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Lightweight and flexible solar cells would do away with many of the limitations of current silicon-based PV cells, which are heavy, breakable, bulky, and relatively expensive to install. The quantum dots could also be used to make thermal photovoltaic cells, tapping infrared radiation from fuel-fired sources, and for medical diagnostics, using infrared light to screen for cancer, according to the researchers.

The ability to harness infrared radiation could make solar energy more practical in more geographic areas, “assuming there’s some total power-production rate threshold that has to be met before the approach becomes economical in a given area,” Sargent said. “There is a mild advantage in that some infrared light makes it better through clouds, but the main point is that harvesting infrared as well as the visible wavelengths results in more power harvested.”

Traditional silicon-based solar panels (above) are often derided for being clunky and expensive. Researchers at Georgia Tech are making organic solar cells that are thin and flexible (below). Photography: © Royalty-Free/CORBIS (top); Courtesy Georgia Institute of Technology (bottom)

The quantum dots represent an early stage in the evolution toward commercially available solar cells. But their internal quantum efficiency—the amount of photons absorbed that actually reach the electrical circuit and are turned into usable energy—is just 3 percent, compared to 90 percent for most PV cells now on the market. The researchers are working on increasing this number, along with the quantum dots’ absorption of external light and their external power efficiency, or the ability to harvest the sun’s power efficiently over the entire spectrum, absorbing more light at multiple wavelengths and ensuring that the efficiencies are additive, Sargent said.

Researchers are also addressing the environmental trade-offs in making solar cells, a process that’s energy-intensive and involves hazardous chemicals. The lead sulfide nanoparticles in the Toronto study “need to be encapsulated, and an end-of-life strategy is needed, such as recycling of the materials,” Sargent said. He noted that the lead sulfide is “a showcase for the technology. The approach illustrates the value of infrared harvesting cheaply and flexibly. Once we or others develop even more innocuous materials that do the same thing, they will be adopted.”

Creating a process for making any material a solar collector by applying quantum dots is a step in the right direction, said Alexis Karolides, an architect and green-building consultant with the Rocky Mountain Institute. “Instead of asking how much can we increase the efficiency of current photovoltaic technology, we need to ask what’s possible,” she said.

Down the road, embedded solar cells and solar sheeting will need to be integrated with building control systems and power storage technologies like hydrogen fuel cells, according to Sargent. “Presumably, the days when the sun is shining don’t correspond identically with your power needs—so you might think of looking at power harvesting and storage problems together, in an integrated fashion.”

Ted Smalley Bowen

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