You can make solar panels with impure silicon, claims Roy Johnson. You just have to know how to isolate the undesirables.
CaliSolar, a solar start-up that derives from research originally conducted at UC Berkeley, has come up with a way to make solar cells out of upgraded metallurgical silicon, which is less pure and less costly than the industry standard electrical grade silicon, according to Johnson, the company's CEO, at a meeting at the Dow Jones Alternative Energy Innovations conference taking place this week.
Electrical grade silicon is 99.99999 plus percent pure, but it costs $150 to $250 a kilogram, he said. Even before the worldwide shortage of silicon the stuff costs $50 or more a kilo because of the ornate processing procedures. Only around 70,000 tons are manufactured worldwide.
By contrast, upgraded metallurgical silicon is only 99 percent or so and goes for $20 to $50 a kilo. Approximately 1.2 million tons get made a year.
While the purity differences may not sound big to you, to a solar panel it's usually a deal breaker. Photovoltaic solar panels generate electricity by separating negative charges from positive charges contained in the sunlight that strikes them. The negative charges (electrons) are then captured, converted to AC power, and then used to power things in your house. Pure silicon is usually needed because impurities prevent electron capture.
CaliSolar gets around this by isolating the impurities during manufacturing. The impurities can be channeled into the borders between crystals, Johnson said, which don't generate electricity anyway. The impurities can also be coaxed, through chemical and physical forces, to "float" to the top of a wafer, where they can be etched off. In other words, it nullifies the effect of the impurities by coming up with ways to ensure that the impurities aren't uniformly distributed.
The solar cells coming out of CaliSolar's labs are already roughly competitive with standard silicon solar cells in terms of efficiency, said Johnson, a former networking executive. If CaliSolar can mass manufacture solar cells with a 14 percent efficiency rating--which means that they will convert 14 percent of the sunlight that strikes them into electricity--these solar cells will cost far less than the 16 percent efficiency cells that are common on the market today.
"We are already making market-competitive solar cells" in CaliSolar's labs, he said.
CaliSolar, however, isn't selling its cells yet. It has delivered samples to potential customers and hopes to have a prototype plant built in the first quarter of 2008. Following that, it will build a mass production plant. (To get that bigger plant, however, it will also have to obtain about $30 million to $50 million in funding, which isn't as tough as it sounds these days.) The company conducts its silicon research in Berlin, but will likely build its mass-production solar plant in California, which will likely become the largest solar market in the world, Johnson said.
Silicon panels weigh a lot, so manufacturers want to build them near the customer base. CaliSolar will make silicon ingots, which get turned into wafers, and solar cells, but it does not plan to make solar panels. Instead, it will sell its solar cells to panel makers.
The technique of using impure silicon was invented by Eicke Weber, a former UC professor now at Germany's Frauhofer Institute.