UNSW researchers found that some POE encapsulants can cause severe corrosion in TOPCon solar panels, causing up to 55% power loss under humid heat. Their research highlights that the reliability of the modules depends on the exact formulation of the encapsulant, and not just the polymer type.
A group of researchers from Australia’s University of New South Wales (UNSW) have investigated the impact of different encapsulants on the corrosion-induced degradation of n-type tunnel oxide passivated contact (TOPCon) modules and have found that some POE encapsulants are more susceptible to causing corrosion than other types.
“In this study, we show that not all POE encapsulants are automatically safe for TOPCon modules,” corresponding author Chandany Sen said. pv magazine. “By combining electrical testing, spectroscopy and electron microscopy, we map out step-by-step how a specific commercial POE formulation can break down under heat and humidity, generate a cocktail of organic acids and then chemically attack the silver-aluminum contacts until the module loses more than half its power.”
For their tests, the scientists fabricated TOPCon mini-modules encapsulated in ethylene-vinyl acetate copolymer (EVA) and three commercially available polyolefin elastomers (POE-A, -B and -C).
Sen highlighted that some POE formulations with appropriate antioxidant and UV stabilizer packages still remained stable. “The key message is that reliability depends on the exact recipe of the encapsulant, not just the label of the polymer family,” she said. “Encapsulants that look very similar on the outside produce completely different results inside the module. One kept the contacts essentially intact, while the other created an acidic environment that severely degraded the metal lattice.”
The teams’ research began by manufacturing TOPCon mini-modules that differed only in the encapsulant. The TOPCon cells therein were off-the-shelf, with a boron-doped emitter (p+ emitter), aluminum oxide (Al2O3)/hydrogenated silicon nitride (SiNx:H) stack, and a screen-printed silver/aluminum H-pattern grille on the front. On the backside was a silicon dioxide (SiO2)/phosphorus-doped polysilicon (n+poly-Si)/SiNx:H stack and a screen-printed H-patterned silver lattice.
All cells were soldered on both sides to connect ribbon/lip wires to the cell rails, forming an array of 8 cells. They were then all encapsulated using various materials, including commercial sources.
According to the manufacturer’s datasheets, the EVA and the three POEs have an optical transmission of at least 90%. POE-C exhibits UV-cut properties, while the others remain completely transparent across the solar spectrum. The coupling level, expressed as gel content, was up to 80% for the EVA, up to 70% for POE-A, up to 75% for POE-B and 55-85% for POE-C. Lamination of all mini-modules was performed using a two-chamber, two-stage thermal process.
Image: University of New South Wales, Solar Energy and Solar Cell Materials, CC BY 4.0
Module performance was measured before and after 1000 hours of moist heat (DH) using a commercial flash tester at 85 C and 85% relative humidity to study moisture-induced failures. “The biggest surprise was that one POE encapsulant, which should have been the safe choice compared to standard EVA, actually performed much worse: in our moist heat test, the EVA module lost about 11% of its power, while the POE-C module lost about 55%,” Sen added. Upon compression, POE-A and POE-B showed reductions of 15.6% and 6%, respectively, and produced no measurable amounts of organic acids.
“An equally surprising result was the role of the UV absorber in the encapsulant. This additive is normally added to protect UV-sensitive solar cells and polymer backsheets from sunlight, but in this POE-C it appears to break down under heat and moisture, creating additional organic acids that join the corrosive cocktail that attacks the metal contacts,” said Sen. “In other words, a chemical intended to protect the module can actually help cause premature failure under the wrong conditions.”
Sen’s team added that a multi-technical analysis of the POE-C mini-module suggests a potential cascade of mutually reinforcing degradation pathways. This included a thermo-oxidative degradation of the POE matrix, producing carboxylic acids; residual azelaic acid from soldering flux; and a hydrolytic degradation of benzophenone UV absorbers into benzoic and phenolic acids.
“We are currently working on follow-up research, where we will deliberately vary only one additive at a time, so that we can determine exactly which chemical ingredients make a module robust and which cause corrosion in TOPCon and other advanced cell technologies,” said Sen. “The long-term goal is to translate our current negative findings into clear design rules and quality control tools that will keep future PV modules reliable for decades.”
The research work appeared in “The Dark Side of Certain POE Encapsulant: Chemical Routes to Metallization Corrosion in TOPCon Modules”, published in Solar energy materials and solar cells. Researchers from Germany’s Fraunhofer Center for Silicon Photovoltaics (CSP) and Anhalt University of Applied Sciences also participated in the study.
Other research by UNSW demonstrated the impact of solder flux on the performance of TOPCon solar cells, degradation mechanisms of industrial TOPCon solar panels encapsulated with ethylene vinyl acetate (EVA) under accelerated moist heat conditionsas well as the vulnerability of TOPCon solar cells to contact corrosion and three types of defects in TOPCon solar modules that have never been detected in PERC panels.
In addition, UNSW scientists investigated the sodium-induced degradation of TOPCon solar cells under moist heat exposure, the role of ‘hidden contaminants’ in the degradation of both TOPCon and heterojunction devices, and the impact of electron irradiation on PERC, the performance of TOPCon solar cells.
More recently, another UNSW research team has developed an experimentally validated model that links UV-induced degradation in TOPCon solar cells to hydrogen transport, charge capture and permanent structural changes in the passivation stack.
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