While SiOnyx, a MA-based startup, uses lasers to create black silicon and improve the efficiency and price of solar cells, researchers at the National Renewable Energy Laboratory (NREL) discovered a simple chemical treatment that could replace the otherwise expensive antireflective solar cell coatings currently used in the industry.
The silicon wafers used regularly in making the solar cells are treated with silicon nitride, to make an antireflective layer. The process of treating the wafer is done under vacuum conditions, adding to the total manufacturing price, and gains more efficiency by reducing the light reflected back into the environment. Even with the best coating, silicon solar cells treated this way usually reflect around 3 percent of the incident light.
Howard Branz and his team from the NREL developed a new way to chemically make black silicon, which has a reflection of almost zero percent. Their process can be performed at room temperature and pressure, using equipment that already exists in solar cell production facilities, thus reducing the manufacturers’ investment to adopt their technology.What’s even better is that their method only implies one step, unlike others.
The silicon wafer is submerged in a bath having a water solution of hydrofluoric acid, hydrogen peroxide and chloroauric acid (hydrogen, chlorine, and gold). The gold in the chloroauric acid (HAuCl4) acts as a catalyst for the yet unexplained chemical reactions, leading to the formation of etching gold nanoparticles, that dig at various depths into the silicon wafer. The gold can be reused several times after a treatment.
At 40 ºC, the etching process takes less than a minute, and at room temperature about three, making black, highly absorbent silicon wafer, riddled with tiny tunnels of varying depths. The porous surface just created doesn’t reflect light. Varying the depth is one of the key elements, because the length of the tunnels determines which wavelength of light it will interact with, making the cell responsive at a large spectrum of light.
Branz says that the process they invented could be applied by using the baths used in today’s solar cell factories to clean the wafers between etching steps. Their one-step etching process would eliminate much of the time spent with other more expensive treatments.