Why and How to Measure the Non-Metallized Contact Resistivity of a Passivating Contact

When a silicon solar cell passivating contact is combined with a metal grid, the lateral resistance of the wafer and of the passivating contact layer, as well as their interface resistances, are decisive for a high fill factor (FF). A particular challenge for characterization and modelling arises for industrial bifacial TOPCon solar cells featuring fire-through metallization on the rear side, in which case three contact resistivities are of relevance: (i) the contact resistivity over the thin oxide under the metallization, (ii) the same in the non-metallized region, and (iii) the contact resistivity between the poly-Si and the metal. We show that a common approach to determine a single lumped contact resistivity via transfer length method (TLM) may result in large errors when predicting its influence on FF. We then present a new approach to determine the three contact resistivities via a modified TLM structure and fitting of Quokka3 simulations, which we call biTLM.