| Work Detail |
Researchers at the University of New South Wales (Australia) claim to have identified new degradation mechanisms of TOPCon contacts significantly influenced by the combination of ions and aluminum-silver paste compositions. The main degradation mechanism was a significant increase in series resistance.
Scientists from Australias University of New South Wales (UNSW) and Chinese-Canadian PV module manufacturer Canadian Solar have investigated the degradation of TOPCon industrial solar cells under accelerated test conditions of 85°C and 85% relative humidity in the so-called Damp Heat Test (DH85).
“Our findings highlight the vulnerability of TOPCon solar cells to contact corrosion, highlighting the electrochemical reactivity of the metallization as a potential risk for long-term module operation,” lead author of the research Bram Hoex told pv magazine . “This study provides crucial insights into the degradation mechanisms of TOPCon cells, essential for optimizing the performance and improving the long-term reliability of these modules.”
Hoex explained that the study complements the research group’s previous work and extends it by using two types of sodium-containing salts, which allowed the researchers to observe differences in the chemical reactions taking place. “This better understanding of the underlying chemistry that explains the degradation is key to further improving the stability of TOPCon,” he added.
The group published a first paper in late April, identifying three new types of failures in TOPCon solar modules that had never been detected in PERC panels. This work aimed to assess the impact of bills of materials (BOM) on the reliability of commercially available PERC and TOPCon solar cells.
“We were quite surprised by the results,” Hoex told pv magazine at the time. “We expected the polyolefin elastomer (POE) to perform well overall, but we found that some POEs performed very poorly. This is probably due to the different additives used in POE, which react with the solder flux and metallization, causing fretting corrosion.”
In the new research, scientists discovered significant differences between the sensitivity of the front and back sides of TOPCon solar cells when degradation was induced by two sodium-containing salts: sodium bicarbonate (NaHCO3) and sodium chloride (NaCl).
The researchers conducted tests using 158.75 mm x 158.75 mm TOPCon G1 n-type solar cells with five busbars. Their average power conversion efficiency was 23.1%.
On the front side, the devices were fabricated with a boron diffuse emitter coated with aluminum oxide (Al2O3) and silicon nitride (SiNx), as well as a silicon oxynitride (SiOyNz) stack providing surface passivation and acting as an antireflection coating (ARC). On the back side, the cells featured a TOPCon architecture based on a silicon oxide (SiO2) tunnel layer paired with a phosphor-doped poly-Si layer, as well as a SiNx ARC layer.
"The front-side metallization comprised a combination of silver (Ag) and aluminum (Al, 3-5 at.%) as the conductive material, while the back-side metal contact consisted solely of Ag for conduction," the researchers explained.
DH85 testing was performed using scanning electron microscopy (SEM) imaging on five cell samples: reference cells treated with deionized water (DIW) only; cells exposed to NaHCO3 on the front; NaHCO3 on the back; cells exposed to NaCl on the front; and cells exposed to NaCl on the back.
As for the first group, the scientists found that exposing TOPCon cells to high humidity at elevated temperatures for extended periods did not intrinsically lead to degradation. However, in the other groups, efficiency degradation was significant.
“Our accelerated test with NaCl causes extreme degradation of power conversion efficiency, with a relative reduction of 92%, while NaHCO3 causes a relative reduction of efficiency of 5%,” says Hoex. “The main degradation mechanism is a significant increase in series resistance (Rs), likely due to electrochemical reactions within the silver-aluminum (Ag/Al) paste.”
“When applied to the backside of the TOPCon solar cell, NaHCO3 increases recombination and deteriorates the contacts, leading to a 16% efficiency reduction after only 100 hours of DH85 testing,” Hoex added. “In contrast, NaCl primarily increases recombination and does not appear to affect the contact, causing a 4% efficiency loss. Further investigation reveals degradation mechanisms at the backside metal contacts, in particular oxidation at the Ag-Si interface.”
The scientists highlighted that Ag/Al pastes are exposed to long-term stability risks when exposed to ionic-level contaminants and stated that other ionic-level combinations should be investigated.
“In summary, this study reveals that TOPCon contact degradation mechanisms are significantly influenced by the combination of ions and paste compositions,” the researchers stated. “It provides crucial information on the degradation patterns and underlying factors affecting the metal contacts of TOPCon solar cells, potentially aiding in the evaluation of module-level reliability.”
Their findings are available in the article “ Unveiling the origin of metal contact failures in TOPCon solar cells through accelerated damp-heat testing,” published in Solar Energy Materials and Solar Cells . The research team also included scholars from China’s Nantong University and Shanghai Jiao Tong University.
In another study published earlier in April, the same research team collaborated with scientists at Chinese panel manufacturer Jolywood to design a laser-assisted baking process for manufacturing TOPCon solar cells that can reportedly increase contact quality and corrosion resistance while reducing production costs.
Called Jolywood Special Injected Metallization (JSIM), the new technique involves a laser-assisted baking process using a customized silver (Ag) paste for front contact formation on TOPCon solar cells. This is a low-temperature baking technique intended to facilitate the penetration of the paste through the cell’s front anti-reflective coating.
“JSIM technology is already being produced on a large scale by Jolywood, and other companies are also developing and implementing their own versions of laser-assisted baking,” Bram Hoex, lead author of the research, told pv magazine at the time . “Jolywood was one of the first manufacturers to implement TOPCon technology in large-scale production.” |
