German research institute Fraunhofer ISE has introduced a new metallization process for heterojunction solar cells that increases energy conversion efficiency by more than 0.1% while reducing silver consumption. A device developed with the new technique achieved 23.2% efficiency.
Researchers at the Fraunhofer Institute for Solar Energy Systems (Fraunhofer ISE) in Germany have improved the front-face metallization of silicon heterojunction (SHJ) solar cells by using a very thin silver layer for multi-wire interconnects.
„This approach is easy to implement, as it requires only the use of sufficiently elegant displays in combination with an optimal phase design,” said corresponding author Andreas Lorenz. pv magazine.
To optimize the metallization process, the researchers took into account three production parameters: the printing technique, the pitch of the fingers and their width. „One of the main challenges in the coming years is the scarcity of critical resources such as silver, indium and bismuth,” explains the research team. „The need to reduce the silver layer is particularly urgent in the case of SHJ solar cells because silver paste is commonly applied to the front and back faces of common cell structures.”
In their work, the scientists only studied the use of silver on the front side. First, they examined knot-free and static print screens. In the first case, they used an advanced knotless fine mesh screen known as 520 X 11 X 0° and in the second, they used a conventional angle mesh screen known as 520 X 11 X 22.5°.
„The metallization of the front side is printed using two types of screens with identical printing conditions and print speed/print flood/flood = 300 mm/s,” they further noted that the knot-free method obtained the average width. The fingers are 1.3 μm narrower compared to standard practice.
For silver finger pitch, the team tested 1.3 mm silver finger pitch, yielding 120 fingers and 1 mm pitch, yielding 156 fingers. At 1.3 pitch, a total of 19 mg of silver paste was required, while at 1 mm, 21 mg was required.
„Decreasing the finger pitch increases the fill factor (FF), while the short-circuit current density decreases due to increased shading,” the academics say. In this particular case the two effects largely offset each other, resulting in a comparable replacement capacity in both groups.
Additionally, the researchers tested three finger widths: 20 μm, 18 μm, and 15 μm. By doing so, they found that it was possible to print a uniform grating arrangement with a width of 15 μm, resulting in a reduction of 5 mg of silver compared to 20 μm and a 0.14% increase in efficiency.
Following this optimization method, the team fabricated optimized solar cells with an advanced 520 X 11 X 0° knot-free fine mesh screen, 1 mm finger pitch and 15 μm width. These cells were compared to unoptimized ones, which used conventional 520
„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. „Additionally, the silver paste deposit of this group can be reduced by ~2 mg. „This underscores the importance of constant optimization of the screen printing process in terms of cell performance and resource utilization for SHJ solar cells.”
Their findingsTowards an Advanced Metallization Process for Silicon Heterojunction Solar Cells with Very Low Silver Laydown” (Towards a state-of-the-art metallization process for silicon heterojunction solar cells with very low silver deposition), published Advances in photovoltaics. The research team included scientists from the German electronic components company Yakio Nexenzos GmbH.
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