Silicon Solar Cells Ditch the Wafers
Startup Crystal Solar hopes to take some of the cost out of high-performance single-crystalline solar cells by eliminating conventional silicon wafers. The company says it has developed a wafer-free process for making 50-micrometer-thick solar cells with over 15 percent efficiency, with the possibility of higher efficiencies. Because the process doesn’t waste much silicon, Crystal Solar expects to produce cells for half or even a third of the cost of conventional cells.
Earlier this month, the National Renewable Energy Laboratory (NREL) announced it would give the company up to $4 million over the next 18 months to fund development of the technology. Crystal Solar, based in Santa Clara, California, will open a small-scale pilot plant by early 2013.
For solar electricity to compete with coal-fired power, silicon solar cells must get still less expensive and more efficient. (NREL’s goal is grid parity by 2017.) In today’s conventional solar cells, silicon accounts for about two-thirds of the materials costs. During the four-day process of creating a pure, single-crystal silicon ingot and sawing it down into thin pieces, about half the starting material is lost. Using less silicon in each finished solar cell would further save on materials costs.
Crystal Solar uses a process called epitaxial growth to deposit silicon films directly from gases, eliminating silicon wafers from the process. Over the past two years, the company has adapted the process make very thin single-crystalline silicon solar cells. Crystal Solar says it can make silicon cells that are highly efficient, but thinner than a piece of paper. The sweet spot, it believes, is 40 to 50 micrometers thick, approaching the lower limit of how thin a solar cell can be while still performing up to the material’s theoretical potential. (Much thinner than this, and it won’t absorb enough light.)
For many years, researchers have tried to adapt epitaxial growth methods to make thin single-crystalline solar cells. The chip industry has been using this method for decades—in fact, modern microelectronics has been made possible by machinery that uses high-temperature vacuum chambers to deposit different forms of silicon on top of silicon wafers. (Before starting Crystal Solar, chief technology officer K. V. Ravi was the director of renewables and environment at Applied Materials, one of the world’s biggest suppliers of semiconductor manufacturing equipment—including equipment used to grow various forms of epitaxial silicon for computer chips, display electronics, and solar cells.)
But the epitaxial method hasn’t been workable for making thin-film single-crystalline solar cells—the kind with the highest performance. To make the process work for single-crystalline solar cells, Crystal Solar had to remake the processing equipment from the ground up.
Crystal Solar says it has now made the process practical. The semiconductor industry utilizes 5 percent of the silicon in trichlorosilane gas. Ravi says Crystal Solar’s equipment uses 60 to 70 percent of the silicon, and can make a solar cell 20 times faster than making one on conventional epitaxial growth equipment. Academic labs have made similar efficiency demonstrations with very small test cells that have never been scaled up. Crystal Solar has made standard-size solar cells with its process.
Making these thin, high-quality silicon films is one thing, but handling them is quite another. Ravi says the company has also developed equipment to handle, finish, and package the thin silicon sheets to make solar cells, though it is not disclosing details on how it does this.
The company has been in stealth mode since its founding in 2008, and Ravi would not name the sources that have provided Crystal Solar with unspecified tens of millions of dollars in two rounds of funding. He says the company does not intend to become an equipment manufacturer, but will partner with another company to make and sell panels.
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