A research group in China has investigated screen-printed silver-nickel (Ag-Ni) rear contacts for tunnel oxide passivated contact (TOPCon) solar cells, assessing device performance at different nickel concentrations. The scientists found that partially replacing silver with lower-cost nickel could reduce metallization costs without significantly compromising cell efficiency.
Silver consumption has become a growing concern for the PV industry, particularly as manufacturers adopt high-efficiency cell architectures such as TOPCon and heterojunction (HJT) technologies, which rely on silver-based metallization pastes. Researchers and manufacturers have been exploring ways to reduce silver consumption through alternative contact materials, including copper-based metallization and hybrid pastes. Silver-nickel contacts represent another potential approach, combining silver’s electrical conductivity with the lower cost of nickel, although contact resistance, grid conductivity, firing compatibility, and long-term reliability remain important considerations.
The scientists noted that silver paste has become the second-largest cost component in solar cell manufacturing, while strong demand from the PV industry is putting upward pressure on global silver prices. “The development of silver-lean or silver-free metallization technologies has emerged as a critical requirement for the sustainable growth of the PV industry,” they said.
“In our study, a novel rear-side contact architecture for TOPCon solar cells uses an Ag-Ni hybrid metallization paste with different Ni contents of 6%–60%,” they added. “The primary objective was to achieve an optimal techno-economic balance between metallization cost and cell efficiency.”
The researchers fabricated TOPCon solar cells using industrial 182 mm × 182 mm precursors with a thickness of around 100 μm. They screen-printed the rear contacts using Ag-Ni pastes containing 6%, 10%, 15%, 30%, and 60% nickel by weight, alongside a reference paste containing only silver.
All pastes were printed under identical conditions using a 430-mesh screen, a snap-off distance of 1.2 mm, and a squeegee pressure of 55 N. The cells were subsequently fired in a belt furnace, with peak temperatures of 711 C on the front side and 660 C on the rear side.
The team assessed cell performance using current density-voltage and Suns-Voc measurements, as well as contact resistivity and grid-line resistivity measurements. The researchers also used electron microscopy to analyze the contact microstructure and the interfaces with silicon.
The results showed that low nickel concentrations largely preserved cell performance. Under front-side illumination, the cells achieved efficiencies of 26.26%, 26.21%, and 25.86% with nickel concentrations of 6%, 10%, 15%, and 30%, respectively.
At a nickel concentration of 60%, however, efficiency dropped sharply to 6.88% because of increased electrical resistance and recombination losses. The corresponding contact resistivities for the five nickel concentrations were 2.62, 2.79, 3.27, 3.43, and 1.63 mΩ·cm², respectively. Despite its comparatively low contact resistivity, the 60% nickel formulation exhibited exceptionally high grid-line resistivity, which severely limited cell performance.
The researchers identified the paste containing 10% nickel as offering the best balance between performance and cost and selected it for further optimization. They adjusted the silver powder composition to improve particle packing and densification and modified the softening point of the glass frit to improve compatibility with the firing process.
In a production trial involving 219 cells, the optimized paste delivered an average conversion efficiency of 26.67%, compared with 26.77% for 215 reference cells manufactured using conventional silver paste. The champion cell achieved an efficiency of 26.87%, an open-circuit voltage of 0.744 V, a short-circuit current density of 41.47 mA/cm², and a fill factor of 87.10%.
“When 10% of silver is replaced by nickel, the silver consumption per watt is approximately 8 mg/W,” the researchers said. “Furthermore, accounting for the paste formulation costs, increased manufacturing complexity, and potential yield losses, the metallization cost per watt for the rear side of cells using Ni-10 paste is reduced by approximately 11%.”
The researchers presented their findings in “Screen-Printed Silver-Nickel Hybrid Contacts for SiOx/n+-Poly-Si Layers in TOPCon Solar Cells,” published in Progress in Photovoltaics: Research and Applications. The research team included scientists from China’s Yangzhou University, Jiaxing University, and Changzhou University.
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