A UNSW-led team found that annealing conditions significantly affect the voltage, strain and microstructure in copper-plated heterojunction solar cell contacts, with fast annealing conditions increasing the microvoltage in both copper and indium tin oxide.
A team of scientists led by Australia’s University of New South Wales (UNSW) has investigated how stress and strain evolve in copper (Cu)-plated contacts on heterojunction (HJT) solar cells under different incandescence conditions. Their work specifically investigated how annealing affects the material properties of Cu, indium tin oxide (ITO) and silicon (Si).
“We applied multiple characterization methods to understand how annealing conditions affect stress and strain in Cu-plated HJT cells,” co-author Pei-Chieh Hsiao told pv magazine. “Our results show that Cu contacts on HJT cells must be carefully assessed to balance adhesion with mechanical integrity.”
Hsiao emphasized the importance of controlling the microscopic structure of copper contacts to limit mechanical stress in HJT solar cells. “Ideally, plated Cu with low defect density and (100) crystal texture is preferred,” he explained. “This reduces the stress in Si after annealing due to a lower Young’s modulus. The desired texture can be achieved by adjusting the electrolyte or plating parameters, and the annealing can then be optimized to minimize the thermal stress while maintaining the (100) orientation.”
The team started with silicon heterojunction G12 half-cut n-type precursors measuring 210 mm x 105 mm. The cells were coated with a resin-based mask to limit copper plating, with selective openings created via a collimated light source. Copper was then plated on the exposed ITO surface using an acid-based plating solution at a current density of 42 mA/cm².
The team compared three annealing methods. In self-annealing, samples were stored at room temperature in a low humidity environment. Rapid annealing (same day) was carried out in dry compressed air at 205 ± 5 C for 45 seconds under about 15 solar rays. Rapid annealing (next day) under the same conditions, but was performed approximately 24 hours after plating.
Image: University of New South Wales, Sydney, Solar Energy and Solar Cell Materials, CC BY 4.0
“Due to the limitation of low-temperature processing of HJT cells, rapid annealing was carried out at 200 °C, which is lower than the grain growth stage at more than 250 °C,” Hsiao said. “It means that annealing plated Cu contacts on HJT cells would clearly outperform that on PERC or TOPCon cells, where higher annealing temperatures are allowed and improved contact adhesion has been demonstrated.”
The team then examined the samples in a series of tests. First, nanoindentation was used to measure the mechanical strength and stiffness of the plated copper. Secondly, X-ray diffraction (XRD) was used to investigate the crystal structure of the copper and the underlying ITO layer. Finally, Raman spectroscopy was used to map the mechanical stress caused by the copper contacts in the silicon, especially near the contact edges.
The analysis revealed that no significant differences were found in the yield strength or plastic response of plated Cu, which was consistent with the similar Cu grain size. Furthermore, XRD patterns showed that fast annealing decreased the Cu lattice parameter and promoted grain growth in the Cu(200) crystallographic orientation, while simultaneously increasing the ITO lattice parameter and full width at half maximum (FWHM).
As a result, microstresses in both Cu and ITO increased under rapid annealing, with the scientists noting that Raman spectroscopy revealed approximately 2 μm wide regions of high local strain in the silicon along the plated Cu fingers, with the strain being lower in self-annealed Cu and higher in fast-annealed Cu.
These results indicate that minimizing defects and promoting preferential (100) texture in plated Cu can reduce stress transfer to Si and ITO. Maintaining uniform plating conditions and careful surface preparation are also essential to achieving optimal texture and adhesion. In general, self-annealing is preferred when comparable contact adhesion can be achieved as it maintains (100) orientation and minimizes thermal stress.
The research work was described in “Stress and strain analysis of Cu-plated contacts on HJT cells under different annealing conditions”, published in Solar energy materials and solar cells. Scientists from Australia’s University of New South Wales and technology company SunDrive Solar contributed to the research.
In early January, a research team from UNSW and Chinese-Canadian solar panel manufacturer Canadian Solar investigated how HJT solar cells are affected by sodium (Na) and moisture degradation under accelerated moisture-heat tests and discovered that most degradation modes primarily affect the cells themselves, making cell-level testing preferable.
A month later, another UNSW team assessed the impact of solder flux on HJT solar cells and found that the composition of this component is critical to preventing large cracks and significant peeling.
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