Solar cell efficiency, currently hovering in the 15 to 20 percent range, can theoretically be boosted to as high as 70 percent by printing specially engineered nanostructures on the cells, researchers say. It has long been thought that conversion efficiency of solar cells cannot exceed 34 percent for a single material or less than about 45 percent for the most efficient cells. A thermodynamic limit is responsible for this practical limitation. By clever light management, Harry Atwater, applied physicist at California Institute of Technology, and his colleague Albert Polman of the FOM Institute for Atomic and Molecular Physics in Amsterdam say efficiencies as high as 70 percent should be achieveable. The conversion process of solar cells is typically not very efficient: a conventional, commercially available silicon solar cell today converts only 15 to 20 percent of the energy of the sunlight to electricity; a large fraction, about 80 to 85 percent, is lost as heat. Blue and green light are converted to electricity with an efficiency of less than 50 percent, while infrared light is not absorbed by a silicon solar cell at all. The highest efficiency record ever realized by a silicon solar cell was 27 percent. “With a slightly more complex solar cell, it becomes possible to convert all colors of the light from the sun to electricity, and an efficiency of up to 70 percent is achievable,” said nanophotonics expert Albert Polman. (Image: iStockphoto) Light that is not converted in the solar cell leads to thermodynamic disorder known as entropy and, as a result, reduced cell voltage. As a result, the maximum achievable efficiency is limited to 34 percent. This is known as the Shockley-Queisser limit. The incomplete trapping of light inside the solar cell and defects in the solar cell material’s crystal structure also cause efficiency loss. By managing the light in a clever way using specially engineered nanostructures printed on the surface of the solar cells, it can be better trapped, and many of the efficiency problems can be solved. In their Nature Materials paper, Atwater and Polman describe several recipes for achieving these improvements. The inspiration for some of these ideas, which are based on the integration of photonic nanostructures and circuits on the solar cell, comes from optical integrated circuit technology, where structures to guide and control light are routinely made. “Before 2000, scientists and technologists had developed materials and devices with high electronic quality, but little attention was paid to optical design,” Atwater said. “But in the last decade, revolutions in photonic material design and large-area nanostructure fabrication have given researchers and technologists tools to enable a new era of ultrahigh-efficiency photovoltaics.” Atwater also explained that the solar cell community has historically assumed that cells could be made with either low efficiency and low cost, or high efficiency at a high cost. Now, however, high-efficiency, low-cost solar cells are achievable. “The solar cell community has driven down solar panel cost, yet is very conservative and has not boosted efficiency significantly,” Polman said. “But solar panels with a high efficiency take up much less space, because you need fewer panels to generate the same amount of power. That saves costs of land, installation and infrastructure. With a slightly more complex solar cell, it becomes possible to convert all colors of the light from the sun to electricity, and an efficiency of up to 70 percent is achievable.” For more information, visit: www.caltech.edu