Improving solar cell performance with narrower contact fingers
A team of researchers from Germany's Fraunhofer Institute for Solar Energy Systems (Fraunhofer ISE) and German technology company LPKF Laser & Electronics SE have developed a low-temperature metallization process to print ultra-fine metal contact fingers used in solar cells.
Narrower ultra-fine line contacts may reduce front-side shading in solar cells, thus enhancing their overall efficiency and performance.
“The industry trend is toward continuously narrower fingers, with a target of achieving a finger width of 15 μm by 2034, as highlighted in the ITRPV 2024 report,” the research group explained. “This push towards reduction is driven not only by efficiency gains but also by the decreased use of costly silver (Ag) in solar cell production.”
In the study “Advanced Fine Line Printing With Glass Stencils: Achieving Metal Contact Fingers Below 10 μm,” published in Progress in Photovoltaics, the scientists explained that the solar industry has been able to reduce finger width from 120 μm in 2005 to less than 20 μm in 2024.
The proposed metallization process is based on glass stencils provided by LPKF Laser & Electronics SE. These stencils rely on the so-called LIDE technology, which is a two-step process that creates deep structures in thin glass with a high aspect ratio in the range of over 1:10 at a high processing speed. “This process is not only rapid but also cost-efficient, allowing for the production of dot- or line-shaped structures down to 5 μm and less,” the academics stressed.
They also explained that they used the LIDE technique to initially modify a 400 μm thick sheet of 10 cm x 10 cm AF 32 eco glass. They then used wet chemical etching to remove the laser-modified glass at a faster rate than the unmodified areas, which they said resulted in highly precise microstructures. Moreover, they used a lasering process to structure the glass for the opening of the final aperture channel at the opposite side of the glass foil.
Using complex cross-sectional geometries of the printing channels into the glass foil, the academics were reportedly able to create 290 μm stencils with mask aperture widths of 10 μm, 7.5 μm, and down to 5 μm, which allowed them to manufacture line contacts with core widths of around 8.4 μm and nominal aperture widths of 7.5 μm and 10 μm, respectively.
The group explained that this low finger height resulted in metal contacts with aspect ratios of around 0.19. The aspect ratio defines the ratio of the width to the height of an image or screen.
“By reducing the aperture channel length in the glass stencils from 50 μm to 25 μm and using thinner glass foils, the finger height increased, resulting in aspect ratios of 0.35,” it further explained. “The aspect ratio was further enhanced to 0.45 by using a nanoparticle-based low-temperature Ag paste, which also improved the homogeneity of the fingers compared with a microparticle-based Ag paste.”
Looking forward, the scientists said they want to implement the novel process through a screen printer, which could enhance printing results through quantitative optimization. “Additionally, quantifiable results on electrical performance and IV measurements on solar cells will be obtained, thereby demonstrating the optoelectronic advantages of glass stencil-printed contacts,” they concluded.
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