A nanoengineered wafer that can be optimally tuned for light absorption is the thinnest, most efficient absorber of visible light to date, report engineers at Stanford University. "Achieving complete absorption of visible light with a minimal amount of material is highly desirable for many applications, including solar energy conversion to fuel and electricity," said Stacey Bent, a professor of chemical engineering at Stanford and a member of the research team. "Our results show that it is possible for an extremely thin layer of material to absorb almost 100 percent of incident light of a specific wavelength." Because they require less material, thinner solar cells cost less. The challenge lies in reducing the thickness of the cell without compromising its ability to absorb and convert sunlight into energy. An ideal solar cell would absorb the entire visible spectrum, as well as the ultraviolet and infrared. These four wafers contain the thinnest light-absorber layer ever built. Courtesy of Mark Shwartz, Stanford University. The Stanford team, which included postdoc Carl Hagglund, found that they could tune gold nanodots to absorb 99 percent of the reddish-orange light at the 600-nm wavelength. "We tuned the optical properties of our system to maximize the light absorption," said Hagglund, lead author of a study on the work. "We also achieved 93 percent absorption in the gold nanodots themselves. The volume of each dot is equivalent to a layer of gold just 1.6 nanometers thick, making it the thinnest absorber of visible light on record — about 1000 times thinner than commercially available thin-film solar cell absorbers." The wafers were fabricated at a nearby Hitachi facility via block-copolymer lithography, with a thin-film coating atop the wafers added via atomic layer deposition. "That allowed us to tune the system simply by changing the thickness of the coating around the dots," he said. "People have built arrays like this, but they haven't tuned them to the optimal conditions for light absorption. That's one novel aspect of our work." The previous record holder required an absorber layer three times thicker to reach total light absorption, he added. "So we've substantially pushed the limits of what can be achieved for light harvesting by optimizing these ultrathin, nanoengineered systems," Hagglund said. The next step is to demonstrate that the technology can be used in actual solar cells, with the ultimate goal being to develop improved solar cells and solar fuel devices by confining the absorption of sunlight to the smallest amount of material possible. "We are now looking at building structures using ultrathin semiconductor materials that can absorb sunlight," said Bent, co-director of the Stanford Center on Nanostructuring for Efficient Energy Conversion. "These prototypes will then be tested to see how efficiently we can achieve solar energy conversion." This is a cross section of the record-thin absorber layer showing three gold nanodots, each about 14 × 17 nm in size and coated with tin sulfide. Courtesy of Carl Hagglund, Stanford University. The three types of coatings they applied — tin sulfide, zinc oxide and aluminum oxide — are not light-absorbing, Hagglund said, "But it has been shown theoretically that if you apply a semiconductor coating, you can shift the absorption from the metal particles to the semiconductor materials. That would create more long-lived energetic charge carriers that could be channeled into some useful process, like making an electrical current or synthesizing fuel." They are also considering nanodot arrays made of less expensive metals. "We chose gold because it was more chemically stable for our experiment," Hagglund said. "Although the cost of the gold was virtually negligible, silver is cheaper and better from an optical point of view if you want to make a good solar cell. Our device represents an orders-of-magnitude reduction in thickness. This suggests that we can eventually reduce the thickness of solar cells quite a lot." Their results are published in the current online edition of Nano Letters. For more information, visit: www.stanford.edu