| Work Detail |
Dutch scientists have reported an increased risk of degradation of n-type TOPCon cells with EVA encapsulant due to possible degradation by moisture.
Researchers from the Netherlands Organization for Applied Scientific Research (TNO) have conducted a series of damp heat tests to assess the impact, in terms of moisture degradation, of different encapsulation materials on the performance losses of bifacial PV modules.
"The aim of this research was to find an explanation for the degradation phenomena observed in small samples of laboratory bifacial modules after prolonged moist heat tests," lead author of the research, Paul Sommeling, explained to pv magazine .
In the study “ Corrosion effects in bifacial crystalline silicon PV modules; interactions between metallization and encapsulation ”, published in Solar Energy Materials and Solar Cells, the scientists made a comparison between three encapsulant materials used in module production photovoltaics: ethyl vinyl acetate (EVA), polyolefin elastomers (POE) and thermoplastic polyolefins (TPO).
The research group claimed to have observed in previous studies that POE and TPO, unlike EVA, do not release acidic components, which means that they may perform better than EVA with respect to acid-induced corrosion. However, there is still no hard evidence that these materials can actually increase the durability of modules in the field, as it takes a long time before significant degradation can be observed in the field.
Using moist heat tests lasting up to 2,500 h, the academics compared the performance of the three materials in glass-encapsulant-cell-encapsulant-glass-based laminates without the use of an edge sealer.
“Commercially available nop-type bifacial cells (TOPCON and PERC respectively) have been used to build these laminates, which have subsequently been tested in a climatic chamber under humid heat conditions (85ºC/85% relative humidity)”, they explained. “Current-voltage (IV) and electroluminescence (EL) measurements have been made at 500h intervals to follow the change in laminate characteristics over time.”
They used a technique known as "coring" to be able to perform post-mortem analysis of all the samples analyzed. To do this, they used scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) to analyze the effects of the tests on the surface of the solar cell. All encapsulants were tested to IEC 61215 PV and beyond, with extended test time.
According to the research group, tests showed that chemically inert, hydrophobic TPO encapsulants provided the highest degree of protection against moisture-induced degradation compared to EVA and POE. They also indicated that the front face metallization of the n-type TOPCon cells tested is more prone to acid- or moisture-induced degradation than their p-type counterparts.
“This front metallization of TOPCon cells degrades more quickly than the rear metallization and also more quickly than the metallization of PERC cells in EVA-based laminates,” the scientists emphasize. "We attribute these differences to the different compositions of the metallization of the cells applied."
They concluded that the main factor causing the delamination of the metal grid is probably the degradation of the lead glass, which they said is part of the metallization grid of the cells and contains lead oxide (PbO).
"These results are partly surprising," Sommeling said. "The effects of corrosion in photovoltaics are known and have been collected in the bibliography that studies both the effects after exposure in the field and after accelerated tests in the laboratory."
He also explained that the most serious corrosion problems are largely related to the release of acetic acid from EVA, the most widely used encapsulant material in photovoltaic panels. The acid causes corrosion of the metallization of the cells and/or of the tabulation material or of the solder joints. Currently, EVA is being replaced by alternatives that do not release acidic compounds, which helps to avoid many problems related to corrosion.
“The surprising or new aspects described in our article take into account the quite remarkable differences observed between different types of metallization and different solar cells with regard to corrosion behavior, which can be correlated with different metallization compositions. of the cell,” added Sommeling. “Also without acids present, corrosion can still occur just from the influence of moisture, and again this varies widely between the different types of plating applied.”
“It can be concluded that the specific combination of n-type TOPCon solar cells studied in our work in combination with EVA is probably a higher-risk combination, compared to other combinations of cell types and encapsulants,” Sommeling said. “The application of EVA in combination with a relatively corrosion-susceptible metallization should and 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.” |
