Other than light-induced degradation and potential-induced degradation, there has been another failure mechanism, which affects the reliability of the PV modules. Snail Trail (also known as snail tracks or worm marks) have been observed in outdoor conditions, which appears as brownish discoloured contact fingers especially around the cell edges and microcrack areas. This phenomenon typically occurs within several months to several years after the initial installation. The formation of snail trails has been related to the ingress of moisture and oxygen via microcracks [1]. It has been reported to affect the modules from a wide range of PV module manufacturers worldwide [2]. It was shown that the discolouration can be caused by a high density of silver nanoparticles within the encapsulation foil just above the silver finger [3], by silver oxide, or silver carbonate nanoparticles [4]. Under operating conditions, the moisture in the surrounding enter the PV modules through the back sheet foil. While a solar cell is generally an effective barrier to protect the moisture coming to the front surface, the cell edges or microcracks are sites where moisture can diffuse to the solar cell front surface. The silver nanoparticles from the grid fingers may dissolve and migrate into the encapsulant causing a chemical reaction to cause the discolouration. Fortunately, Canadian Solar demonstrated that snail trails do not affect actual solar module performance [5]. They have shown some evidence that the glass frits of silver paste and peroxide additives in the EVA (ethylene vinyl acetate) were the cause to form snail trails [5].
The has been several testing methods developed for susceptibility for snail trails. For example, a 50-100 hour damp-heat test at 85 °C and 85% relative humidity in combination with a forward bias current of 8 A has been shown to be able to create snail trails in susceptible PV modules [3][6].
[1] S. Richter, M. Werner, S. Swatek, and C. Hagendorf, “Understanding the snail trail effect in silicon solar modules on microstructural scale,” 27th Eur. Photovolt. Sol. Energy Conf. Exhib., no. January 2016, pp. 3439–3441, 2012.
[2] R. H. A. Xu, S.L. Lee, “Snail Trail,” Today Energy Solut., vol. 5, 2012.
[3] S. Meyer et al., “Silver nanoparticles cause snail trails in photovoltaic modules,” Sol. Energy Mater. Sol. Cells, vol. 121, pp. 171–175, 2014.
[4] P. Peng et al., “Microscopy study of snail trail phenomenon on photovoltaic modules,” RSC Adv., vol. 2, no. 30, p. 11359, 2012.
[5] H.-C. Liu, C.-T. Huang, W.-K. Lee, S.-S. Yan, and F.-M. Lin, “A Defect Formation as Snail Trails in Photovoltaic Modules,” Energy Power Eng., vol. 7, no. 8, pp. 348–353, 2015.
[6] S. Meyer, S. Richter, S. Timmel, M. Gläser, S. Swatek, and C. Hagendorf, “Snail trails : root cause analysis and test procedures,” vol. 0, 2013.