Influence of micro- and macrostructure when determining the contact resistivity of interconnects based on electrically conductive adhesives

The contact resistivity of interfaces in solar module interconnects has a direct impact on the series resistance of the entire module and fill factor. Thus, this impacts the performance of entire photovoltaic systems. Accurate measurement of the contact resistance is a key component of optimizing the performance of such interconnects. However, this is difficult for electrically conductive adhesive (ECA) based interconnects. This work shows that transmission line method (TLM) test structures based on ECA display non-negligible inhomogeneities leading to inaccuracies when determining the contact resistivity. Seven methods based on two models - the front- and end-contact TLM models (or a combination of both) - were investigated for four commercially available ECAs used in solar modules and their impacts on the extrapolation of the contact resistivity were quantified. It was determined that even when macroscale inhomogeneities (e.g., variations in the thickness of the ECA) are not present, microscopic structural effects influence the sheet and contact resistance. In particular, variations in the distribution of fillers significantly alter the bulk resistivity of the composites and this variability is also clearly correlated with differences in the geometry of the fillers. It is concluded that the best approach to reduce inaccuracies in the determination of the contact resistivity of ECA-based interconnects is to calculate the sheet and contact resistance locally (using three consecutive contacts) employing a redundant and modified test structure. Afterwards, the contact resistivity ought to be computed using the end-contact TLM model and the median should be assigned as the contact resistivity of the sample.