Silicon is the material of choice for microchips, sensors, solar cells and every other kind of electronics today, save a few high-frequency applications using the even more expensive gallium arsenide. Solar cell manufacturers try to minimize costs by making the silicon wafers they use thinner, but they are still meticulously oven-grown perfect crystalline disks of ultra pure silicon semiconductor.
Schematic diagram of the light-trapping elements used to optimize absorption within a silicon wire solar cell.
Using less-expensive polymers instead of silicon in solar cells is already being done today, but at much lower efficiencies than silicon solar cells. Now, Caltech claims to have married the efficiency of silicon to the low cost of plastic, yielding the best of both worlds. The technique grows a forest of tiny silicon wires atop a plastic substrate. Each silicon wire is just 1 micron in diameter, but as long as 100 microns (0.1 millimeter). Each wire is coated with a nonreflective coating so that light particles (photons) can more easily penetrate them. Almost no light is reflected back, as in normal solar cells. Instead, the light tends to bounce around among the forest of silicon wires until it is absorbed by one. The overall measured quantum efficiency is over 90 percent.
The thinnest traditional silicon solar cells today are about the same overall thickness as Caltech's new material, but costwise it is drastically reduced, since the new material only contains 2 percent as much silicon, the rest being inexpensive polymers. The new material can also be mass-produced using inexpensive roll-to-roll manufacturing techniques instead of expensive semiconductor ovens.
The researchers predict that their technique not only will be less expensive, but Caltech also claims the forest of silicon wires acts like a solar concentrator, converting more photons and a wider range of wavelengths. Silicon solar cells only work well for certain wavelengths of light, and have to be tuned to the reddish hue of sunny skies in California or the bluish hue of overcast skies in Oregon. Caltech, on the other hand, found that its new material could be tuned to absorb 96 percent of the incident sunlight at a single wavelength, plus was wide-band enough to convert 85 percent of the total collectible sunlight across the spectrum.
Also, because the new silicon-wire solar cells are flexible, they can be formed onto the surface of other products to give them photovoltaic capabilities, such as window coverings, roofing and even the outside car body of an electric vehicle.
Caltech's lead researcher on the project is professor Harry Atwater, along with fellow professor Nathan Lewis and doctoral candidate Michael Kelzenberg. Funding is provided by BP, the Energy Frontier Research Center program of the Department of Energy, the National Science Foundation and the Kavli Nanoscience Institute at Caltech.