To get to the root of polyamide's degradation problem, Gu and her team acquired backsheet samples from solar panels deployed in regions around the globe, including sites in the U.S., China, Thailand and Italy. Most of the panels, which were in use from three to six years, showed clear signs of premature cracking.
With the weathered backsheets in hand, the researchers conducted a gamut of chemical and mechanical tests to examine the patterns and severity of degradation throughout the depth of the sheets. The results, described in the journal Progress in Photovoltaics: Research and Applications, showed that the areas of the sheets that had undergone the worst cracking were those that had become the most rigid. And curiously, the most brittle areas were on the inner side of the sheets, Gu said.
How could the quality of the walled-off interior diminish more quickly than the exposed outer layer? Gu and her team speculated that the sunlight-induced degradation of the top side of the encapsulant - a film that surrounds the solar cells - produced damaging chemicals that descended toward the backsheets, speeding up their decay. If true, the proposed mechanism would explain why cracks form in between solar cells, as chemicals could find passage to the back through these regions.
The researchers identified acetic acid as a prime suspect, as it is known to be harmful to polyamide and is produced during the degradation of a polymer commonly used as an encapsulant, called ethylene vinyl acetate (EVA). To test their hypothesis, the researchers stowed several polyamide strips away in vials of acetic acid and then, after five months, analyzed how they decayed compared with strips placed in either air or water.
Under the microscope, cracks mirroring those from the weathered backsheets appeared on the surface of the plastic strips exposed to acetic acid, which appeared much worse than on those that had been in air or water. Chemical analysis showed that degradation products of polyamide were higher in the acetic-acid-exposed strips, providing further evidence that the acid accelerates the deterioration of the backsheet material.
The study highlights the interplay between solar panel components (the EVA encapsulant and polyamide backsheet in this case) as a potentially critical factor to consider when designing solar panels that are built to last.
These new insights into premature failures could also become valuable to NIST researchers and others who seek to replicate the degradation process in the lab as a way to test and predict the longevity of solar panel components.