Evergreen Solar Advances String Ribbon Process

Evergreen Solar, Inc. said Wednesday that it has advanced its proprietary “String Ribbon” technology for manufacturing solar cells that could result in more than doubling the output of its crystal growth furnaces.

Marlboro, Massachusetts – December 5, 2002 [SolarAccess.com] The development allows the simultaneous growth of two silicon ribbons, rather than one, from each furnace, the company said. Evergreen expects to introduce double ribbon into production in late 2003 with new furnaces and retrofitted existing furnaces and has filed a patent application for the process. “This important milestone reinforces Evergreen’s position as a leading technical innovator in the solar industry,” said Mark A. Farber, president and CEO of Evergreen Solar. “We have taken our unique core technology, originally developed at MIT, and expanded and improved it. Doubling the throughput of each furnace leverages capital cost, floor space, energy consumption, labor, and consumables, which are expected to have an important impact on reducing costs.” Farber said sales of the Massachusetts-based company’s products have increased ten-fold in the last six quarters while unit costs have been “substantially lowered.” “With the invaluable contributions of our research scientist, Richard Wallace, we have made significant progress on this particular project over the past nine months,” said Jack I. Hanoka, vice president and CTO of Evergreen Solar. “We are exploring further advancements of our String Ribbon technology, including the potential to grow more than two ribbons from one furnace, as well as thinner ribbon.” Hanoka said furnace productivity was doubled in 2001 by increasing ribbon width from 2.2 to 3.2 inches and boosting growth speed by 40 percent, without increasing the furnace size. String Ribbon combines the most attractive attributes of conventional crystalline silicon and emerging thin films. It achieves the reliability, stability, high efficiency, and market acceptance of crystalline silicon – without the inherent cost and waste of sawing solid silicon blocks, according to the company. The technique embodies the efficient material utilization and potential for continuous processing of thin films, but with a more manufacturable process. In the String Ribbon technique, two high temperature strings are pulled vertically through a shallow silicon melt, and the molten silicon spans and freezes between the strings to form a ribbon of silicon. With the new development, four strings will form two ribbons. The process is continuous: long strings are unwound from spools; the melt is replenished and the silicon ribbon is cut into strips for further processing, without interrupting growth. This advantage in material efficiency means String Ribbon can yield over twice as many solar cells per pound of silicon as conventional methods, according to the company.
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