This 300-nanometer-wide silicon wire (left) generates electricity from sunlight. Such nano­wire solar cells would initially be useful in tiny sensors, or in robots whose electronics might need built-in power. But arrays of microscopic wires could change the economics of solar power by enabling solar cells built from cheap materials such as low-grade silicon or even iron oxide–rust.

A number of such cheap materials absorb light and generate electrons, but defects in the materials usually “trap” the electrons before they can be collected. Microscopic wires, though, can be made thin enough to allow electrons to slip out easily and generate current, even if the material has defects. And the wires can be long enough to absorb plenty of photons from sunlight hitting them at an angle.

The image is colored to highlight functional layers of the nanowire, which was made in the lab of Harvard University chemist Charles Lieber. The layers are made of silicon modified in ways that give them properties useful for generating and harvesting charged particles. To make solar panels, the microscopic wires could be grown in dense arrays. The below image shows a cross section of a silicon-wire array fabricated in the labs of chemist Nathan Lewis and physicist Harry ­Atwater at Caltech. Each wire is two or three micrometers in diame­ter. Both groups are in the early research stages, but arranging microscopic wires in a forestlike configuration could lead to new materials that harvest sunlight cheaply and efficiently.

Courtesy of Brendan Kayes and Michael Filler