Three Stanford University engineers have developed a coating technology that improves on solar panel performance. Their invention shunts away heat generated by a solar cell under sunlight, and cools it in a way that permits it to convert more photons into electricity. The work, by Shanhui Fan, a professor of electrical engineering at Stanford, research associate Aaswath P. Raman and doctoral candidate Linxiao Zhu, is described in the current issue of Proceedings of the National Academy of Sciences. Their discovery addresses a long-term problem solar power has faced: the hotter solar cells get, the less efficient they are at converting the photons into useful electricity. Their solution uses a thin, patterned silica material laid on top of a traditional solar cell. The material is transparent to the visible sunlight that powers solar cells, but captures and emits thermal radiation, or heat, from infrared rays. “Solar arrays must face the sun to function, even though that heat is detrimental to efficiency,” says Prof. Fan. “Our thermal overlay allows sunlight to pass through, preserving or even enhancing sunlight absorption, but it also cools the cell by radiating the heat out and improving the cell efficiency.” In 2014, the same researchers developed an ultrathin material that radiated infrared heat directly back toward space without warming the atmosphere. They called this ‘radiative cooling,’ because it shunted thermal energy directly into the cold void of space. In their new paper, they applied this notion to improve solar array performance when the sun is at its hottest. They tested their technology on a custom-made solar absorber, which mimics the properties of a solar cell without producing electricity. Covered with a micron-scale pattern, it maximizes the capability to dump heat, in the form of infrared light, into space. Their experiments showed that the overlay allowed visible light to pass through to the solar cells, but that it also cooled the underlying absorber by as much as 23 deg. F. For a typical crystalline silicon solar cell with an efficiency of 20 percent, 23 °F of cooling would improve absolute cell efficiency by over one percent, a figure that represents a significant gain in energy production. The researchers say their thermal overlays work best in dry, clear environments, which are also the preferred sites for large solar arrays. They believe that commercial and industrial applications are feasible, perhaps using nanoprint lithography, which is a common technique for producing nanometer-scale patterns. “That’s not necessarily the only way,” said Raman, a co-first-author of the paper. “New techniques and machines for manufacturing these kinds of patterns will continue to advance. I’m optimistic.” The technology could also be used for cooling cars out in the sun. The coating’s transparency should not affect vehicle color.