Solar Power Lightens Up with Thin-Film Technology

　　The sun blasts Earth with enough energy in one hour—4.3 x 1020 joules—to provide all of humanity's energy needs for a year (4.1 x 1020 joules), according to physicist Steven Chu, director of Lawrence Berkeley National Laboratory. The question is how to most effectively harness it. Thin-film solar cells may be the answer: One recently converted 19.9 percent of the sunlight that hit it into electricity, surpassing the amount converted into power by mass-produced traditional silicon photovoltaics and offering the potential to unleash this renewable energy source.
　　Prices for high-grade silicon (that can generate electricity from sunlight) shot up in 2004 in response to growing demand, reaching as high as $500 per kilogram (2.2 pounds) this year. Enter thin-film solar cells—devices that use a fine layer of semiconducting material, such as silicon, copper indium gallium selenide or cadmium telluride, to harvest electricity from sunlight at a fraction of the cost.
　　"The fundamental advantage of thin film comes in the form of the amount of material you need," says electrical engineer Jeff Britt, chief technology officer of thin-film manufacturer Global Solar Energy in Tucson, Ariz. "These are direct bandgap semiconductors. You can get by with one or two microns and still absorb 98 percent of the sunlight." (In other words, it takes at least 100 times less thin-film material to absorb the same amount of sunlight as traditional silicon photovoltaic cells.)
　　Global Solar uses a technology known as copper indium gallium selenide (CIGS) to make its thin-film solar cells. The company has already supplied the U.S. military and outdoor enthusiasts with portable field chargers, largely for communication and other small electronic devices powered by such cells. In March, the company opened a new factory in Tucson, where it plans to produce enough thin-film CIGS solar cells to generate 40 megawatts of electricity next year—enough to power roughly 15,000 average American homes; it hopes to boost the juice to 100 megawatts by 2010 in response to what it predicts will be a growing market.
　　"We're focusing on low-cost terrestrial power generation," Britt says. "It's intended for large-scale, ground-based arrays." In other words, the types of solar farms previously dominated by traditional silicon photovoltaics now used to generate electricity from sunshine in states like Arizona and California.
　　Global Solar is not alone. A host of companies, including HelioVolt, Nanosolar and others, are using CIGS technology in an attempt to cut the cost of producing photovoltaic cells. But there are other challenges. "The first hurdle is cost," says materials scientist B. J. Stanbery, CEO of HelioVolt in Austin, Tex., which is in the process of opening its first CIGS solar cell factory. "The second is efficiency [how much sunlight can be converted to power] and the third is the reliability, [which means the] lifetime of the device."
　　Researchers at the U.S. Department of Energy's (DoE) National Renewable Energy Laboratory have succeeded in producing CIGS cells that can convert nearly 20 percent of the sunlight that falls on them into electricity. But manufacturers note that mass production reduces their efficiency because chemical processes are not as easy to control on an industrial assembly line.
　　"Benchtop is a great thing to measure because it tells you about the potential of the technology. It tells you nothing, however, about what people are actually making or can make," says Paul Wormser, senior director of product development for the Solar Energy Solutions Group at electronics manufacturer Sharp Electronics, headquartered in Osaka, Japan. "By the time you go into production, you're going to get about half" of the efficiency demonstrated in a lab under perfect conditions.
　　Sharp pairs amorphous silicon (fine layers of randomly arranged silicon) with layers of crystalline silicon (whose atoms are in a more structured lattice) to make its thin-film cells. It plans to increase its manufacturing capacity at its plant in Katsuragi, Japan, to produce enough cells to make 160 megawatts of electricity by October—and to bring its total annual output to enough cells to produce 1,000 megawatts by 2010 by building another factory in Sakai, Japan.
　　He denies speculation that thin-film solar cells will eventually kill the traditional crystalline silicon phtotvoltaics end of the business, noting that they are designed to supplement, not supplant, the old standbys. "Rumors of crystalline's demise are highly exaggerated," Wormser says. "We see thin-film as highly complementary to crystalline and concentrators."
　　But Wormser says that thin-film cells have the potential to produce more power over time than the older technology, because they resist the sun's heat better and produce more power when the temperature spikes.
　　Durability may be an issue, however. Consequently, thin-film cells intended for large arrays use lower grade silicon (read: glass) to protect the delicate photovoltaic layers. For example, Tempe, Ariz.–based First Solar, Inc., which employs cadmium telluride in its thin-film solar cells, sells its modules encased in glass for either large arrays or rooftops. "The elegance of the solar business is that you construct a product and it just sits there generating power for 20 to 25 years," says company president, Bruce Sohn.
　　In addition to offering solar modules at $1.25 a pop (compared with at least  double that per module for traditional photovoltaics), First Solar has also instituted a process for recycling them at the end of their active lives.
　　"Glass can be returned to the glass industry. Metals can be repurified and given back to us in the form of the cadmium telluride compound. Even the wires can be reused," Sohn says. "We really can recycle in excess of 90 percent of the weight of the product today in a perpetual, environmentally friendly life cycle."
　　In fact, cadmium telluride solar cells are currently the most ecofriendly devices, even though they use a toxic heavy metal, primarily because they require the least energy—typically provided by burning fossil fuels—to manufacture, says environmental engineer Vasilis Fthenakis, senior scientist at Brookhaven National Laboratory's National Photovoltaic Environment Research Center in Upton, N.Y., and Columbia University.
　　Yet, cadmium telluride commands only about 30 percent of the thin-film market, according to DoE statistics, compared with amorphous silicon cells (such as those produced by Sharp and ECD Ovonics), which account for more than 60 percent; CIGS cells make up just about 1 percent of this market.
　　But CIGS has the most potential efficiency (converting as much as 25 percent of incoming sunlight to electricity) of any of the thin-film technologies as well as of traditional photovoltaic cells, Heliovolt's Stanberry says. Würth Solar in Germany has mass-produced such cells that can convert as much as 13 percent of sunlight, according to Lawrence Kazmerski, director of the DoE's National Center for Photovoltaics in Golden, Colo.
　　All of the thin-film technologies also offer the potential for ubiquity. That is, says Sharp's Wormser, "you have the opportunity with thin film to make what people refer to as a semitransparent photovoltaic module in place of a window on a building. It allows you to see out through the window, but from the outside it looks like tinted glass."
　　The thin-film solar cells can be used in more flexible applications, such as so-called solar shingles, roofing materials that double as electricity generators. "It's going to serve the purpose of keeping out the elements, but it's also going to generate power for you," Global Solar's Britt says. This also eliminates the significant cost—typically at least doubling the price of a given module—of adding solar photovoltaic systems to already existing buildings.
　　Alternative forms of electricity generation—or some kind of efficient energy storage, such as better batteries—would be necessary for those times when the sun is not shining. But thin-film solar cells hold the promise of harnessing the sun's power in an efficient and sustainable way—and displacing the burning of fossilized sunlight for energy that is contributing climate change–causing carbon dioxide to the atmosphere.
　　"Combining this highest efficiency, lowest cost and most reliable thin-film technology directly into building construction materials will be the beginning of a revolution in solar power," HelioVolt's Stanbery says. "I worried that I wouldn't live to see the day when solar became an economically substantive part of our energy mix, but I think we're on the road to that happening finally. The best is yet to come."