Replacement of Silver in Silicon Solar Cell Metallization Pastes Containing a Highly Reactive Glass Frit: Is it Possible?

Massive savings in silicon solar cell production could be achieved by replacing the costly Ag in front side metallization pastes against cheaper metals like Ni or Zn. Regarding the attempt to split the metallization process up into a two step process, first printing a paste containing Ni or Zn instead of Ag for contact formation and second printing a conducting paste containing e.g. Cu, this work is restricted to the investigation of the contacting paste. Simple pastes containing only Ni or Zn respectively, rudimentary glass frit, organic binder and solvent were tested on wafers with a low resistivity emitter using various fast firing profiles and annealing. Our first attempts using Ag compatible frits have shown that it is possible to create a contact to Si yet it is significantly worse than with comparable silver pastes. SEM measurements of the contact area after selective metal/glass etch have shown a strong Si etching by the investigated pastes and only insufficient crystallite formation. This explains the reduced open circuit voltage, pseudo fill factor and the high contact resistivity measured on the cells. Our investigations show that it is very hard to exchange Ag by another metal without fundamentally changing the paste formulation and/or the firing conditions.