Harnessing quantum dots for solar panels

Solar start-up Stion announced Tuesday that it has received $15 million in venture funding in an effort, sources say, to combine nanotechnology with alternative energy.

Formerly called NStructures, Stion plans to make thin-film solar cells that can compare in performance with silicon solar cells but cost less. The big question is what the active material in the solar panels will be that will convert sunlight into electricity.

"It is not silicon based. It is not cadmium telluride. It is not CIGS (copper indium gallium selenide)," said Frank Yang, manager of business development. "In due time, we will make it publicly available."

Sources, however, say the company is probably working with quantum dots, tiny particles measuring a few nanometers, or tens of atoms, in diameter. Partly because of their small size, quantum dots can be highly sensitive to physical phenomena and can be used to trap electrons. Since solar panels work by wiggling electrons out of sunlight and transferring them to a wire, quantum dots in theory could work well in solar panels. Quantum dots, however, remain highly experimental.

Howard Lee, Stion's chief technology officer, worked for years as a solar researcher at Lawrence Livermore National Laboratory and also obtained a number of patents on quantum dots at Ultradot. Stion's CEO is Chet Farris, who once served as president of Shell Solar.

Coming up with a solar material that can be applied to thin foils or sheets of plastic is one of the major goals in the solar industry. Most solar cells on the market today extract electricity from sunlight with silicon and are integrated into glass substrates, which is relatively heavy. First Solar uses a glass substrate too, but the active ingredient in its cells is cadmium telluride, which is currently cheaper.

Several companies are trying to bring CIGS to market, but they probably won't start selling their products until next year. There are several ways to make CIGS or CIS cells, but it remains an open question which one will work best for mass manufacturing.

CIGS panels likely won't be as efficient as silicon, but will cost less because CIGS cells can be integrated into inexpensive foils, proponents say. Silicon solar cells on the market today can hit 22 percent efficiency and can go up to 29 percent. CIGS panels have hit 19.5 percent in the lab but will likely hit efficiencies only in the mid- to low-teens when they first hit the market. (Multi-junction solar cells made up of layers of different materials and lenses to concentrate sunlight can boost efficiency rates higher, but also add costs.)

There are a lot of materials and combinations of materials that Stion could be looking at, said Rommel Noufi, a researcher at the National Renewable Energy Lab. Finding the right combination of chemicals, though, isn't easy. Some last longer than others. The material that absorbs sunlight also must be paired with a material that can funnel electrons to a building's electrical system.

Stion won't likely come out with products until 2010, said Yang. The company, however, is aiming for efficiencies that will compare well with silicon.

"As efficiency goes up, costs go down," he explained. Homeowners need fewer solar cells to generate the same amount of electricity with higher efficiency and installation costs drop.

Steve Chan, of Chinese giant Suntech Power Holdings estimates that an increase of 1 percent efficiency leads to a 1.5 percent improvement in gross margins.