Protecting solar cells from UV light

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Lamination and encapsulant materials play a key role in protecting PV modules’ inner workings from heat, cold, dust, damp and, somewhat ironically, from the light they are built to absorb. Ultraviolet light is a factor in several types of degradation and performance loss observed in the field, and is likely still a problem with the latest cell technologies.

For the encapsulant material itself, yellowing over long periods of exposure to light is a common problem that can cause performance loss by itself, and also reduce the level of protection against other degradation. And this was the focus of a group of scientists led by France's National Solar Energy Institute (INES), a division of the French Alternative Energies and Atomic Energy Commission (CEA), who conducted accelerated testing on modules built with a range of different encapsulation materials.

The group fabricated single-cell modules, using heterojunction solar cells laminated at 160 degrees with five different commercially available PV encapsulant materials – two based on ethylene vinyl acetate (EVA) and three based on polyolefins (POE). These modules were illuminated for 4,200 h, and samples were also tested for up to 1,300 h under ultraviolet radiation and an elevated temperature, to test the effects of UV light alone.

The experiments are described in full in the paper “Solar cell UV-induced degradation or module discolouration: Between the devil and the deep yellow,” published in Progress in Photovoltaics. After testing, the modules were inspected visually and measured for fluorescence and current/voltage performance.

Results showed that the EVA encapsulant was most affected, and two of the POE samples also exhibited minor yellowing. Notably, the three samples that did have yellowing were those that contained additives designed to absorb UV light. After 4,200 h of testing, the EVA module was shown to have lost 4.2% of its initial performance.

The study concludes that UV-absorbing additives tend to degrade, affecting PV module performance over time. However, the researchers note that the protection they provide against other degradation mechanisms would likely still justify their use – as long as the solar cells they are protecting remain vulnerable to UV-induced degradation. “The destruction of UV absorbers is an issue affecting the integrity of the whole PV module and can lead to accelerated delamination, among other critical types of damage,” the researchers explain. “There is then a challenge to find new ways to bolster the photoprotection of the device, especially for the most stringent environments, such as those located in deserts.”

As for solutions to this challenge, they suggest trialing mineral-based rather than organic UV absorber materials, and also note that quantum dots may eventually become an alternate solution, though this is still in earlier stages of research. Finally, they suggest investigating UV-absorbing glass, replacing the need for encapsulant additives.

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