Backsheet Chalking - Theoretical Background and Relation to Backsheet Cracking and Insulation Failures

This paper assesses the phenomenon of PV backsheet chalking and its effect on solar module reliability. We review the current state of literature on the chalking effect in general and discuss the commonly accepted photocatalytic mechanism of polymer degradation in paints and coatings. We then apply this general knowledge to solar backsheets and discuss its relevance for solar modules reliability and performance, including the experience we have gained from investigations on field-exposed modules. After a physical and chemical analysis identifying the powder as titanium dioxide (TiO2), the important factors for the occurrence of backsheet chalking are determined as (a) the type of polymer layers, (b) the polymer formulation, and (c) the amount and location of TiO2 in the backsheet. Finally, two cases of field-aged modules exemplify different degradation pathways, that both originate from the same degradation mechanism. Although backsheet chalking can, in some cases, be an indication for further degradation, an analytical investigation of the backsheet is necessary to determine the actually associated risk.