A group of researchers led by France’s National Solar Energy Institute (INES) – a division of the French Alternative Energies and Atomic Energy Commission (CEA) – has investigated the reliability of heterojunction (HJT) solar panels under heat damp (HD) conditions and has found that the use of thin indium tin oxide (ITO layers) in combination with capping layer made of silicon nitride (SiNx) help maintain good reliability levels.
“For our testing, we used different cell structures, with different ITO thicknesses and different capping layers,” the research’s corresponding author, Lucie Pirot-Berson, told pv magazine. “The study was conducted at the module scale, not just the cell scale, and showed that reducing the ITO thickness accelerates the degradation mechanisms induced by sodium and moisture.”
“The results showed that reducing the indium consumption, which is necessary for the HJT technology, may therefore be difficult,” she went on to say. “Fortunately, some capping layers made of SiNX or silicon oxynitride (SiOyNz) provide protection against sodium-induced degradation and can be used in combination with ITO.”
The DH test is an accelerated test that tests the reliability of modules under extreme humidity and heat. In its standard form, the PV is placed in a controlled chamber with a temperature of 85 C and humidity of 85% for at least 1,000 h.
The academics used cells based on n-type 160 μm-thick M2 wafers for their experiment. They tested three different ITO layers with thicknesses of 15 nm, 30 nm and 100 nm and different SiOX and SiNX dielectric capping layers that were added on top of the thin ITO layers. The TCO layers were deposited via plasma-enhanced chemical vapor deposition (PECVD) equipment provided by Switzerland’s Meyer Burger.
“After TCO deposition, screen printing was performed using a pattern with 6 busbars,” the research group explained. “Capping layers were then added by PECVD technique in certain configurations: SiNX (100 nm) or SiOX (100 nm). The optimum thickness of the capping layers was calculated using the CROWM simulation software. After production, the cells were cut in half with an IR laser. Depending on configuration, two or three strings of two half-cells per batch were interconnected with electrically conductive adhesive (ECA).”
The cells were then encapsulated through thermoplastic polyolefin (TPO) in modules with both glass-glass and glass-backsheet configurations. The modules were laminated via a 3S laminator at 160 C with a lamination time of 18 m.
Under DH conditions, the glass-glass modules were found to be particularly susceptible to moisture degradation at their edges, while for glass-backsheet panels the main moisture ingress was found to be at the rear side. Furthermore, the scientists found that, in glass-glass panels, sodium ions are gradually released along the glass, on both sides, while in glass-backsheet modules they are released only on the front side.
“In general, moisture ingress is greater in glass-backsheet modules than in double-glass modules due to penetration through the backsheet layer,” they emphasized. “This greater moisture ingress results in more glass leaching and moisture-induced degradation, as well as more sodium-induced degradation in glass-backsheet modules.”
The front capping layers, meanwhile, were found to provide protection against sodium-induced degradation and “drastically” reduce short-circuit current losses. “However, these capping layers can be degraded by the laser cutting process at the half-cell edges, which requires further optimization,” the group added. “Finally, these capping layers do not reduce the FF losses of the modules, and even increase them. This remains to be investigated.”
Their findings are available in the study “Study and mitigation of moisture-induced degradation in SHJ modules by modifying cell structure,” published in Solar Energy Materials and Solar Cells. The research group also comprised scientists from EDF R&D and EDF Renouvelables, which are both units of French energy company EDF. “Finally, we provide a way to reduce indium by 85% while maintaining the performances and durability of heterojunction modules,” they concluded.
