CaliSolar
Most solar cells are made from very pure--and thus expensive--silicon. But ­CaliSolar can make do with silicon that has a thousand times the impurities and can cost a sixth as much. That could mean real savings, since silicon can represent a quarter to a third of the cost of making solar cells. Usually, impurities lower cells' efficiency by reabsorbing freed electrons. But CaliSolar's manufacturing process herds impurities together, so electrons are less likely to hit them. The company's cells convert 15 percent of solar energy into ­electricity, putting them in the ballpark of conventional cells.

Product: Solar cells made from "dirty" silicon

CEO: Roy Johnson

Location: Sunnyvale, CA

Funding: $13.4 million

Funders: Advanced Technology Ventures, GlobeSpan Capital Partners

URL: calisolar.com

Wakonda Technologies
Thin-film solar cells are inefficient but cheap. Where there's room to put up a lot of them, they're cost ­effective, but to compete elsewhere, they'll have to get more efficient. Wakonda has found a way to coat a metal foil with some of the most efficient photovoltaic materials known, such as gallium arsenide. The company says that its thin-film cells will be even more efficient than sili­con panels. Gallium arsenide is expensive, but Wakonda claims that its films will cost as little as its competitors'.

Product: Thin-film solar cells from superefficient semiconductors

CEO: Les Fritzemeier

Location: Fairport, NY

Funding: $9.5 million

Funders: Advanced Technology Ventures, General Catalyst Partners, Polaris Venture Partners, Applied Ventures

URL: wakondatech.com

Prism Solar Technologies

Prism has engineered a new type of solar concentrator by inscribing holograms in a transparent medium sealed between pieces of glass. Much of the light that strikes a conventional solar cell generates waste heat rather than electricity, but Prism's holograms can divert unproductive wavelengths of light away from the cell. As a consequence, it can concentrate the productive wavelengths more intensely without fear of overheating. The company claims that it can concentrate light two to three times as much as its competitors can.

Product: Holographic solar concentrators

CEO: Rick Lewandowski

Location: Lake Katrine, NY

Funding: $8.5 million

Funders: Counter Point Ventures, I2BF Venture Capital, Phoenix-Fire II, Walt Robb

URL: prismsolar.com