Dutch researchers have reported a higher risk of degradation of n-type TOPCon cells with EVA encapsulation due to potential moisture degradation. Metallization of the front side makes n-type cells more vulnerable than p-type cells according to wet heat experiments.
“The purpose of this study was to find an explanation for the degradation phenomena observed in small laboratory bifacial modulus samples after prolonged wet heat testing,” said Paul Sommeling, lead author of the study. pv magazine.
In the study “Corrosion effects in bifacial crystalline silicon PV modules; interactions between metallization and encapsulation”, was published in Solar energy materials and solar cellsresearchers compared three encapsulation materials used in PV module production: ethylene vinyl acetate (EVA), polyolefin elastomer (POE), and thermoplastic polyolefins (TPO).
The research team said they found in previous studies that POE and TPO, unlike EVA, do not release acidic components, meaning they may perform better than EVA in terms of acid corrosion. However, there is still no evidence that these materials can actually increase the durability of modules in the field, as it takes a long time before relevant field degradation can be observed.
Using moist heat testing lasting up to 2,500 hours, the researchers compared the performance of the three materials in glass-encapsulation-cell-encapsulation-glass-based laminates without an edge seal.
“Commercially available n- or p-type bifacial cells (TOPCON and PERC) have been used to construct these laminates, which have subsequently been tested in a climate chamber under humid thermal conditions (85 C / 85% RH).” explained. “Current voltage (IV) and electroluminescence (EL) measurements have been taken every 500 hours to monitor the change in laminate properties over time.”
They used a “coring” technique to enable post-mortem analyzes of all the samples tested. For this they used scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) to analyze the effects of the tests on the solar cell surface. All encapsulants were tested according to the IEC 61215 PV standard and later with an extended test time.
According to the research team, testing showed that the hydrophobic and chemically inert TPO encapsulants provided the highest protection against moisture degradation compared to EVA and POE. It also showed that the front-side metallization of the tested n-type TOPCon cells is more susceptible to degradation by acid or moisture than their p-type counterparts.
“This front-side metallization of TOPCon cells degraded faster than back-side metallization and also faster than metallization of PERC cells in EVA-based laminates,” the researchers emphasized. “We create these differences from the different compositions of the cell metallization used.”
They concluded that the main factor causing mesh delamination is likely the breakdown of lead glass, which they said is part of the cellular metallization mesh and contains lead oxide (PbO).
“These results are partly surprising,” Sommeling said. “The corrosion effects of PV are known and reported in the literature, which examines both effects after field exposure and accelerated laboratory testing.”
He also explained that the most serious corrosion problems are largely related to the release of acetic acid from EVA, the most commonly used encapsulation material for PV panels. The acid causes corrosion of cell metallization and/or edging material or solder joints. EVA is currently being replaced by alternatives that do not release acidic compounds and this helps prevent many corrosion related problems.
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“The surprising or ‘new’ aspects described in our paper account for the quite striking differences in corrosion behavior observed between different metallization types and different solar cells, which can be correlated to the different compositions of cell metallization,” Sommeling added. “Even without acids, corrosion can still only occur due to the influence of moisture, and this again varies greatly with different types of metallization.”
“It can be concluded that the specific combination of n-type TOPCon solar cells together with EVA studied in our paper is probably more risky compared to other combinations of cell types and encapsulants,” confirmed Sommeling. “The use of EVA together with relatively high corrosion sensitive metallization should and indeed can be avoided. Alternatively, if the identified more corrosion-resistant metallization types can be applied to TOPCon solar cells, this should also help build more corrosion-resistant n-type PV panels.