Researchers poke holes in solar cells to improve efficiency

www.designnews.com
3 Jun 2013

Researchers have been working on a variety of ways to improve the efficiency of solar cells, including the use of a spray-coating technique and by working with electrons and photons. Now, government researchers have come up with yet another way--poking holes in silicon wafers so they don't reflect and waste valuable sunlight.

The method, called "dark solar", has been developed by the US Department of Energy's National Renewable Energy Laboratory (NREL), and it enables the cells to reflect only 2% of the sunlight that reaches them. The solar cells they've produced through this technology can work at an 18.2% efficiency rate, which is competitive for the type of black silicon cells researchers are working with.

The development of the black solar cells is "quite simple", but is enhanced by the hole-etching technique, according to information about the work provided to Design News by NREL spokesman William Scanlon.

To develop the cells, researchers first put the silicon silver/gray-colored wafer in a liquid solution of silver nitrate and hydrofluoric acid. In that bath, the ionic silver changes to silver nanoparticles. The wafer is then placed in a second bath of hydrofluoric acid and hydrogen peroxide, where the silver nanoparticles act as a catalyst for the oxidation of silicon, according to researchers.

While this process of making black silicon is not new, it's the etching of holes in the second bath of hydrofluoric acid and hydrogen peroxide that NREL has developed to bolster cell efficiency. These holes maximize the cell's sunlight conversion by preventing reflection of sunlight off the cell. To ensure the cells achieve top efficiency through the process, researchers had to consider the depth and size of the cells--they had to be deep and small enough to suppress reflections, but not too deep to impact the conversion efficiency.

"As the wavelengths move deeper and deeper into the holes they change gradually from wavelengths of air to wavelengths of silicon", researchers said in the NREL document provided by Scanlon. "That gradualness is the key. The photons never perceive an abrupt change from gas (air) to solid (silicon), and thus don't bounce back toward the atmosphere". That lack of bounce-back is what allows the photos to be absorbed in the silicon holes and thus non-reflective.

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