally fired cells. Our microstructural observations suggest that a tunneling mechanism (likely assisted by nano-Ag colloids) is responsible for current extraction in these cells fired with optimal conditions.

Microstructures of front-side Ag contact of crystalline Si solar cells fired at temperatures from below to above optimal were systematically investigated using advanced electron microscopy. Ag pastes studied included commercial pastes and an experimental paste containing nano-sized metallic Zn additive. Microstructures of optimally fired cells determined from cross-sectional and top-view images were found to be consistent with a primary tunneling mechanism for current flow (a “nano-Ag colloids assisted tunneling” model) due to the lack of Ag crystallites connecting the silver conductor line to the silicon emitter. We mapped the evolution of the interfacial microstructures across the firing temperature window and correlated it with cell performance. Our result sheds light on the relative importance of the two current transport models as the peak firing temperature was swept from below to above optimal.