Massachusetts Institute of Technology (MIT) researchers have developed a device that converts the wavelength of sunlight into a band that is more easily used by photovoltaic panels, marking the way for a breakthrough in solar cell technology. The devices, called thermophotovoltaics, act as both absorbers and emitters, and address a problem that has plagued traditional solar cells since their genesis—that they can only absorb certain wavelengths of sunlight, which restricts their ability to create electrical energy to about 33.7 percent efficiency— known as the Shockley-Queisser limit. As reported by the IEEE Spectrum, the researchers, led by graduate student Andrej Lenert, reasoned that if they could manipulate the wavelength of the sunlight to what the solar cells could handle, they could theoretically raise efficiency to more than 80 percent. The outer layer of the absorber-emitter uses an array of multiwalled carbon nanotubes, and the emitter portion is a photonic crystal layer made of silicon and silicon dioxide.

However, early tests have returned only a 3.2 percent efficiency. This is because the thermophotovoltaics get extremely hot. The new device’s absorber-emitter reached a temperature of 962°C (1764°F); at those temperatures, the devices are difficult to optimize and operate.

If these issues can be resolved, the devices also have the potential to aid in energy storage, since heat is an easier stored form of energy than electricity.

Photo courtesy Andrej Lenert, Evelyn Wang, Marin Soljacic, Ivan Celanovic, David Bierman, Walker Chan, and Youngsuk Nam.

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