Fraunhofer ISE scientists constructed a bifacial heterojunction solar cell with a power conversion efficiency of 21.7% by rotary screen printing. They used a machine capable of achieving a cycle time of just 0.6 seconds per solar cell and allowing a large reduction in the use of silver.
A group of researchers from Fraunhofer Institute for Solar Energy Systems (ISE) in Germany has developed a new metallization technique for bifacial silicon heterojunction (SHJ) solar cells that they believe could significantly help reduce the amount of silver needed to produce them while maintaining high levels of solar efficiency. power conversion.
Scientists used rotary screen printing (RSP) for front and back side metallization of SHJ bifacial solar cells via the “Rock-Star” demonstration machine developed by the Fraunhofer ISE itself in partnership with the German automation company ASYS Automatisierungssysteme GmbH.
This machine is able to achieve a cycle time of only 0.6 seconds per solar cell, which means an increase in throughput by a factor of 1.5. It can also achieve a throughput of up to 8,000 wafers per hour in an in-line configuration. “However, some technical challenges need to be overcome until these technologies can seriously challenge the dominance of FSPs. [flatbed screen printing] in the field of solar cell metallization,” the scientists said.
The RSP unit is based on a cylindrical mesh screen with a partially open emulsion layer which serves as the impression form. During the printing process, the cylinder rotates and the paste is printed through the open areas of the rotating screen by a polyurethane (PU) squeegee inside the screen. The wafer is transported through the printing unit on a support with vacuum fixing.
“The impression form is based on a fine mesh of stainless steel which is reinforced with a thin layer of electrolytic nickel to increase the stability of the mesh,” the research team said. “Compared to the fine meshes used for FSP screens, rotating screens need a considerably thicker wire thickness to allow sufficient stability during the printing process.”
According to the researchers, the RSP technique was successfully performed on the demo machine at 50% of the maximum machine speed (0.86 s per cell) and 70% of the machine speed (0.65 s per cell). “In a virtual production line, these cycle times would correspond to a raw throughput of approximately 4100 (50% speed) and 5500 (70% speed) cells h−1 on one track,” they pointed out.
Using the proposed technique, the academics were able to produce a fully metallized silicon bifacial heterojunction solar cell with a power conversion efficiency of 21.7%, which compares to a cell’s 22.1% efficiency. benchmark built with FSP – currently the dominant technology for the industry. metallization of crystalline silicon solar cells with a market share of more than 98%.
“Using RSP for front and/or back side metallization reveals an impressive reduction in the average wet silver paste layer of 60-70% (RSP fully printed cells) and 49% ( RSP back side, FSP front side) compared to the FSP reference process,” they further explained. “This corresponds to a very low total silver consumption of 6 to 9 mg W for fully or partially bifacial solar cells. metalized with RSP versus 17 mg W.”
The group presented their findings in the study “Rotating Screen-Printed Metallization of Heterojunction Solar Cells: Towards High-Throughput Production with a Very Low Silver Layer,” Posted in energy technology.
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