UNSW unveils new ageing method to assess TOPCon solar module degradation

A research team from the University of New South Wales (UNSW) has developed a new cell-level accelerated ageing method for TOPCon solar technologies.

“Conventional solution-based accelerated tests like cetic acid soaking impose chemically unrealistic conditions and often fail to reproduce degradation trends at the module level,” the research’s lead author, Bram Hoex, told pv magazine. “We introduced a chemically selective, pH-controlled, nitrate-based cell-level aging method that replicates the mildly acidic environment within EVA-encapsulated modules.”

The scientists explained that to accelerate the assessment of TOPCon cell and module stability, solution-based ageing methods, particularly immersion in acetic acid (CH₃COOH), have been widely used. While these approaches provide valuable mechanistic insights, they have limitations, as both sides of the cell are exposed simultaneously, and the chemical conditions are often harsher than the mildly acidic environment that develops inside EVA-encapsulated modules.

Alternative methods, such as spraying salts like sodium chloride (NaCl) or sodium bicarbonate (NaHCO₃) onto cell surfaces, fail to replicate these realistic conditions. Nitrate species are however naturally occurring and can produce tunable acidic environments depending on the cation used, making them well suited for chemically relevant accelerated ageing.

“Building on these insights, we develop a nitrate-based, single-side ageing method in which controlled-pH contaminants are applied to the front surface before damp-heat exposure,” Hoex said. “This approach enables targeted evaluation of front-side metallisation stability and reliably reproduces module-level degradation trends, providing a framework for chemically realistic accelerated testing of TOPCon solar cells.”

The research team conducted the tests on TOPCon solar cells measuring 182 mm × 183.75 mm and based on n-type Czochralski silicon wafers with two front-contact variants: conventional silver/aluminum (Ag/Al) paste and a low-Al Ag paste processed using a laser-assisted firing technique (Ag/LAF). All cells were half-cut to form 144-cell modules, encapsulated with EVA – UV-blocking on the front and UV-transparent on the rear – and completed with a transparent backsheet featuring a white grid.

Module-level damp-heat testing was conducted according to the IEC TS 62782 standard, with electrical output measured and electroluminescence imaging used to identify degradation. At the cell level, accelerated stress tests included immersion in 0.1 M CH₃COOH or CH₃COONa at 85 C, as well as spray application of salt solutions with controlled pH, followed by damp-heat testing at 85 C and 85% relative humidity (DH85). Solution pH was determined at 25 C, and relative acidity trends were maintained during high-temperature ageing.

The electrical performance was measured before and after tests using a LOANA system, while photoluminescence and series resistance maps were acquired with a BT Imaging R3 system. All experiments included multiple replicates to ensure reproducibility, enabling comprehensive assessment of front-side metallisation stability and module-level degradation fingerprints.

The tests showed that the Ag/Al and Ag/LAF cells showed different degradation behaviors, with Ag/LAF contacts exhibiting higher sensitivity to acidic conditions and pronounced losses in efficiency and fill factor due to front-contact delamination.

Scanning electron microscopy (SEM) and focused ion beam-scanning electron microscopy (FIB-SEM) analyses revealed that Ag/Al contacts rely on Al spikes and high glass frit content, which provide mechanical robustness and slower degradation, while Ag/LAF contacts depend on silver nanoparticles (AgNPs) with thin lead oxide (PbO)-rich glass frit layers that dissolve under acidic or chloride-rich conditions.

Furthermore, single-sided salt treatments highlighted the role of solution pH and specific ions in front-contact corrosion, showing severe degradation under aluminium nitrate (Al(NO₃)₃) and chloride (Cl⁻) salts. Neutral salts were found to cause minor effects, whereas acidic nitrate solutions accelerated PbO glass-frit dissolution. Moreover, Cell-level DH85 with zinc nitrate (Zn(NO₃)₂) demonstrated consistent trends with module-level performance, with fill factor loss as the dominant factor.

“Our approach provides a fast and physically meaningful screening tool to identify reliability risks at the solar cell stage, before committing to full module assembly and long-term damp-heat testing exceeding 1,000 hours,” Hoex said. “It enables rapid optimization of metallization and bill-of-materials (BOM) choices, reducing development time and costs while avoiding misleading conclusions that can arise from overly aggressive or non-representative accelerated tests. By establishing a clear link between cell-level testing and actual module degradation mechanisms, this method enhances the predictive capability for long-term performance.”

The new methodology was presented in “Bridging accelerated cell-level degradation to module-relevant failure mechanisms in TOPCon solar cells and modules,” published in the Chemical Engineering Journal. “In essence, this work demonstrates that well-designed, chemically relevant cell-level tests can significantly accelerate reliability assessment, while still capturing the key degradation pathways observed at the module level,” Hoex concluded.

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