A Comparison of Key PV Backsheet and Module Performance from Fielded Module Exposures and Accelerated Tests

The performance of photovoltaic (PV) modules and their component materials under the stresses in the outdoor environment is the most important indicator of reliability, durability, and safety of PV modules throughout their service life. Tedlar polyvinyl fluoride films have been a key component in backsheet component structures and used in PV modules that have been in the service environment for over 25 years. The performance of fielded modules and components can be compared with the performance in durability tests using other backsheet materials including polyethylene terephthalate. Accelerated testing protocols are described including UV exposure based on solar irradiance in different climates and relevant albedo levels for exposure of the back of PV modules. The change in critical performance properties in durability tests including damp heat and UV is compared with backsheets that are extracted from fielded modules. Analysis of mechanical and chemical properties of the inner and outer layers of backsheets that are removed from fielded modules is examined. Area-specific coring techniques that are followed by layer composition analysis were also used to understand fielded module failure mechanisms/defects. The impact of extended damp heat and UV on backsheet properties and module performance that is observed in the field is further quantified through measurement of mechanical, optical, electrical, and permeability properties of the backsheet and power, electrical insulation, and physical properties of the modules. Correlations between field performance and accelerated testing are demonstrated. The first results for sequential and combined stress testing are described and compared with the field performance. A comparison of two large and diverse sets of modules from two different locations (EU and Japan) is discussed, and the power loss with field exposure period and the impact of backsheet is described.