Researchers from the University of New South Wales and Jolywood found that corrosion-induced degradation in TOPCon solar cells is mainly driven by glass frit chemistry in low-aluminum silver metallization. Their findings show that barium-zinc modified frits significantly improve resistance to acetic acid and moisture-heat stress, allowing for more stable silver-silicon interfaces and reducing power loss at the panel level.
A research team from the University of New South Wales (UNSW) and Chinese solar module manufacturer Jolywood have investigated the causes of corrosion-induced degradation in TOPCon solar cells fabricated with a layer of aluminum (Al) silver (Ag) paste and ethylene vinyl acetate (EVA) and EVA/polyolefin/EVA (EPE) encapsulant and found that glass frit chemistry plays a key role in the degradation process.
“Our work establishes a direct correlation between acetic acid corrosion at the cell level and moist heat degradation at the module level in laser-assisted baking (LAF) processed TOPCon devices,” said corresponding author Bram Hoex. pv magazine. “We have shown that glass frit chemistry is a crucial parameter for metallization reliability in EVA-based TOPCon modules.”
“The research also provides practical guidance for designing low Al-Ag corrosion-resistant pastes compatible with cost-effective glass-backsheet module architectures and supports the broader industrial transition to reliable EVA-compatible TOPCon technologies,” he continued. “We believe this work provides important insight into how metallization design and encapsulation chemistry interact under moist heat (DH), especially as the industry strives for lower-cost bills of materials (BOMs) for TOPCon modules.”
The researchers conducted a series of tests, both at the cell and module levels.
As for the cells, they were fabricated on G10 n-type Czochralski (Cz) silicon wafers with two types of low aluminum silver pastes. Both cell types underwent baking and LAF processes, differing only in front-contact metallization, referred to as Pastes A and B. Paste compositions were analyzed using inductively coupled plasma mass spectrometry (ICP-MS) and inductively coupled plasma optical emission spectroscopy (ICP-OES) to quantify silver and trace elements.
To simulate the acidic conditions caused by EVA degradation, cleaned cells were immersed in 0.10 mol/L acetic acid at room temperature. Electrical performance was monitored before and after exposure using IV measurements, photoluminescence (PL) imaging, series resistance mapping, and contact resistance testing. Additional microstructural and chemical analyzes were performed using scanning electron microscopy (SEM), focused ion beam (FIB) milling, energy dispersive spectroscopy (EDS) mapping and elemental quantification to investigate corrosion effects at the Ag-Si interface.
At module level, glass/EPE/TOPCon/EVA/backsheet structures were subjected to moisture-heat tests at 85 C and 85% relative humidity in accordance with IEC 61215 standards. Periodic electroluminescence (EL) imaging and electrical measurements monitored degradation during accelerated aging. After 1500 hours of exposure to moist heat, the modules were reevaluated to quantify performance loss and identify degradation patterns.
Image: UNSW, Progress in Photovoltaics, CC BY 4.0
The cell-level acetic acid tests showed strong performance differences between TOPCon solar cells using pastes A and B, despite similar initial efficiencies of approximately 25.2%. Paste A degraded rapidly and lost between 80% and 90% efficiency within 120 minutes due to a sharp increase in series resistance and severe loss of fill factor caused by corrosion at the Ag-Si interface. In contrast, Paste B showed much slower degradation, with stable open-circuit voltage and short-circuit current for 240 minutes and only a moderate increase in series resistance.
“Under accelerated exposure to acetic acid, Paste A showed rapid degradation, including catastrophic increases in series resistance and severe loss of electrical performance,” Hoex pointed out. “In contrast, Paste B maintained stable contact resistance, preserved the integrity of the Ag-Si interface, and showed significantly slower degradation.”
Contact resistance measurements confirmed severe, non-uniform degradation in Paste A, while Paste B maintained stable contacts with low resistance. Structural analysis also revealed that Paste A contains a lead-phosphorus (Pb/B)-rich glass frit without barium (Ba), which is highly vulnerable to acid-induced dissolution. Paste B contains Ba- and zinc (Zn)-modified glass chemistry, which improves resistance to acid corrosion. “ Ba-containing glass frits showed significantly improved chemical durability, suppressing ionic leaching and maintaining interfacial continuity under acidic and humid conditions,” Hoex explains.
Meanwhile, SEM and FIB imaging showed that Paste A exhibited almost complete dissolution of the interfacial glass layer, leading to voids and contact failure, while Paste B retained a continuous Ag-Si interface. Additionally, module-level moist heat testing confirmed the same trend, with Paste A modules losing between 28% and 30% of power due to fill factor-induced losses, while Paste B modules degraded by only 4% to 5%. EL imaging further revealed severe, non-uniform resistive damage in Paste A modules compared to stable performance in Paste B.
“Overall, these results demonstrate that the chemical durability of the glass frit rather than the metallic Ag phase determines metallization reliability in LAF TOPCon devices,” the scientists concluded. “Incorporating alkaline earth modifiers such as Ba and Zn into the glass network provides an effective means to reduce degradation caused by acetic acid and moisture, enabling stable Ag-Si interfaces and extended module life.”
Their findings were presented in “Enabling EVA for TOPCon: How Glass Frit Composition Controls Resistance to Acetic Acid-Induced Corrosion”, published in Progress in photovoltaics.
Other research by UNSW showed the impact of POE encapsulants on the corrosion of TOPCon modules, 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. Also UNSW and Jolywood investigated how effectively Jolywood’s proprietary laser-assisted baking process, called Jolywood Special Injected Metallization (JSIM), improves the efficiency of industrial-scale TOPCon solar cells by reducing the recombination of Si-metal contacts and found that the manufacturing step can increase cell efficiency by approximately 0.6% absolute compared to the basic one-step baking process.
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