Nickel powder cuts TOPCon rear-finger raw-metal costs by 6.8%

By Grace Kim

A 10% nickel loading in the metal-powder fraction, using Ni-4000 with an approximate particle size of 3.8 μm, cut rear-finger raw-metal costs by 6.8%, or $561,500 per gigawatt, while maintaining 26.325% cell efficiency. The University of New South Wales team produced that result on industrial n-type TOPCon cells across two production runs.

What does the result change for TOPCon manufacturers?

Corresponding author Ning Song told pv magazine: “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 behavior.”

The study addresses the manufacturing constraint: nickel offers relatively low cost and good electrical conductivity, but oxidation during firing and powder variation can impair electrodes. Earlier work had established Ag-Ni hybrid paste feasibility while leaving practical cost benefits uncertain.

For TOPCon producers, the result identifies a narrow formulation target rather than a general rule to maximize nickel. The preferred mix used Ni-4000, not the finer, more oxygen-rich Ni-1500, and held nickel at 10% of the metal-powder fraction.

Why did powder selection decide the economics?

Researchers tested Ni-1500 and Ni-4000, with approximate particle sizes of 1.5 μm and 3.8 μm. They replaced 5% to 15% of the metal powder in industrial TOPCon rear-finger pastes, then measured electrical performance, contact resistance, electrode microstructure, printing behavior, paste consumption and finger geometry.

The finer Ni-1500 powder had higher specific surface area and oxygen content than Ni-4000. At the same nickel loading, Ni-1500 produced greater resistive losses, making powder identity consequential beyond the substitution percentage.

At 5% Ni-1500, cells maintained the 26.349% silver-only baseline efficiency. Raising Ni-1500 to 10% lowered efficiency by 0.225 percentage points, to 26.124%; 10% Ni-4000 instead delivered 26.325%, only 0.024 percentage points below baseline.

Measurements connected the small efficiency gap to higher contact and finger resistivities, more electrode voids, a less continuous silver network and different oxide-related contact chemistry. Nickel substitution also widened printed fingers and increased paste consumption, particularly at higher loadings.

Why did more nickel not produce more savings?

The 10% Ni-4000 formulation provided the strongest balance, saving 6.8% in rear-finger raw-metal costs, equivalent to $561,500 per gigawatt. At 15% Ni-4000, efficiency remained similar at 26.302%, but paste consumption rose 17% above the silver-only baseline and savings fell to 0.3%.

Song added: “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.”

How did the researchers test the formulations?

The five rear-side metallization pastes contained 90% metal powder, 2% glass frit and 8% organic vehicle. One used silver only; four used 5% to 15% nickel in the metal fraction with Ni-1500 or Ni-4000.

Researchers screen-printed the pastes onto 182 mm by 183.75 mm industrial n-type TOPCon cells. They then dried the cells, co-fired them at a 724 °C peak set temperature and applied laser-enhanced contact optimisation.

Across two industrial production runs, the team used at least 12 cells per formulation. Calibrated equipment and imaging measured cell efficiency, series resistance and spatial electrical characteristics.

Contact resistivity came from transfer-length measurements, while finger resistivity came from four-point probes. Scanning electron microscopy, energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy examined electrode microstructure and contact interfaces.

The team also measured finger geometry, paste viscosity and actual paste consumption. It calculated silver and nickel consumption per watt from paste deposition and average cell power, then estimated rear-finger raw-metal costs using representative metal prices.

The researchers concluded that further optimization must consider interactions among nickel powder properties, glass-frit chemistry and paste rheology, alongside long-term metallization reliability. Their recommendation requires evaluating powder characteristics, electrode formation and actual paste consumption together.

The findings appear in “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 team included scientists from Chinese physical vapor deposition materials manufacturer Shenzhen APG and Australia-based SunDrive.

Where do copper and other silver reductions fit?

SunDrive, one of the organizations represented in the study, separately develops proprietary copper electroplating that replaces silver in cell metallization. The company aims to reduce manufacturing costs and improve efficiency, but its technology is distinct from the nickel-paste work on TOPCon cells.

SunDrive has partnered with China’s Maxwell Technologies and Jiangsu Vistar Equipment Technology to develop and commercialise industrial-scale direct-copper-plating equipment for heterojunction solar cells. It previously demonstrated a 26.41%-efficient HJT cell with copper metallization and secured $25.3 million ($16.5 million) in Australian government funding to develop a 300 MW commercial-scale production tool.

UNSW has also pursued silver-free contact routes at cell scale. A screen-printing method combined intermittent silver contacts with silver-free fingers and busbars, cutting rear-side silver consumption in TOPCon cells by 85%.

In a separate project with Trina Solar, the university developed silver-free back-contact cells using screen-printed aluminum contacts. UNSW also has investigated copper-plated contacts for TOPCon and heterojunction cells, focusing on metallization reliability, corrosion resistance and mechanical stability.

The next market milestone is long-duration metallization reliability data, alongside confirmation that measured paste consumption holds in repeated production runs. The 6.8% raw-metal saving becomes commercially meaningful only if Ni-4000 preserves contact resistance and cell performance without eroding its printing advantage; until then, the two-run result identifies a candidate formulation, not a finished transition away from silver.