STANFORD, Calif., July 24, 2014 — A new technique allows solar cells to passively cool themselves by driving away unwanted thermal radiation. Developed by a team from Stanford University, the new solar cells offer high efficiency and more longevity than conventional cells (most commonly silicon semiconductors), which typically convert only about 30 percent of incident sunlight into usable electricity. Pyramid structures made of silica glass provide maximal radiative cooling capability for a solar cell. Courtesy of Linxiao Zhu/Stanford University. Solar energy that is not converted generates waste heat, which, according to the researchers, unavoidably lessens a solar cell’s performance. Solar cells also age faster when their temperatures rise — for every increase of 18 °F, the aging rate of traditional solar panels doubles. Solar cell efficiency is negatively impacted as well, declining about half a percent for every 1.8 °F increase in temperature. In their research, the investigators compared two different silica covering designs: one a flat surface approximately 5 mm thick, and the other a thinner layer covered with pyramids and cones a few microns thick and tuned to refract and redirect unwanted IR wavelengths away from the solar cell. “The goal was to lower the operating temperature of the solar cell while maintaining its solar absorption,” said lead researcher Dr. Shanhui Fan, an electrical engineering professor at Stanford. “We were quite pleased to see that while the flat layer of silica provided some passive cooling, the patterned layer of silica considerably outperforms the 5-mm-thick uniform silica design, and has nearly identical performance as the ideal scheme.” The research was published in Optica (doi: 10.1364/OPTICA.1.000032). For more information, visit www.stanford.edu.