Firing behavior of lead-containing and lead-free metallization silver paste for monocrystalline silicon solar cells

In this work, PbO-TeO2-Bi2O3-SiO2 lead-containing glass and TeO2-Bi2O3-ZnO-Li2O lead-free glass were synthesized in order to investigate the difference between them and find out the reason why lead-free paste can't be used on monocrystalline wafer commercially. Their flowability and reactivity with SiNx were characterized. Lead-free glass presents lower flow temperature, better wettability and stronger etching ability to SiNx than lead-containing glass. Button test was carried out and large white particles was identified on the interface for lead-free glass. These results indicates the excessive flowability and etching to SiNx for lead-free glass. The morphology of grid line and interface between silver electrode and silicon wafer were investigated on dependence of temperature. The lead-containing and lead-free glasses went through similar evolution of silver sintering and interface microstructure with lead-free glass better facilitating silver sintering through liquid phase sintering due to its lower flow temperature. The thicker glass layer and more Ag crystallites with large size can be observed from cross-sectional micrographs for lead-free glass. Therefore, lead-free paste exhibits lower Voc and FF than lead-containing glass owing to its ecxessive corrosion of SiNx, thicker glass layer and large Ag crystallites.