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 percent of the sunlight that reaches them. The solar cells they’ve produced through this technology can work at an 18.2 percent efficiency rate, which is competitive for the type of black silicon cells researchers are working with.
Hao-Chih Yaun, a researcher at the National Renewable Energy Laboratory, pours a mixture of hydrofluoric acid and hydrogen peroxide onto a silicon wafer as part of the process to create a dark solar cell.
(Source: Dennis Schroeder/NREL)
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.
Researchers now are working on getting the efficiency of the cells over the 20 percent hurdle, which is more in line with what commercial solar cells produce. In the meantime, NREL already has commercially licensed this black solar technique to Red Bank, N.J.-based company Natcore, which will combine it with its own solar cell-passivation technique to create more efficient solar panels.
Natcore has developed a way to passivate solar cells -- or treat them to keep from trapping light-generated electric charges moving toward the cell’s contacts, which takes output power from the cell -- through a bath process not unlike NREL’s technique. Normally, cells would have to be treated through a thermal process that required heating them in a 1,000C oven.
“To make solar cells cost-competitive, we must reduce their cost and increase their output,” Natcore President and CEO Chuck Provini said in a
press release on the company’s website. “The combined NREL-Natcore technologies will reduce cost by eliminating the need for thermal oxidation. And they'll increase output by enabling cells to be more productive throughout all daylight hours.”