Another potential problem that LaPierre thinks may inhibit higher efficiencies is the electron transport capacity of the wires. When photons of sunlight are captured by a wire, they produces electrons that must then escape from the material to produce an electric current. Electrons, however, are easily trapped along the surface of the wires, reducing their overall efficiency. Thin-film solar cells have to overcome this challenge as well, but the problem is especially acute in thin wire cells because they have a much larger surface area per volume than planar films.
The wires that Lewis and colleagues grew, however, are 1.6 micrometers in diameter, three orders of magnitude thicker than typical solar cell nanowires. The thicker microwires have a lower surface area to volume ratio that, according to modeling conducted by the group, boosts the electron transport capacity of the wires.
Matthew Beard, a senior scientist at the National Renewable Energy Laboratory in Golden, CO, says the relatively high surface area of the wires could be a plus for converting solar power into hydrogen fuel. The high surface area and low cost of raw materials of the silicon microwires means they could be used directly as electrodes to hydrolyze water into hydrogen.
Still, Beard says microwire solar technology will have a tough time competing as a source of power against currently available thin films that are relatively inexpensive and already achieve 10 to 12 percent efficiencies. But Beard adds that silicon, the raw material for the wires, is more readily available than metals such as cadmium and telluride that make up today’s most efficient thin films. “This technology has a long way to go, but potentially can compete, as silicon is more abundant than those materials and potentially cheaper,” he says.