A simple chemical treatment could replace expensive antireflective solar cell coatings, bringing down the cost of crystalline silicon panels. The treatment, a one-step dip in a chemical bath, creates a highly antireflective layer of black silicon on the surface of silicon wafers, and it would cost just pennies per watt, say researchers at the National Renewable Energy Laboratory (NREL). They’ve used it to create black silicon solar cells that match the efficiency of conventional silicon cells on the market.
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–it suppress reflection so that more photons actually enter the silicon wafer instead of bouncing off its surface, increasing 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, principal 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.
To replace the vacuum-deposition processes used to treat the surface of a silicon wafer, Branz’s lab developed a chemical process that can be performed at ambient temperature and pressure using equipment already on site at solar-panel factories. A wafer is submerged in a bath containing a water solution of hydrogen peroxide, hydrofluoric acid, and chloroauric acid, which is made up of hydrogen, chlorine, and gold. The small amount of gold in the acid bath acts as a catalyst for chemical reactions. It’s not clear exactly what the chemical reactions are, but they lead to the formation of gold nanoparticles that drill nanoholes at varying depths into the wafer. Branz says the gold can be reused again and again.