New metallization tech for HJT solar cells minimizes silver use, increases efficiency

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 very low silver laydown for multi-wire interconnection.

“This approach is easy to implement as it does only require the utilization of sufficient fine line screens in combination with an optimized grid layout,” corresponding author, Andreas Lorenz, told pv magazine. 

In optimizing the metallization process, the researchers have accounted for three production parameters – the printing technique, the finger pitch, and the finger width. “One of the primary challenges for the coming years is the increasing shortage of critical resources, namely, silver, indium, and bismuth,” said the research group. “The need to reduce silver laydown is particularly urgent for SHJ solar cells as the silver paste is usually applied on the front and rear sides of typical cell architectures.”

In their work, the scientists examined only the application of silver on the front side. Firstly, they examined knotless versus standard printing screens. In the first case, they used an advanced knotless fine-mesh screen known as 520 X 11 X 0°, and in the second, a conventional angled fine-mesh screen known as 520 X 11 X 22.5°.

“The front side metallization is screen printed using both screen types with the same printing conditions and a printing/flooding speed of print/flood = 300 mm/s,” they added, noting that the knotless method obtained a mean finger width narrower by 1.3 μm compared to the standard procedure.

As for the pitch of the silver fingers, the group tested a silver finger pitch of 1.3 mm, resulting in 120 fingers, and a 1 mm pitch, resulting in 156 fingers. In the case of the 1.3 pitch, a total of 19 mg of silver paste was needed, while in the case of 1 mm, it rose to 21 mg.

“Reducing the finger pitch results in an increased fill factor (FF), while the short-circuit current density decreases due to increased shading,” the academics stated. Both effects offset each other to a large extent in this specific case, resulting in a comparable conversion efficiency for both groups.

Furthermore, the researchers tested three finger widths – 20 μm, 18 μm, and 15 μm. In doing so, they found that it is possible to print an even grid layout with a width of 15 μm, resulting in a silver reduction of 5 mg compared to 20 μm, in addition to an increased efficiency of 0.14%.

Following 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. Those were compared to unoptimized 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 obtained 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. “Furthermore, the silver paste laydown of this group could be reduced by ~ 2 mg. This emphasizes the significance of consistent optimization of the screen-printing process in terms of cell performance and resource utilization for SHJ solar cells.”

Their findings were presented in “Towards a cutting-edge metallization process for silicon heterojunction solar cells with very low silver laydown,” published in Progress in Photovoltaics. The research group included scientists from German electronic components company Yageo Nexensos GmbH.