Electron microscopy study of front-side Ag contact in crystalline Si solar cells

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.