Silver nanoparticles cause snail trails in photovoltaic modules

•The origin of snail trails in PV modules is discovered.•Snail trails can be induced at test modules in the lab.•Ag nanoparticles cause the typical discolorations at the contact grid fingers.•The formation of Ag nanoparticles can be simulated in a test tube.•The polymer foil additive composition is a way to avoid snail trails on PV modules.

After some months of operation, a number of PV modules develop a discolouration defect called “snail trails”, or “snail traces”, which appear as irregular dark stripes across the cells. Whereas these traces were soon identified as discoloured silver contacts along the cell edges or at micro cracks, the chemical and mechanistic reasons for this phenomenon have not yet been resolved in detail. In this work we show that silver nanoparticles accumulating within the encapsulation foil cause the brownish discolouration, and that certain additives of encapsulation and back sheet foils trigger the formation of these nanoparticles.