Laser transfer printing process from BASF and Schmid

25th September 2009

BASF LTP

ES Admin

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BASF and Schmid have jointly developed a new, contact-free laser transfer printing (LTP) process for metallizing the front and back of solar cells. At present, it is mainly the screen printing process that is used for metallization. The printer and paste system now developed by BASF and Schmid is based on a contact-free laser process and not only offers customers advantages with regard to handling, but also increases the efficiency of cell production. Thus, for example, the breakage rate is reduced and the throughput increased, and it is possible to handle solar cells below 180 µm. Furthermore, because they contain no lead, the BASF pastes are especially environment-friendly. The silver and aluminum pastes are also available as water-based products, i.e. free of solvents, which should lead to significant production improvements, especially in cell manufacturing.

As a company with many years’ experience in the field of metal pastes for a variety of applications, BASF started work on the development of silver pastes for the photovoltaic industry in 2005: “We wanted to meet industry’s requirements in this field and not only offer ‘conventional’ lead-free silver pastes, but also try to improve the metallization process itself,” says Dr. Frank Kleine Jäger, project manager for silver pastes at BASF. This led to the idea of developing a laser printing technology and solvent-free silver pastes specifically tailored to it. BASF soon found the right partner in the form of aurentum Innovationstechnologie GmbH, a company based in Mainz, Germany, which developed the contact-free laser printing process for printing with the customized lead-free pastes.

In order to be able to test the technology and pastes on an industrial scale, BASF, at the end of 2008, concluded a joint development agreement with the Schmid Group, one of the world’s leading manufacturers of machinery for the production of solar cells. “With Schmid we managed to gain one of the most innovative partners, which has a great deal of expertise not only in machine manufacturing, but also in the solar industry itself,” says Kleine Jäger, “only together were we able to transform our idea so quickly and flexibly into a real production machine that actually worked.”

The LTP PV 1500 laser transfer printer was developed by the Schmid Group to completely replace the existing screen printing process. It can be used for coating both the backs of solar cells with aluminum and aluminum/silver and the fronts with silver. The system can be integrated into existing production lines and is suitable for all types and sizes of cells. There are plans for technical interlinking for combined printing of fronts and backs. The LTP printer achieves a throughput of 1500 wafers per hour and works without any contact at all, in order to prevent any mechanical stress on the wafers. A wafer for printing is prepositioned, held in place on a vacuum conveyor and then passed through under the laser. The laser beam separates the metallization paste from the carrier film and transfers the paste to the wafer. The paste on the film is constantly replaced in a cycle. An integrated ink supply with viscosity control ensures that pastes with different compositions can be used. Because the printing is done digitally, any patterns can be printed and serialization of the individual printed wafers is possible.

As from 2010 BASF will be offering customized metallization pastes for mono- and multicrystalline solar cells that are designed specifically for the new LTP process and can also be used in conventional screen printing. The portfolio will comprise both silver pastes for front side printing and aluminum-based pastes for printing of back sides.