These tiny indentations, each less than a micrometer across, can trap rays of light as effectively as conventional solid silicon surfaces that are 30 times thicker, according to their findings. The new findings are reported by MIT postdoc Anastassios Mavrokefalos, professor Gang Chen, and three other postdocs and graduate students, all of MIT's Department of Mechanical Engineering. "We see our method as enhancing the performance of thin-film solar cells," Mavrokefalos said, but it would actually work for any silicon cells. "It would enhance the efficiency, no matter what the thickness".
Graduate student Matthew Branham, a co-author of the paper, said, "If you can dramatically cut the amount of silicon (in a solar cell) ... you can potentially make a big difference in the cost of production. The problem is, when you make it very thin, it doesn't absorb light as well."
The operation of a solar cell occurs in two basic steps: First, an incoming particle of light, called a photon, enters and is absorbed by the material, rather than reflecting off its surface or passing right through. Second, electrons knocked loose from their atoms when that photon is absorbed then need to make their way to a wire where they can be harnessed to produce an electrical current, rather than just being trapped by other atoms. Unfortunately, most efforts to increase the ability of thin crystalline silicon to trap photons, such as by creating a forest of tiny silicon nanowires on the surface, also greatly increase the material's surface area, increasing the chance that electrons will recombine on the surface before they can be harnessed.
The new approach avoids that problem. The tiny surface indentations described a "inverted nanopyramids" greatly increase light absorption, but with only a 70 percent increase in surface area, limiting surface recombination. Using this method, a sheet of crystalline silicon just 10 micrometers (millionths of a meter) thick can absorb light as efficiently as a