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Nano Solar Cells

The story of Nanosys begins with Larry Bock, a former biotech entrepreneur who is now the company’s executive chairman. In the 1980s and ’90s, Bock helped start 14 biotech companies, including Athena Neurosciences, which was acquired by Elan Pharmaceuticals for $700 million in 1996. But by the late 1990s, Bock had soured on opportunities for startups in biotech. “It used to be you could cut five deals with big pharma and go public,” he says. “Then all of a sudden, there weren’t even five big pharma companies around.” Barely in his 40s, Bock went into retirement.

Browsing through the journal Science one day, Bock was astounded to see so many articles devoted to nanotechnology-“something I had never even heard of,” he says. Intrigued, he spent a year meeting with nanotech experts to identify business opportunities. His conclusion: materials known as inorganic nanocrystals held great potential for near-term products. Unlike more exotic nanomaterials like carbon nanotubes, inorganic nanocrystals were made of silicon and other materials already familiar to electronics makers. Plus, in theory at least, the properties of nanocrystals could be easily manipulated to make useful devices. All of a sudden, Bock was out of retirement and back in the game.

By August 2001, Bock had founded Nanosys together with Empedocles, CEO Calvin Chow, and a handpicked team of top scientists from MIT, Harvard University, and the University of California, Berkeley. The game plan was simple but ambitious: turn this scientific expertise into real products for existing markets-and think big. First on the list: revolutionize energy technology.

One of the recruited scientists was UC Berkeley chemist Paul Alivisatos, who was already using nanotech to try to develop a cheap, renewable energy source. In his basement lab at Berkeley, Alivisatos was building new kinds of materials for solar cells, made of bar-shaped semiconductor rods just two to five nanometers wide and 60 to 100 nanometers long. In 2002, Alivisatos showed that by mixing these “nanorods” with an electrically conducting polymer, he could make a flexible material that behaved much like a traditional solar cell.

Each nanorod absorbs sunlight and turns it into a highly efficient flow of electrons along its length. If the material is sandwiched between two electrodes-say, above and below-then any rods oriented vertically contribute to a usable electric current. And because the nanorods can be grown in one step and processed like plastic-without the high heat, vacuum ovens, or precise layering silicon wafers require-the material is five to ten times cheaper to make than a conventional solar cell.

But it is the overall energy efficiency of the material that really counts. To be a viable product, nano solar cells need to convert 10 to 15 percent of the solar energy they receive into usable electricity. They’re not there yet, but possible solutions are in the works. Alivisatos found, for instance, that if he grew “nanotetrapods” shaped like a child’s jacks instead of rods, the nanomaterial yielded a higher efficiency. It turns out that these new tetrapods are better at herding electrons, so they produce a greater electric current.

In a back room at Nanosys’s Palo Alto labs, Erik Scher is attempting to turn these scientific discoveries into materials suitable for products. To concoct the nano solar cells, he uses a syringe to inject semiconductors into a heated, soapy solution of other chemicals. As the solution cools, the semiconductors crystallize into tiny nanostructures. Empedocles compares the process to making rock candy by supersaturating hot water with sugar-but on the nanoscale. The exact recipe determines the dimensions and solar-conversion properties of the crystals. Then another team of scientists measures how much light each type of crystal absorbs and how much electricity it produces. The result: a sheet of material coated with nanorods and optimized to convert sunlight into electricity.

Recharging Your Roof

Nano solar cells could soon turn sunlight into electrical power for your home. These supercheap solar cells-made of nanocrystal structures in an electrically conducting plastic, sandwiched between flexible electrodes-could be laminated in a thin coating onto ordinary roofing tile. Here’s how it would work (drawing not to scale):

1. Sunlight penetrates the top electrode and is absorbed by the nanostructures (brown).
2. The solar energy excites electrons in the nanostructures, giving rise to an electric current that flows between the electrodes through the nanostructures and the polymer (blue).
3. The electric current is collected by wires and used to charge a battery on the underside of the roof that provides power for appliances, lights, and heating systems.

(Infographic by +ism)

Unlike conventional solar panels, which can be bulky and unsightly, Nanosys’s finished product could be laminated onto regular roofing tiles or embedded in architectural glass (see “Recharging Your Roof,” above). Wires connected to electrodes that sandwich the material would transmit electric current to a battery or back to the power grid. Spread over large surfaces, these solar cells could provide enough electricity to run home appliances, office equipment, and even buses. Working with Nanosys, Matsushita Electric in Osaka, Japan-a large maker of solar-integrated building materials-plans to put the solar cells into its roofing tiles within a few years.

This could change the future of energy, experts say. Although their market is growing, conventional solar cells have been mainly limited to high-end homes and niche applications like satellites, because they are so pricey to manufacture. For most Americans, solar energy is still five times more expensive than electricity from the power grid. But at one-tenth to one-fifth the cost of conventional solar cells, the Nanosys material could finally make solar power competitive with fossil fuels. “That’s a remarkable claim,” says John Benner, an expert on photovoltaics at the National Renewable Energy Laboratory in Golden, CO. “That changes the face of a lot of things.”

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