This etching process takes three minutes at room temperature, and less than a minute at 40 ºC. The result is a black, highly absorbent silicon wafer whose surface is riddled with tiny tunnels of varying depths. The pores create a surface with no sharp edges to reflect the light, and the variation in their depth is key, because the length of the tunnels determines which wavelength of light it will interact with. Variation in tunnel length allows the surface to trap a broad spectrum of light.
Several groups are developing black silicon for solar cells and other optical devices. Others have used a multistep process to create the silicon-tunneling reactions, starting by placing metal particles over the surface of a wafer and then adding the acids. “We’re doing it in a single, liquid step that requires no vacuum processes at all,” says Branz. Beverley, MA-based startup SiOnyx uses laser pulses to generate tiny cones on silicon surfaces. Their formula for black silicon creates a different material than NREL’s, but with similar properties. Company cofounder and principle scientist James Carey says the company hopes to enter the market within a year. Carey would not disclose what photovoltaic efficiencies SiOnyx has achieved using its process.
The baths used in today’s factories to clean solar cells between etching steps could be used to create the black silicon coating, says Branz, which means this process could be adopted by manufacturers at very low cost. The agency has applied for patents on the process.