The sun never shines in the basement laboratory on the University of California, Berkeley campus, where graduate students Delia Milliron and Ilan Gur spend their days. But that isn’t stopping them from doing some of today’s most advanced work in solar-cell technology.
In one corner of the lab Gur brews toxic ingredients to make a half-vial of crystals, each measuring a millionth or so of an inch, of cadmium selenide, a light-sensitive semiconductor. He covers a postage-stamp-size piece of glass with the nanocrystal goop and hands it over to Milliron. In a mini-vacuum-chamber she adds tiny strips of aluminum. Finally they carry the tiny chip into a black room and shine light on it:The chip spews back power–1.5 milliwatts for this fledgling solar cell, which on some bright future day could translate to 15 watts per square meter.
Even brighter days are ahead. The pursuit of an improved photovoltaic cell, the cornerstone of solar energy production, has the whiff of a gold rush about it. Since 1999 shipments of photovoltaic cells and modules have been growing an average of 30% a year, reaching $3.5 billion in 2002, according to Clean Edge, a San Francisco research group.
Manufacturers, led by Japanese companies such as Sharp, are convinced that within sevenyears traditional solar-cell technology will deliver power as cheaply and conveniently as steam turbines. The economics of solar, once derided as hippie wishful thinking, are getting pretty compelling. “When I started in the early 1970s, the going price for solar modules was about $200 per watt,” recalls Arthur Rudin, director of engineering for Sharp’s solar systems division in Huntington Beach, Calif. “Today the average price for solar modules is about $5 a watt.”
That is, a solar cell that generates a watt of power when the sun is at its peak for four hours a day sells for about $5. Figure in night, clouds, winter, various supporting hardware and the depreciable lives of cells, and you find that sun-made electricity is still pretty expensive, between 20 cents and 50 cents per kilowatt-hour. Get the $5 cost down to $1, though, and the sun could compete with natural gas.
The best commercial solar cells today convert 15% of sunlight into electricity, but they are made of brittle, expensive materials such as silicon. Cells can be made from cheaper, more malleable plastics, but those typically now turn a mere 3% of light into electricity. “We know it’s possible to do better,” contends Paul Alivisatos, the chemist who oversees that lab at UC-Berkeley.
In March 2002 Alivisatos put his reputation–and his venture capitalists’ money–where his mouth is, turning over much of his research to a Palo Alto firm called Nanosys. The company has amassed $75 million and is devoting much effort to figuring out how toembed nanofilaments of semiconductors in cheap, bendable plastic sheets. Nanosys’ goals: 10% efficiency, $1 per watt.
Solar energy cells are typically made of crystalline silicon, the kind used in computer microprocessors, sandwiched between electrodes (see box).Silicon’s orderly atomic structure allows electrons to shoot rapidly through the crystal to the electrodes. But silicon is expensive and must be processed in elaborate clean-room facilities, just like computer chips. Organic materials such as light-absorbing polymers are easier and cheaper to process, but typically fritter away 97% of the sun’s energy because the electrons must navigate a tortuous path.
Solar entrepreneurs have many ways to attack the problems. In Los Gatos, Calif. a nine-person startup called Solaicx is building a furnace for making cheap crystalline silicon substrate. Pour in elemental silicon and out will come little bricks that are the foundation material for 90% of today’s solar modules. “We have far less waste than the usual manufacturing process, lower labor costs–we’re just attacking every factor of production,” says Robert Ford, chief executive of the company.
He says that Solaicx’s furnace may be able to cut the cost of making wafers by as much as 50% when the company is in full-scale production, about two years from now. He bets that Solaicx will make it possible for solar modules to generate power at 8 cents per kilowatt-hour by 2007.
Nanosys and two other new firms, Konarka and Nanosolar, are taking the radical approach of replacing chunky solar-cell modules with rolls of flexible plastic that have photovoltaic elements built in. “You don’t have to invent a new factory to do this. You use coating and printing machines,” says William Beckenbaugh, chief executive of Konarka. Production costs would drop by maybe 80%.
Which light-sensitive nanomaterials to use? Nanosys’ solar-cell work is based on Alivisatos’ tinkering with so-called nanorods in his Berkeley lab. When embedded in plastic, the rods act like roads for the electrons, shortening their route toward the electrode andproducing power more efficiently. Early batches of nanorods were as organized as pickup sticks. In a happy accident Milliron and other Berkeley postdocs made an errant batch of nanorods that looked like jacks–with four legs–rather than rods. No matter which end was up, one leg always pointed toward the electrode. Another perk:Nanorods can be “tuned” to absorb light from different parts of the spectrum just by growing them bigger or smaller. Although Nanosys is still exploring just the right shape for its nanorods, it has committed to a deal with Matsushita Electric Works to make plastic photovoltaic laminate for materials like Spanish-style roof tiles, slated for the Japanese market by late 2006.
In Lowell, Mass. two-year-old Konarka is using titanium dioxide nanocrystals coated with a thin layer of a light-sensitive dye. The dye-sensitized solar cells suck up photons even in dim light. Konarka’s work is based on a decade of research by Michael Graetzel at the Ecole Polytechnique Fédérale de Lausanne. Konarka’s plastic-based solar modules have already shown efficiencies of up to 6.5%, beating the scores of thin- film (or amorphous) silicon solar cells, Beckenbaugh says. (For a story on a thin-film enthusiast, see p. 86.) Konarka has teamed up with Groupe Electricité de France, ChevronTexaco and Eastman Chemical to make products by 2005. Earlier this year Aisin Seiki, in Japan, demonstrated a solar module that’s more efficient than the best silicon versions and can be made for one-tenth the cost.
Nanosolar, in Palo Alto, is building “brushes” of semiconducting metal oxide nanowires that it says improve efficiency even more. Its work is based, in part, on technology developed at Sandia National Laboratories, with the help of former students from Alivisatos’ laboratory. R. Martin Roscheisen, the company’s founder, says Nanosolar has a working prototype and plans to raise funds to build a factory.
With reporting by Benjamin Fulford.