Saving silicon

Evergreen Solar, a Marlboro, MA-based manufacturer of solar power products, has announced what it describes as a major advance in the development of its String Ribbon manufacturing process.


Evergreen Solar, an Marlboro, MA-based manufacturer of solar power products, has announced what it describes as a major advance in the development of its String Ribbon manufacturing process. The new process, for which Evergreen has filed a patent application, has the potential to produce silicon wafers thinner than 150 microns.


“Evergreen Solar’s mission since its inception has been to produce high-quality solar products at a fraction of the cost of conventional methods,” said Richard M. Feldt, President and Chief Executive Officer. “With the latest enhancement to our ribbon growth process, Evergreen Solar has demonstrated in its pilot operations that it can manufacture wafers using one third of the silicon required by conventional methods, significantly reducing the overall cost of producing solar panels.”


In Evergreen’s 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, creating a ribbon.


The process is continuous: long strings are unwound from spools; the melt is replenished and the silicon ribbon is cut to length for further processing, without interrupting growth. Evergreen Solar said that this advantage in material efficiency means String Ribbon yields over twice as many solar cells per pound of silicon as conventional methods.


“By changing the growing conditions in our Gemini II furnaces, we can produce wafers that are half the thickness of our current standard,” said Dr. Brown F. Williams, Vice President, Research and Development. “This achievement directly cuts the use of silicon in half for us and by a factor of three when compared to conventional methods.”


“Our competitors produce wafers by sawing slices of silicon off of large ingots,” continued Dr. Williams. “The silicon lost in the width of the saw cut means that other wafer manufacturers are effectively throwing away half of their silicon starting material.


“Our standard String Ribbon process already avoids that shrinkage. Moreover, because current cutting techniques have difficulty producing wafers as thin as 150 microns, we truly have a unique, defensible competitive advantage.”


Feldt continued, “There is some concern that the tightening silicon supplies may limit the growth of the photovoltaic industry for the next year or two. Our technology’s potential to further reduce silicon requirements is really quite dramatic and holds great promise for our industry. We expect that production of our thin wafers will entail other savings in material costs and the thin wafers will perform at a higher level of efficiency.”


“The yield from the pilot quantities of our thin wafers processed into solar cells and assembled into complete panels has been encouraging. As a result, we will methodically scale our pilot operations and debug our wafer, cell and panel manufacturing processes for thin ribbons.” Feldt concluded.