A new type of solar cell may one day prove so inexpensive and flexible, it could be used to turn your clothes into portable power sources—keeping you warm or cool, or charging your phone. Materials scientist Paul Alivisatos and colleagues at the University of California, Berkeley, mixed P3HT, a plastic that conducts electricity, with nanorods made from cadmium selenide, a semiconductor material. This mixture was then "spin-cast" onto a glass base, a process similar to swirling a wineglass so that the wine spreads into a thin film. This film was then sandwiched between two conducting layers (one transparent, to admit sunlight) that acted as electrodes. Light striking the middle layer generated an electric current as electrons flowed along the nanorods. The result: a hybrid nanorod-polymer solar cell about one-thousandth as wide as a human hair. The cells could theoretically be spin-cast onto almost any material, including fabric, and since they're based on plastic, the cells withstand bending. Plastic is cheap, too, so the cells could be very cost-effective.
Alivisatos and company will have to improve their efficiency, however. Today's advanced cells convert about 35 percent of the energy in sunlight into electricity, and even cells based on cheap, simple designs routinely convert about 10 percent. The new cell converts less than 2 percent.
But the team already has ideas for improvement. They could align the nanorods more carefully (in the current design, they are dumped randomly into the plastic). They might lengthen the rods to improve their ability to convey electric charges through the middle layer. And they could adjust the thickness of the rods, allowing them to capture energy from a broader spectrum of the sun's rays. "This is a big breakthrough," says Michael McGehee, a Stanford engineer who specializes in nanotechnology. "I think 2 percent is just the beginning."