Researchers at Harvard University have made solar cells that are a small fraction of the width of a human hair. The cells, each made from a single nanowire just 300 nanometers wide, could be useful for powering tiny sensors or robots for environmental monitoring or military applications. What's more, the basic design of the solar cells could be useful in large-scale power production, potentially lowering the cost of generating electricity from the sun.

Each of the new solar cells is a nanowire with a core of crystalline silicon and several concentric layers of silicon with different electronic properties. These layers perform the same functions that the semiconductor layers in conventional solar cells do, absorbing light and capturing electrons to create electricity. To make the cells, Charles Lieber, a professor of chemistry at Harvard University, modified methods he'd previously used to make nanowires that could serve as sensors or transistors. He then demonstrated that his solar cells can power two of his earlier nanowire devices, a pH sensor and a set of transistors.

"This paper provides the very first example of using a single silicon nanowire for harvesting solar energy," says Zhong Lin Wang, professor of materials science and engineering at Georgia Tech. He calls Lieber's work "breakthrough research in the field of nanotechnology."

At first, the nanowire solar cells will most likely be useful in niche applications where their small size is key, such as extremely small sensors, or robots whose sensors and electronics might benefit from an integrated power source. "There has been a lot of talk recently about making independent nanomachines and nanosystems," says Phaedon Avouris, a fellow at IBM Research. "The issue has always been, how are you going to power them? If you want to have an independent nanosystem that's self-contained, that's not plugged into a central power supply, then you need something like this."

The ultimate goal would be to build electronic components that can self-assemble into devices that might not be possible to make otherwise. (Lieber has shown that it's possible to make such components from nanowires, which can then be assembled into regular arrays in solution.) "We'd like to incorporate memory, a nanoprocessor, maybe a sensor, and a power source to drive that," Lieber says. "If you try to put together all of these pieces with conventional technology, it gets pretty cumbersome."

In addition to powering tiny machines, solar cells made from microscopic wires might eventually be bundled together into large arrays to replace conventional rooftop solar panels. Lieber's research is still at an early stage, but his new nanowires suggest that a theoretical solar cell proposed by researchers at the California Institute of Technology could be viable. Harry Atwater, a professor of applied physics and materials science at Caltech, and Nathan Lewis, a professor of chemistry there, have suggested that solar cells made of microscopic wires would be much cheaper than conventional solar cells, since they could be made from less expensive materials--including, Lewis says, rust.