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Adhesive stringer for the industrial interconnection of heterojunction solar cells.
© teamtechnik Maschinen und Anlagen GmbH
Solar cells – New processing method
03.05.2018

The conductive adhesive paste is applied using a screen-printing mask.
© Fraunhofer ISE

Solar cells with three, four or five busbars can be connected in the adhesive stringer.
© Fraunhofer ISE

Adhesion rather than soldering of electrical contacts

To connect individual high-efficiency solar cells, it is now possible to adhere rather than solder these on an industrial scale. The technology is market-ready. The method is the result of joint research conducted between the Fraunhofer Institute for Solar Energy Systems (ISE) and system manufacturer teamtechnik. Innovative solar cells with temperature-sensitive layers can thus now be gently connected. Even with narrow cell thicknesses below 160 micrometers, the lower thermo-mechanical stress demonstrates crucial benefits.

In the “KleVer” research project, researchers refined the adhesion technology such that it now represents a viable alternative to widespread soft-soldering techniques in cell interconnection. The new procedure is being trailed in a “stringer”. This is an automated system that electrically connects solar cells from which solar modules are constructed. Using the trial system, the cooperation partners have been able to demonstrate the feasibility of conductively bonding solar cells with three, four or five front contacts (busbars) as full or half-cells, with a throughput of approx. 1,600 cells per hour. The throughput in industrial production is only marginally below that of soldering cells. The reliability of adhesive connection has been confirmed by testing in climate chambers.

The considerably lower process temperatures of below 180 °C in comparison to soldering allows temperature-sensitive high-efficiency cells in particular to be gently interconnected with material savings. This is also shown to be the case with heterojunction modules, manufactured with adhesive connections. This cell type, which currently demonstrates the highest level of efficiency, cannot be soldered in the usual way at temperatures in excess of 200 °C. Thermo-mechanical stresses too are down considerably due to the reduced rigidity of the adhesive layer and tension in the material is lessened. This reduces the risk of microfractures and cracking in the cells. The cycle time for interconnecting a cell in the teamtechnik stringer is less than 2.3 seconds and is largely determined by the curing speed of the adhesive material applied.

“Having ramped up our expertise in the conductive bonding of silicon solar cells in recent years with testing on a laboratory scale, the stringer now makes it possible for us to further optimise lead-free connection technology with our partners on an industrial scale,” explains Dr. Achim Kraft, project manager and head of the Connection Technology team at Fraunhofer ISE. Axel Riethmüller, project manager and Chief Operating Officer at teamtechnik, likewise seems pleased: “As a manufacturer of high-performance stringers, we are proud to be at the very forefront of the industrial interconnection of delicate heterojunction solar cells with the adhesive technology unit developed. We have already welcomed orders for the very first stringers with adhesive technology. This wouldn’t have been possible without the joint project with Fraunhofer ISE.”

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System and process technology
teamtechnik Maschinen und Anlagen GmbH

Qualification and characterisation
Fraunhofer ISE