Alongside rapid developments in PV cell technology, the other components and materials that go into a module are constantly in need of improvement. Polymer materials commonly used for backsheets and encapsulants play an important role in protecting a module’s inner workings from the elements, and with ever higher performance to protect, and growing expectations as to a PV module’s lifetime and extended performance, suppliers of these materials are moving with the times.
“The choice of polymers has a significant influence on the properties of the PV modules not only for efficiency but more importantly for reliability, as most PV module degradation modes are directly linked to polymer degradation and material interactions with polymer components,” said the scientists behind new work on understanding polymer degradation in PV modules.
The research, led by the OFI Austrian Research Institute for Chemistry and Technology, aims to quantify the influence of these polymers on PV performance and degradation over time and to develop models for degradation that could be useful to manufacturers in choosing new materials that have little or no track record in PV installations.
“Many gigawatts of modules with new technologies and materials can be produced and installed without having sufficient experience about long-term reliability,” the group said. “In the worst case, this has led to unexpected degradation mechanisms several years after field deployment, which were not predicted in laboratory accelerated testing, such as potential-induced degradation (PID) or backsheet cracking.”
The group fabricated and tested six-cell modules with varied combinations of encapsulant and backsheet – including the current industry standard encapsulant material ethylene vinyl acetate (EVA), and two others currently seeing rapid commercial development – polyolefin elastomer and thermoplastic polyolefin. Backsheet materials included in the study are polyethylene terephthalate laminated with a fluorinated polymer layer (PPF), and coextruded polypropylene (CPO).
All possible combinations of these materials were then put through accelerated aging tests, operated at 85 C and 85% relative humidity for up to 6,000 hours. The results from the tests are available in the paper “Quantifying the influence of encapsulant and backsheet composition on PV-power and electrical degradation,” which was recently published in Progress in Photovoltaics.
With the data from the testing, and further measurements from sample modules collected from the 2015-18 period, the group used statistical modeling to find links between the backsheet and encapsulant performance and overall module power and degradation. The results showed that the combination of TPO encapsulant and CPO backsheet achieved the highest initial performance, and that modules with EVA encapsulant showed significantly faster degradation than either of the polyolefin based materials.
The group said that its approach offers an example of how the role of materials not directly involved in the module’s energy generation can be quantified, and that with further developments the modelling approach could be used to identify materials that could help manufacturers meet requirements for longer lifetimes, with some now expecting modules to keep generating power for 40 years or even longer.
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