Suntech has developed a way to form monocrystalline material using a modified version of the multicrystalline process.
It uses seed crystals, but instead of being gradually drawn out of the silicon (as with the conventional monocrystalline process), they are arranged at the bottom of a crucible and completely covered with melted silicon. Then heat is extracted through the bottom of the crucible, ensuring that crystallization begins at the bottom, where the seeds are.
This is essentially the process patented decades ago. Refinements Suntech has made help overcome one of the main challenges the process presents: the molten silicon in contact with the edges of the container forms its own seeds, and as a result, the final slab of silicon is monocrystalline on the inside and multicrystalline toward the outside.
Suntech figured out how to keep the multicrystalline area to a minimum: the resulting ingot is 70 percent monocrystalline. Pure monocrystalline wafers are made from the center of the ingot. The material on the edges, which is half monocrystalline and half multicrystalline, is also used. Cells made of this turn out to be about 10 percent more efficient than ordinary multicrystalline cells, which is almost as efficient as purely monocrystalline cells.
Wenham says the process can use existing wafer-processing equipment, so it can be scaled up quickly. “The process could be quite a game-changer in photovoltaics, as it offers much higher performance at reduced costs,” he says.
Wenham says that Suntech expects much of the industry to adopt similar technology in the next two years. Because the basic principle is no longer under patent, many companies have been able to develop their own versions of it. He says Suntech is looking to patent technologies related to using the new materials to make solar panels.