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Hole-studded black silicon ups solar cells efficiency

Posted: 24 Sep 2010     Print Version  Bookmark and Share

Keywords:black silicon  solar cell  aqua regia  chloroauric acid 

The US Department of Energy's National Renewable Energy Laboratory (NREL) researchers have created black silicon solar cells that can absorb more solar energy using lots of tiny holes—up to a trillion in a silicon wafer as large as a compact disc—using a newly developed low-cost etching process. NREL estimates that the discovery could lead to the creation of cheaper and more efficient solar cells.

NREL's Black Silicon Nanocatalytic Wet-Chemical Etch process, named because the holes make the silver-grey silicon black, won the R&D 100 award from "R&D Magazine" as one of the year's top scientific inventions.

The research team used a vacuum deposition technique pioneered by scientists from Munich's Technical University to create black silicon. A thin layer of gold was deposited onto the wafer and acid was then poured on it, creating numerous tiny holes with an average depth of 500nm and a diameter slightly narrower than the smallest wavelength of light. The hole-filled black silicon wafer absorbs light more efficiently than regular silicon wafer.

Principal investigator Howard Branz said the team then discovered that mixing the gold with the etching acid mixture and spray painting it instead of using the vacuum deposition technique worked well and reduced the process cost. Another breakthrough was the use of aqua regia, which reacts with gold to form chloroauric acid, which is less expensive than colloidal gold.

Black silicon, which is made in three minutes or less than a minute at 100 degrees Fahrenheit using the NREL process, reduces unwanted reflection of sunlight to less than 2 per cent, compared to 3 to 7 per cent using conventional silicon nitride anti-reflection layers. Black silicon is also better at halting reflection of low-angle morning and afternoon sunlight, the NREL team said.

NREL optics expert and senior scientist Paul Stradins and NREL electron microscopists Bobby To and Kim Jones explained that the holes' diameter and varying depths made black silicon a better solar cell. If the holes were bigger or of a uniform depth, they would appear to be a "sharp interface" to the light rays, which would then just bounce off the surface, they said.

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