Taking advantage of the limitless cold of the universe, a transparent overlay could enable solar cells to convert more photons into electricity. As solar cells heat up in the sunlight, they become less efficient. The overlay helps cool solar cells by sending the heat into space as IR radiation. At the same time, the material is transparent to the visible light that powers the cells. “Solar arrays must face the sun to function, even though that heat is detrimental to efficiency,” said Dr. Shanhui Fan, an electrical engineering professor. He developed the technology along with research associate Dr. Aaswath P. Raman and doctoral candidate Linxiao Zhu at Stanford University in California. “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 group had developed an ultrathin material that radiated IR heat directly back toward space without warming the atmosphere. They described the technology as “radiative cooling” in an earlier paper, published in Nature (doi: 10.1038/nature13883). In the current paper, published in Proceedings of the National Academy of Sciences (doi: 10.1073/pnas.1509453112), the previous findings were applied to improve the efficiency of solar cells. Because a solar absorber must face the sky, it also has radiative access to the coldness of the universe, the researchers say. They investigated the idea of using the sky as a heat sink while preserving the solar absorption properties of applications such as solar cells. A transparent material improves the efficiency of solar cells by radiating thermal energy into space. In a rooftop experiment, the team demonstrated a transparent thermal material based on silica photonic crystal. The micron-scale pattern of the material was designed to maximize the overlay’s ability to dump heat, in the form of IR light, into space. The overlay was placed on a custom-made solar absorber during sunlight; the absorber mimicked the properties of solar cells without producing electricity. The overlay not only preserved and slightly enhanced the sunlight absorption, it also reduced the temperature of the absorber by as much as 23 °F as a result of radiative cooling. For a typical crystalline solar cell with an efficiency of 20 percent, cooling by 23 °F improves its absolute cell efficiency by over one percent, representing a substantial improvement in energy production, the researchers said. The thermal material works best in the dry, clear environments that are also favored for large solar arrays. As for the future, the technology could be scaled up for commercial and industrial applications, perhaps by using nanoprint lithography or other ways to produce the patterns in the material. “New techniques and machines for manufacturing these kinds of patterns will continue to advance. I’m optimistic,” Raman said. Cooler cars could also be among the benefits of the technology. Zhu noted that the overlay potentially could be used on any outdoor device or system that needs cooling, but requires the preservation of the visible spectrum of sunlight for either practical or aesthetic reasons. “Say you have a car that is bright red. You really like that color but you’d also like to take advantage of anything that could aid in cooling your vehicle during hot days,” Zhu said. “Our photonic crystal thermal overlay optimizes use of the thermal portions of the electromagnetic spectrum without affecting visible light, so you can radiate heat efficiently without affecting color.”