| Work Detail |
French researchers have studied the reliability of heterojunction solar modules in a humid heat environment and found that sodium ions are the main source of degradation.
A group of researchers from the French research centre Institut Photovoltaïque dIle-de-France (IPVF) and EDF R&D, a unit of French energy giant EDF, have conducted a series of tests to assess the reliability of heterojunction (HJT) solar panels in a humid heat environment and have identified sodium ions as the main degradation factor.
“Our study shows that sodium ions induce degradation of the cell passivation, especially on the front side,” lead author of the research Lucie Pirot-Berson told pv magazine . “We also found that the use of 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 that they investigated six HJT module deck configurations, each based on 160 µm-thick n-type M2 wafers and half-cut cells connected in pairs with electrically conductive adhesive (ECA) tapes and pads.
All modules were encapsulated using 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 reveal the degradation mechanisms,” they stressed.
The six module cover architectures were: glass-lime; low-sodium glass-lime; sodium-free glass-lime; front sheet-back sheet; glass-back sheet; and front sheet-glass. The proposed configurations used opaque aluminized back sheets and transparent sheets as front or back sheet.
In total, 17 bifacial modules and one monofacial panel were fabricated on a 3S laminator at 160°C. They were then transferred 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) imaging were used to analyze I-V curves.
“In the case of EL, carriers are generated by injecting current into the module. In this case, a dark zone can correspond to different types of degradation. It can come from resistive losses that limit the injected current or it can be depassivated zones that favour non-radiative rather than radiative recombinations,” the academics explain. “In the case of PL, 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.”
Tests showed that modules with a glass-soda-lime glass configuration suffered the greatest losses of open-circuit voltage and short-circuit current, due to the depassivation of the cell by the action of sodium ions originating from the leaching of the glass.
The analysis also showed that the action of sodium ions in inducing cell passivation degradation is particularly strong on the front face, while humidity was found to induce degradation of the transparent conductive oxide (TCO) and contacts and cause losses in fill factor.
“The study shows a higher sensitivity to sodium-induced degradation of the front side of the cell compared to the back side, with significant recombination of the front surface being visible on the front side,” says Peirot-Berson. “This difference could be explained by the nature of the amorphous silicon layers and the morphology of the TCO layers, as already demonstrated in previous publications.”
Looking ahead, he said his findings should be validated by testing other HJT module architectures.
The research team also included scientists from Frances National Institute of Solar Energy (INES), a division of the French Alternative Energies and Atomic Energy Commission (CEA). |
