Researchers at SiOnyx and Harvard are still investigating why black silicon produces much more current than does normal silicon when exposed to the same light. The theory is that this happens because of a mechanism called photoconductive gain. In regular silicon, each photon will knock loose only one electron to contribute to electric current. But in the new material, each photon sends multiple electrons cruising through the circuit, boosting the current 200 to 300 times. “We believe this is really the first time photonic gain has been seen in silicon,” Saylor says.
The material’s potential for photovoltaic solar cells remains to be seen. In a light detector, an external voltage is applied to the silicon. When a photon hits the material, it knocks loose an electron. The voltage sweeps the electron out into an external electric circuit to produce current. But photovoltaic materials have to create a voltage in response to light. It is not clear if black silicon can be coaxed into doing that efficiently, says MIT mechanical-engineering professor Tonio Buonassisi.
Buonassisi is now exploring the material for photovoltaic applications. He and his group are trying to understand the atomic structure of the material so that they can harness it to make a solar cell. The material’s high absorbance makes it a promising candidate. “This is a very interesting material, and it certainly is intriguing for solar cells … although a lot of the mysteries have yet to be unraveled,” Buonassisi says.
SiOnyx, meanwhile, is developing a black-silicon fabrication process. Saylor says that the company wants to develop a scalable way to make uniform black-silicon wafers. Then it plans to license the manufacturing method to companies that make silicon light detectors and solar cells.