To increase the light absorption of solar cells, researchers from the University of California, Berkeley have perfected a nanostructure that could make up cheaper and more efficient solar cells and light detectors. Their structures absorb 99 percent of visible light and use 1 percent of the silicon semiconductor needed for flat crystalline silicon solar cells.
Their material consists of an array of nanopillars that are shaped in a manner that they are 2 micrometers high, with the base of 130 nanometers and the tip of 60 nanometers. This shape allows light to protrude the solar cell without reflecting back. An earlier nanopillar design could only extract 85 percent of the incoming light, because it had the same thickness along the pillars’ entire length. Anyway, there’s no way to compare this to a classic flat film which would only absorb 15 percent.
The production process starts by creating a mold for the pores in a 2.5-millimeter-thick aluminum foil. First they anodize the film to create an arrangement of pores that are 60 nanometers wide and one micrometer deep long. They then expose the foil to phosphoric acid to broaden the pores to 130 nanometers – the longer the foil is exposed to the acid, the broader the pores get. Anodizing the film again makes the existing pores one micrometer deeper, and this additional length has the original 60-nanometer diameter. Trace amounts of gold are then deposited in these pores as a catalyst to grow crystals of semiconductor material – in this case germanium, which is good for photo detectors – inside each pore. Finally, some of the aluminum is etched away, leaving behind an array of germanium nanopillars embedded in an aluminum oxide membrane.
It is yet to early to determine if the process could be scaled up for mass manufacturing, but the proof of principle has been demonstrated and the solar cell that would be obtained this way could be way cheaper than any other silicon cell and much more effective because of its special anti-reflexive properties.