A 2-D photonic crystal could prove key in advancing the absorption and use of solar energy. The new material, developed by researchers at MIT, is a metallic-dielectric crystal that absorbs sunlight from a wide range of angles and can withstand extremely high temperatures. The material has already shown that it can withstand 1000 °C over 24 hours without severe degradation. With the thermophotovoltaic device, the sunlight’s energy is first converted to heat, causing the material to glow and emit light that can be converted to electrical current. Creating the new material for solar energy-to-heat conversion relies on tuning the absorption spectrum. This rendering shows the metallic dielectric photonic crystal that stores solar energy as heat. Courtesy of Jeffrey Chou/MIT. “It’s a very specific window that you want to absorb in,” said postdoctoral scholar Jeffrey Chou, adding that getting the right spectrum of both absorption and emission is essential to efficient performance. “We built this structure and found that it had a very good absorption spectrum.” The researchers worked on an earlier thermophotovoltaic device that took the form of hollow cavities. “They were empty; there was air inside,” Chou said. “No one had tried putting a dielectric material inside, so we tried that and saw some interesting properties.” The new material should absorb virtually all wavelengths of light from the sun that reach Earth’s surface, but “not much of the rest of the spectrum, since that would increase the energy that is re-radiated by the material and thus lost to the conversion process,” according to the researchers. The new material can absorb sunlight efficiently from a wide range of angles, making solar trackers unnecessary. Solar trackers would significantly add to the complexity and expense of a solar power system. The material can also be used with existing manufacturing technology. The work was funded by the Solid-State Solar Thermal Energy Conversion Center and the U.S. Department of Energy. The research was published in Advanced Materials (doi: 10.1002/adma.201403302). For more information, visit www.mit.edu.