German research institute Fraunhofer ISE has unveiled a new metallization process for heterojunction solar cells that reportedly increases energy conversion efficiency by more than 0.1% while reducing silver consumption. A device built with the new technology achieved an efficiency of 23.2%.
Researchers from Germany’s Fraunhofer Institute for Solar Energy Systems (Fraunhofer ISE) have optimized the front-side metallization of silicon heterojunction (SHJ) solar cells using a very low silver layer for multi-wire connections.
“This approach is easy to implement, as it only requires the use of sufficiently fine line screens in combination with an optimized raster layout,” corresponding author Andreas Lorenz told us. pv magazine.
When optimizing the metallization process, the researchers took three production parameters into account: the printing technique, the finger distance and the finger width. “One of the key challenges for the coming years is the increasing shortage of crucial resources, namely silver, indium and bismuth,” the research group said. “The need to reduce silver deposition is particularly pressing for SHJ solar cells because the silver paste is typically applied to the front and back of typical cell architectures.”
In their work, the scientists only investigated the application of silver on the front. First, they examined knotless versus standard pressure screens. In the first case they used an advanced fine-grained screen without knots, known as 520 X 11 X 0°, and in the second case a conventional fine-grained screen, known as 520
“The front metallization is screen printed using both screen types with the same printing conditions and a print/flood speed of print/flood = 300 mm/s,” they added, noting that the knotless method obtained an average finger width that 1.3 was narrower. μm compared to the standard procedure.
As for the silver finger pitch, the group tested a 1.3mm silver finger pitch, resulting in 120 fingers, and a 1mm pitch, resulting in 156 fingers. The 1.3 pitch required a total of 19 mg of silver paste, while at 1 mm this increased to 21 mg.
“Reducing the finger pitch results in an increased fill factor (FF), while the short-circuit current density decreases due to the increased shading,” the academics said. Both effects largely cancel each other out in this specific case, creating comparable conversion efficiency for both groups.
In addition, the researchers tested three finger widths: 20 μm, 18 μm and 15 μm. In doing so, they discovered that it is possible to print a uniform raster layout with a width of 15 μm, resulting in a silver reduction of 5 mg compared to 20 μm, in addition to an increased yield of 0.14%.
According to this optimization method, the group fabricated optimized solar cells with the advanced knotless fine mesh screen 520 X 11 X 0°, with a finger itch of 1 mm and a width of 15 μm. These were compared with non-optimized cells, which used the conventional angled fine-mesh screen 520 X 11 X 22.5°, with a finger itch of 1.3 mm and a width of 20 μm.
“The optimized group achieved an average conversion efficiency of 23.2%, which corresponds to a gain of 0.17% compared to the reference cells without the described optimization,” they concluded. “In addition, the amount of silver paste of this group could be reduced by ~2 mg. This highlights the importance of consistent optimization of the screen printing process in terms of cell performance and resource usage for SHJ solar cells.”
Their findings were presented in “Towards a groundbreaking metallization process for silicon heterojunction solar cells with very low silver deposition,” published in Progress in photovoltaics. The research group consisted of scientists from the German electronic components company Yageo Nexensos GmbH.
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