Making Silicon Less Reflective
Silicon solar cells reflect about a third of the light that they could potentially convert into electricity. A new nanostructured coating developed by researchers at Rennselaer Polytechnic Institute (RPI), in Troy, NY, cuts these reflections to only 4 percent. Applied-physics professor Shawn-Yu Lin and his colleagues calculate that the coating could boost a silicon solar cell’s efficiency to convert light into electricity by nearly 43 percent. They presented their results in an October 29 Optics Letters paper.
The new coating provides antireflection for sunlight coming in at nearly all angles and all sunlight wavelengths, Lin says. It’s a 700-nanometer-thick structure of different material layers that the researchers grow on a silicon wafer. The multilayered coating reduces the reflection of visible and near-infrared wavelengths that silicon can convert into electricity.
Manufacturers of solar cells currently use a thin layer of silicon nitride to cut reflection. But the layer eliminates the reflection of a very narrow range of wavelengths–its thickness dictates the range. Another problem is that it only works well for light coming in at certain angles. Silicon wafers coated with silicon nitride reflect close to 20 percent of the useful light falling on them.
Lin and his colleagues had previously designed a similar coating for aluminum nitride, which is used to make light-emitting diodes. The new coating, however, is designed specifically for silicon, the material widely used in solar cells. The coating has seven layers: the bottom two are made of titania, the middle three of varying mixtures of silica and titania, and the top two of slanted nanoscale silica rods.
Reflection at the interface between two materials, in this case air and silicon, depends on the difference between their refractive indexes–the ability of the material to bend light. Reducing the difference reduces reflection. Air has a refractive index of 1, while silicon’s is 3.5. The researchers use the layers to break down this gap into smaller steps. The top silica nanorod layer has an index of 1.09, and, going down, each layer has a successively larger index. “The sequential arrangement of the multilayered nanostructure allows sunlight to have a soft landing on the solar cell,” Lin says. Light gradually bends more and more as it goes through each layer, and less of it gets reflected back into air.
The researchers deposit the slanted silica nanorods in the top two layers by holding the wafer at an angle. By controlling the nanorods’ angles, the researchers control the material’s porosity, creating layers with extremely low refractive indexes.
Lin and his group at RPI are not the only ones working on the problem. Peng Jiang, a chemical-engineering professor at the University of Florida, is making antireflective coatings by etching the surface of silicon, creating an array of tiny silicon pillars that are less than 300 nanometers tall. “Our best samples reflect less than 1 percent of light, so the surface looks completely dark,” Jiang says. His work has generated interest from European solar-cell manufacturers, and he is now creating a startup company to manufacture the coating on a larger scale.
Lin says that the coating would increase the cost of a solar cell by a few percentage points–not more than current silicon nitride layers contribute. The researchers are now planning to make more-robust coatings suitable for use with solar cells.
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