Survey of Mechanical Durability of PV Backsheets

The ability to maintain mechanical properties after applied environmental stress is a good indicator for long term durability of a backsheet. This work surveys a set of 56 backsheet films, some of which are known to fail in the field and evaluates them utilizing the proposed photovoltaic weathering standard IEC 62788-7-2. While the complete study examined optical reflectance characterization, tensile testing, dielectric testing, and a mandrel bend test, this paper focuses on the mandrel bend test results. All seven of the transparent specimens failed. A polyamide based backsheet known to fail in the field did not fail the bend test. It is believed that the field failure is due to an interaction with the encapsulant and/or the cyclic mechanical exposure experienced in the field. When cracks occur, they happen predominantly in the polyethylene terephthalate for transparent backshees or the “E” layer for opaque films, never in a fluoropolymer layer.