A research team at the University of New South Wales (UNSW) in Australia has investigated the partial replacement of silver (Ag) with nickel (Ni) powders in TOPCon solar cell rear-side metallization and has found that Ni powder characteristics significantly affect electrical performance, printability and cost savings.
“One of the most important findings is that replacing Ag with Ni is not simply a matter of increasing the Ni content. The characteristics of the Ni powder have a significant impact on both electrical performance and printing behaviour,” corresponding author Ning Song told pv magazine.
“With an appropriate nickel powder, we demonstrated a 6.8% reduction in rear-finger raw-metal costs while maintaining almost the same solar cell efficiency. However, increasing the Ni content further did not necessarily deliver greater savings, because additional paste consumption during printing offset the expected benefits,” she added.
The scientists explained that Ni has emerged as a promising Ag substitute due to its relatively low cost and good electrical conductivity. However, Ni oxidation during firing and variations in powder characteristics can affect electrode performance. They also noted that previous studies have demonstrated the feasibility of Ag-Ni hybrid pastes, although their practical cost benefits remain uncertain.
Against this backdrop, the researchers investigated two commercial Ni powders with particle sizes of approximately 1.5 μm and 3.8 μm, replacing 5% to 15% of the metal powder in industrial TOPCon rear-finger pastes. They examined electrical performance, contact resistance, electrode microstructure and printing behavior, while also assessing paste consumption and finger geometry to determine actual raw-metal cost savings.
The team prepared five rear-side metallization pastes containing 90% metal powder, 2% glass frit and 8% organic vehicle. An Ag-only paste served as the baseline, while the other four formulations contained 5% to 15% Ni in the metal powder fraction, using two commercial powders designated Ni-1500 and Ni-4000. The pastes were screen-printed onto industrial n-type TOPCon solar cells measuring 182 mm × 183.75 mm, followed by drying, co-firing at a peak set temperature of 724 C and laser-enhanced contact optimization (LECO).
The experiments were conducted over two industrial production runs, with at least 12 cells per formulation. The researchers measured solar cell efficiency, series resistance and spatial electrical characteristics using calibrated testing equipment and imaging techniques. They evaluated contact and finger resistivities using the transfer length method and four-point probe measurements, respectively. Scanning electron microscopy, energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy were used to examine electrode microstructure and contact interfaces.
The team also measured finger geometry, paste viscosity and actual paste consumption to assess printing behavior. Ag and Ni consumption per watt was calculated from measured paste deposition and average cell power, while rear-finger raw-metal costs were estimated using representative metal prices to evaluate the economic benefits of Ni substitution.
The analysis showed that Ni powder characteristics significantly affected the electrical performance and printing behavior of the TOPCon cells. The finer Ni-1500 powder had a higher specific surface area and oxygen content than the coarser Ni-4000 powder, and exhibited greater resistive losses at the same Ni loading.
At 5% Ni loading, the Ni-1500 formulation maintained the baseline efficiency of 26.349%. However, increasing the loading to 10% reduced efficiency by 0.225 percentage points, to 26.124%. By contrast, the formulation containing 10% Ni-4000 achieved an efficiency of 26.325%, just 0.024 percentage points below the baseline.
Electrical measurements revealed higher contact and finger resistivities with the 10% Ni-1500 formulation, while microscopy identified more voids and a less continuous Ag network. Surface analysis also indicated differences in oxide-related contact chemistry between the formulations. Ni substitution increased printed finger widths and paste consumption, particularly at higher loadings.
The researchers found that the 10% Ni-4000 formulation offered the most favorable balance between cost and efficiency, reducing rear-finger raw-metal costs by 6.8%, equivalent to $561,500/GW. Increasing the Ni-4000 loading to 15% maintained a similar efficiency of 26.302%, but increased paste consumption by 17% compared with the Ag-only baseline, reducing cost savings to just 0.3%.
“These findings highlight the need to evaluate Ni substitution through the combined effects of powder characteristics, electrode formation and actual paste consumption,” the researchers stated. “Further optimization should consider the interactions among Ni powder properties, glass-frit chemistry and paste rheology, together with long-term metallization reliability.”
The findings are presented in the study “Powder-dependent electrical and printing constraints on Ag substitution in Ag-Ni pastes for TOPCon rear metallization,” published in Solar Energy Materials and Solar Cells. The research team included scientists from Chinese physical vapor deposition (PVD) materials manufacturer Shenzhen APG and Australia-based SunDrive.
SunDrive has developed a proprietary copper electroplating technology that replaces silver in solar cell metallization, aiming to reduce manufacturing costs and improve efficiency.
As previously reported by pv magazine, the Australian company has partnered with China’s Maxwell Technologies and Jiangsu Vistar Equipment Technology to develop and commercialize industrial-scale direct-copper plating equipment for heterojunction (HJT) solar cells. The company previously demonstrated a 26.41%-efficient HJT solar cell using its copper-based metallization process. More recently, it secured AUD 25.3 million ($16.5 million) in Australian government funding to support the development of a 300 MW commercial-scale production tool.
UNSW has also developed several approaches to reducing silver consumption in solar cell metallization. As previously reported by pv magazine, its researchers demonstrated a silver-lean screen-printing technique that reduced rear-side silver consumption in TOPCon cells by 85%, using intermittent silver contacts combined with silver-free fingers and busbars. In a separate study with Trina Solar, the university developed silver-free back-contact solar cells using screen-printed aluminum contacts. UNSW has also investigated copper-plated contacts for TOPCon and heterojunction solar cells, focusing on metallization reliability, corrosion resistance and mechanical stability.
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