"Solar Cells at $1 a Watt," Input/Output, ME Departments, May 2008

input/output

Solar Cells at $1 a Watt?

by Alan S. Brown, Associate Editor

Solar cells, which convert free sunlight into electricity, are the ultimate green technology. Yet until now, even the most efficient solar cells have been far too expensive to compete with conventional power sources without large tax subsidies.

That may be about to change. Over the past few months, venture capitalists have bet hundreds of millions of dollars on a different approach to photovoltaics. Instead of trying to make solar cells more efficient, they have been trying to make them cheaper.

In fact, at least one company is hoping to sell solar cells for as little as $1 a watt. At this price, they can compete with electrical power from coal, gas, and nuclear generators without any subsidies.

The problem in the past has been silicon, which accounts for more than 90 percent of all commercial solar cells. Pure silicon wafers are ordinarily expensive, and a worldwide shortage that pits solar cell producers against semiconductor manufacturers only exacerbates the problem.

The new approach relies on thin films of copper indium gallium selenide, or CIGS, for short. Although CIGS solar cells are not yet as efficient as silicon or cadmium telluride (another expensive option), they are far less costly to manufacture. Engineers can deposit the active CIGS layer on everything from glass and aluminum foil to plastic.

Take Nanosolar Inc. of San Jose, Calif., which printed its first commercial CIGS solar cells on aluminum film in December 2007. In 2006, it raised $75 million in a private stock sale and more than $25 million in government subsidies. Last December, the U.S. Department of Energy's Solar America Initiative awarded it another $20 million.

The company is using most of that money to build 647,000 square feet of factory space. A 200,000-square-foot facility in San Jose has begun production and has already sold out its output for the next 18 months. A second plant will start up soon in Germany.

Solar cell producers hope to drive costs down to $1 per watt by printing ink-based solar cells on aluminum foil and, eventually, plastic substrates.

Nanosolar's technology is intriguing. The company starts with a nanoparticle ink, which consists of an organic molecule that bonds copper, indium, gallium, and selenium atoms in just the right proportions to form CIGS. It prints the ink onto an aluminum foil electrode using a roll-to-roll process running at hundreds of feet per minute. It then converts the ink into a true CIGS layer.

The process looks more like printing a newspaper than making semiconductors. It also eliminates costly crystalline silicon and expensive (and slow) vacuum deposition techniques. Nanosolar predicts that it can sell solar panels for as little as $1 per watt and still turn a profit. The most cost-effective conventional silicon solar cells cost about three times as much.

Other companies are also pursuing CIGS thin films. DayStar Technologies Inc. of Halfmoon, N.Y., raised more than $70 million in its first public offering in October 2007. That followed $77 million in funding from venture capitalists in August 2007.

Although DayStar ultimately plans to print CIGS on flexible sheets, it will make its first solar cells on glass. In May, it claimed it could reproducibly achieve 10 to 12 percent efficiencies on large-area glass solar cells. This is significantly less than 15 to 20 percent for most commercial silicon systems. Yet DayStar's lower costs could give it an advantage in the market.

Germany's Juwi Solar GmbH, which expects to install 120 megawatts of solar power in 2008, seems to think so. It has signed a letter of intent to help Daystar commercialize its glass modules and has agreed to buy 25 percent of its output through 2011.

Only weeks after DayStar went public, HelioVolt Corp. raised $24 million from venture capitalists to top off the $77 million it had raised two months earlier. It is planning to build a new 20- megawatt facility in Austin, Texas. It claims its process is 10 to 100 times faster than other CIGS processes, and says it can print solar cells on glass, metal, polymers, and composites. Its first products will be based on glass.

Nanosolar, Daystar, and HelioVolt are only some of the companies that drew investor interest in 2007. According to alternate energy consultant Cleantech Group, venture capitalists pumped $654 million into 33 photovoltaic companies in California alone in 2007, up from $253 million in 16 firms in 2006.

Backed by hundreds of millions in investor capital, the solar cell industry may be well on its way to transforming free sunlight into affordable electricity.