More Coverage. Scientists at the US Department of Energy’s National Renewable Energy Laboratory (NREL) together with Natcore Technology, Inc. have created the “blackest” solar cell to date, a black silicon solar cell that absorbs 99.7% of the sunlight. The scientists believe black silicon technology will make photovoltaics competitive with fossil fuels without subsidy. Could the combination of lower cost and higher energy output quickly make black silicon the anti-reflection control technology of choice?
In this collaborative project, NREL and Natcore are aiming at reducing the cost of silicon solar cells by about 2 to 3% while increasing energy output from 3% to 10% over the course of a day without a tracking mechanism. Black silicon technology is believed to be promising in two ways: “First, black silicon solar cells can potentially get higher solar energy conversion efficiency since they reflect less light, so they capture more sunlight to convert to electricity. Second, black silicon as an anti-reflection technology costs much less than conventional anti-reflection technologies that don’t use liquid etches,” says Hao-Chih Yuan, NREL’s lead scientist working on the project. What’s unique about this “blackest solar cell” is that with a low reflectance of only about 0.3%, it will likely perform very well in cloudy weather when the sunlight is diffused or when the sun is at a steeper angle at morning or sunset, which represents a significant advantage.
To make a black solar cell, crystalline silicon solar wafers are drenched in a liquid that forms nanoparticles of metal — gold or silver. “The metal catalyzes an etch that creates millions of holes on the surface of the silicon,” Dr. Yuan explains. “The holes are so tiny and their depths so random that incident sunlight does not perceive abrupt change when it enters the silicon absorber,” the NREL scientist explains the extremely low reflectivity, adding, “This liquid-etched porous silicon surface, by capturing most of the sunlight, makes the wafer appear black to human eyes.” A silicon wafer can turn “black” in minutes.
A key feature is that the nanoporous surface must be passivated by a film of silicon dioxide or other similar material. While NREL’s devices create the SiO2 film by thermal oxidation, Natcore deposits the oxide with its patented liquid phase deposition process. “The etch bath is a mixture of silver nitrate, hydrofluoric acid and hydrogen peroxide,” explains Dr. Dennis J. Flood, Co-founder and Chief Technical Officer at Natcore, the New Jersey (US)-based company that is working to develop a commercial-scale process for creating black silicon surfaces at standard rates of up to 3000 wafers per hour.
The new light absorption record could have significant impact on future solar applications. Currently, a typical anti-reflection coating would reflect 3% of the sunlight. “So if Natcore can move that reflective loss to less than 1%, that’s about 2% relative gain in efficiency,” Dr. Yuan says. While the record efficiency for this “blackest solar cell” using metal-assisted etched black silicon with Natcore’s liquid-based passivation is 16.5%, the NREL team used a different passivation process — silicon dioxide grown at high temperatures — and reached 18.2 percent in their lab setting, according to Dr. Yuan, who is optimistic they can reach even higher efficiencies than that while keeping the cost very low.
In fact, the cost reduction — particularly in the fabrication of the solar cell —would likely present the new technology’s biggest impact on the future of solar energy. Dr. Yuan estimates conventional anti-reflection applications cost about 4 cents per watt. “We are confident that we can get the black silicon and liquid surface passivation cost to about one cent a watt — that is a significant saving,” he says. “Near term, all the crystalline silicon solar cell technologies can benefit from the lower-dollar-per-watt black silicon — mono-crystalline or multi-crystalline Si. Eventually, it can be applied to Si thin wafers and thin-film crystalline Si.”
Written by Sandra Henderson, Research Editor, Solar Novus Today
Photo: NREL Research scientist Hao-Chih Yuan is reflected in an untreated silicone wafer (left) compared to a "black" silicone wafer that has been etched (right).
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