UNSW and Jolywood studied the thermal stability of laser-assisted TOPCon solar cells during module fabrication and high-temperature stress, identifying hydrogen-related defect dynamics as the key factor behind degradation and recovery. They found that lamination causes temporary efficiency losses that self-recover under light exposure, while optimized LAF can restore deteriorated contacts, providing practical guidance for reliable module production and testing.
A group of researchers from the University of New South Wales (UNSW) in Australia and Chinese module manufacturer Jolywood conducted a comprehensive assessment of how laser-assisted baking (LAF) processes affect the behavior of TOPCon cells under the thermal conditions encountered during soldering, laminating, and high-temperature stress.
“Our ffindings provide clear guidance for module fabrication and reliability testing workflows, particularly with regard to lamination protocols, LAF optimization and the interpretation of cell-to-module (CTM) losses,” said Bram Hoex, the study’s lead author. pv magazine. “We have provided new mechanistic insight into why LAF TOPCon cells behave differently from commodity devices during thermal processing, as well as practical recommendations to ensure stable performance during production and field use.”
In a previous joint research project, UNSW and Jolywood investigated how effectively Jolywood’s proprietary laser-assisted baking process, the so-called Jolywood Special Injected Metallization (JSIM), improves the efficiency of TOPCon solar cells on an industrial scale by reducing the recombination of Si-metal contacts. Specifically, the researchers found that the manufacturing step can increase cell efficiency by about 0.6% absolute compared to the basic one-step baking process.
In the new article, entitled “Thermal stability of laser-assisted TOPCon solar cells: crucial insights for module production, certification testing and field conditions‘ and published in Solar energy materials and solar cellsHoex and his colleagues explained that LAF processes such as the laser-enhanced contact optimization (LECO) technique have been adopted in the commercial production of solar modules in recent years and have increased concerns about the thermal stability of the cells.
“We investigated the thermal stability of LAF TOPCon solar cells under both moderate and high temperatures,” Hoex explains. ‘We have found evidence of that hydrogen redistribution introduced during LAF is a major contributor to defect activation and contact instability. A three-state defect model is proposed to explain the degradation and recovery dynamics.”
The tests were performed on 182.2 mm x 183.75 mm TOPCon cells fabricated with 130 μm n-type wafers. On the back side, a tunneling layer of silicon oxide (SiOx) and phosphorus-doped polycrystalline silicon (n⁺ poly-Si) were formed by plasma oxidation and plasma-assisted in situ doping (POPAID). Surface passivation was achieved using an aluminum oxide layer (AlOx) deposited via atomic layer deposition (ALD) and silicon nitride (SiNx) deposited via plasma-enhanced chemical vapor deposition (PECVD) on the front, and PECVD SiNx on the back. Commercial paste and LAF were used for metallization.
A moderate temperature thermal test was conducted to evaluate cell to module (CTM) losses, focusing on two key manufacturing steps: soldering and laminating. One group of cells underwent both processes, while a second group was subjected to lamination alone for comparison.
The analysis showed that soldering does not adversely affect cell performance, but lamination causes an efficiency loss of 0.29%, mainly due to fill factor reduction coupled with increased J02-type recombination, a type of recombination that usually affects fill factor and open-circuit voltage, especially at low and moderate voltages.
The researchers also found that one minute of sunlight in the sun completely restores performance, indicating that modules in the field naturally repair themselves. “Interestingly, recovery during light soaking and degradation after dark storage are very similar to the behavior observed in samples exposed to ultraviolet-induced degradation light (UVID), suggesting a possible link or similarity in the underlying mechanisms,” the scientists said.
The assessment also showed that rapid thermal annealing at 450 C leads to contact deterioration, with fill factor losses as high as 21.6%, but a subsequent LAF step effectively restores contact and restores performance.
Based on these findings, a three-state defect model and contact degradation mechanisms were proposed for industrial implementation. “These findings provide new insights into the reliability of LAF TOPCon cells and highlight key considerations for industrial processing and module reliability testing,” they concluded.
In June, researchers from the University of Oxford in the United Kingdom and Chinese metallization paste specialist Changzhou Fusion New Materials identified a new failure mode in LECO-based TOPCon solar panels.
Other research by UNSW showed the impact of solder flux on the performance of TOPCon solar cells, that of UV-induced degradation (UVID) in TOPCon 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.
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