Solar Cells Cheaper

The crystalline silicon wafers used to make today's solar cells are treated to create a textured surface, then coated with an antireflective layer, usually silicon nitride, using high-vacuum processes. This additional layer increases the value of a solar cell by improving its efficiency--the more times a photon bounces around inside a solar cell's active layer, the greater the chances it will contribute to the flow of electricity off the cell. But the extra layer also adds to the expense. "We believe it can be cheaper," says Howard Branz, principle scientist in silicon materials and devices at NREL. Even with a coating, the best-quality silicon solar cells typically reflect 3 percent of the light that hits them. Branz's lab is developing inexpensive ways to create black silicon, which reflects almost no light.

Prototype solar cells made at NREL have the best efficiency ever reported for black silicon cells. Monocrystalline silicon cells with the black surface, and no additional antireflective coating, convert 16.8 percent of the light that hits them into electricity, about the same efficiency offered by a typical crystalline silicon solar cell coated with antireflective material. The previous record for black silicon cells was 13.9 percent.