Researchers at the University of New South Wales (UNSW) in Australia evaluated the effects of different flux types on the corrosion of metal contacts in tunneling oxide passivated contact (TOPCon) solar cells under damp heat conditions. The results showed that "no-clean" fluxes can cause severe corrosion of front-side silver-aluminum (Ag-Al) contacts.
Damp heat (DH) testing is an accelerated aging test that subjects photovoltaic devices to 85°C and 85% humidity for at least 1000 hours to assess module reliability under these extreme conditions. "Our research provides photovoltaic manufacturers with a rapid, low-cost method to identify flux-related reliability issues early in production, thereby reducing warranty claims and performance losses caused by moisture-induced corrosion," said Bram Hoex, corresponding author of the paper.
Flux is used during module assembly to remove the oxide layer from the surface of the solder ribbon to ensure a strong metal bond. The research team focused on "no-clean" fluxes, which do not require cleaning and can remove the oxide layer and form a strong bond, but leave a small amount of non-conductive residue.
The tests employed two commercial fluxes: Flux A, based on carboxylic acid, and Flux B, based on malic acid. Three n-type TOPCon cells were produced using the Laser Enhanced Contact Optimization (LECO) process in 2019, 2022, and 2023. The researchers noted that the cells had a similar structure, with a front-side boron-doped emitter covered with aluminum oxide (Al₂O₃) and silicon nitride (SiNx), and screen-printed silver grid lines; the back-side consisted of silicon dioxide (SiO₂), phosphorus-doped polysilicon, SiNx, and the same silver grid lines.
The samples were divided into five groups: front-side Flux A, front-side Flux B, back-side Flux A, back-side Flux B, and an unfluxed control. The flux was applied by spray and dried on an 85°C hotplate for up to 10 minutes.
Analysis revealed that "no-clean" flux residues caused significant corrosion of the TOPCon front-side Ag–Al contacts under wet heat conditions, increasing series resistance and reducing efficiency. Hoex noted, "Halogen-containing Flux A is significantly more corrosive than Flux B, but both can cause significant degradation."
The research team also found that backside silver paste exhibited little degradation due to its greater chemical stability, while a denser metallization structure and lower aluminum content improved corrosion resistance.
To address these issues, the researchers recommend conducting damp heat testing on unpackaged cells before module packaging to quickly identify flux-related risks. They also recommend selecting a low-halogen, acid-optimized flux formulation and optimizing the composition and structure of the metallization paste to limit flux penetration.
The research results have been published in the journal Solar Energy Materials and Solar Cells, titled "Assessing the impact of solder flux-induced corrosion on TOPCon solar cells."
Previously, a joint study by UNSW and Canadian Solar confirmed that flux selection is crucial to the reliability of TOPCon and heterojunction (HJT) cells. A separate team from the Korea Electronics Technology Institute (KETI) found that commercial fluxes can corrode the indium tin oxide (ITO) electrodes in HJT cells, posing a risk of premature degradation. UNSW has also explored the degradation mechanisms of TOPCon cells under UV induction, ethylene vinyl acetate (EVA) encapsulation, and sodium ion exposure, revealing various failure modes not seen in PERC modules.
Researchers at the University of New South Wales (UNSW) in Australia evaluated the effects of different flux types on the corrosion of metal contacts in tunneling oxide passivated contact (TOPCon) solar cells under damp heat conditions. The results showed that "no-clean" fluxes can cause severe corrosion of front-side silver-aluminum (Ag-Al) contacts.
Damp heat (DH) testing is an accelerated aging test that subjects photovoltaic devices to 85°C and 85% humidity for at least 1000 hours to assess module reliability under these extreme conditions. "Our research provides photovoltaic manufacturers with a rapid, low-cost method to identify flux-related reliability issues early in production, thereby reducing warranty claims and performance losses caused by moisture-induced corrosion," said Bram Hoex, corresponding author of the paper.
Flux is used during module assembly to remove the oxide layer from the surface of the solder ribbon to ensure a strong metal bond. The research team focused on "no-clean" fluxes, which do not require cleaning and can remove the oxide layer and form a strong bond, but leave a small amount of non-conductive residue.
The tests employed two commercial fluxes: Flux A, based on carboxylic acid, and Flux B, based on malic acid. Three n-type TOPCon cells were produced using the Laser Enhanced Contact Optimization (LECO) process in 2019, 2022, and 2023. The researchers noted that the cells had a similar structure, with a front-side boron-doped emitter covered with aluminum oxide (Al₂O₃) and silicon nitride (SiNx), and screen-printed silver grid lines; the back-side consisted of silicon dioxide (SiO₂), phosphorus-doped polysilicon, SiNx, and the same silver grid lines.
The samples were divided into five groups: front-side Flux A, front-side Flux B, back-side Flux A, back-side Flux B, and an unfluxed control. The flux was applied by spray and dried on an 85°C hotplate for up to 10 minutes.
Analysis revealed that "no-clean" flux residues caused significant corrosion of the TOPCon front-side Ag–Al contacts under wet heat conditions, increasing series resistance and reducing efficiency. Hoex noted, "Halogen-containing Flux A is significantly more corrosive than Flux B, but both can cause significant degradation."
The research team also found that backside silver paste exhibited little degradation due to its greater chemical stability, while a denser metallization structure and lower aluminum content improved corrosion resistance.
To address these issues, the researchers recommend conducting damp heat testing on unpackaged cells before module packaging to quickly identify flux-related risks. They also recommend selecting a low-halogen, acid-optimized flux formulation and optimizing the composition and structure of the metallization paste to limit flux penetration.
The research results have been published in the journal Solar Energy Materials and Solar Cells, titled "Assessing the impact of solder flux-induced corrosion on TOPCon solar cells."
Previously, a joint study by UNSW and Canadian Solar confirmed that flux selection is crucial to the reliability of TOPCon and heterojunction (HJT) cells. A separate team from the Korea Electronics Technology Institute (KETI) found that commercial fluxes can corrode the indium tin oxide (ITO) electrodes in HJT cells, posing a risk of premature degradation. UNSW has also explored the degradation mechanisms of TOPCon cells under UV induction, ethylene vinyl acetate (EVA) encapsulation, and sodium ion exposure, revealing various failure modes not seen in PERC modules.
