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Researchers in France have investigated heterojunction solar module reliability in damp heat environment and have found that sodium ions are the main source of degradation.
A group of researchers from French research center Institut Photovoltaïque d’Ile-de-France (IPVF) and EDF R&D, a unit of French energy giant EDF, has conducted a series of tests to assess the reliability of heterojunction (HJT) solar panels in a damp heat environment and has identified sodium ions has the main degradation factor.
“Our study shows that sodium ions induce degradation of cell passivation, especially on the front side,” the research’s lead author, Lucie Peirot-Berson, told pv magazine. “We also ascertained that using sodium-free glass significantly reduces module degradation.”
In the study “Failure modes of silicon heterojunction photovoltaic modules in damp heat environment: Sodium and moisture effects,” published in Solar Energy Materials and Solar Cells, the researchers explained they investigated six HJT module cover configurations, with each based on 160 μm-thick M2 n-type wafers and half-cut cells connected in pairs with ribbons and pads of electrically conductive adhesive (ECA).
All modules were encapsulated with thermoplastic polyolefin, which the scientists said had a high water absorption coefficient. “This material was chosen to enhance the migration of moisture and ions and to better highlight the degradation mechanisms,” they emphasized.
The six module cover architectures were: soda-lime glass-glass; low sodium glass-glass; sodium-free glass-glass; frontsheet-backsheet; glass-backsheet; and frontsheet-glass. Opaque aluminized backsheets and transparent films were used as frontsheet or backsheet in the proposed configurations.
Overall, 17 bifacial modules and one monofacial panel were manufactured in a 3S laminator at 160 C. They were then passed to an ESPEC DH85 chamber with standard aging parameters of 85 C and 85% relative humidity (RH). A Spire 5600 SPL flasher was used to measure module degradation and photoluminescence (PL) and electroluminescence (EL) images were utilized to analyze I-V curves.
“For EL, carriers are generated by injecting current into the module. In this case, a dark area can correspond to different kinds of degradation. It can come from resistive losses that limit the injected current or it can be depassivated areas that favor non-radiative recombinations instead of radiative ones,” the academics explained. “For PL, the carriers are generated by exposure to light thanks to the photovoltaic effect. In this case, a dark zone only corresponds to a lower quality of passivation.”
The tests showed that the modules with soda-lime glass-glass configuration suffered the highest open-circuit voltage and short-circuit current losses, due to the depassivation of the cell by the action of sodium ions originated from glass lixiviation.
The analysis also showed that the action of sodium ions in inducing the degradation of cell passivation is particularly strong on the front side, while moisture was found to induce degradation of the transparent conducting oxide (TCO) and contacts and result in fill factor losses.
“The study shows a greater sensitivity to sodium-induced degradation of cell front side compared to the rear side, with significant front-surface recombination being visible on the front side,” Peirot-Berson said. “This difference could be explained by the nature of the amorphous silicon layers and the morphology of the TCO layers as already shown in previous literature.”
Looking forward, said their findings should be validated by testing other HJT module architectures.
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