Blackbody radiation improves solar cell efficiency

A black body is an idealized material that absorbs all radiation that hits it. When it comes to emitting radiation, materials have different limits at different temperatures. Metamaterials, a special type of material that has been much reported, can exhibit a negative refraction index, which allows them to bend light around a space or object, making for much-reported "invisibility cloaks." But designer metamaterials have the potential to extend far beyond visible light. They can be tailored to respond to radiation in a variety of carefully modulated forms, and, thanks to a team of researchers at Duke University, one of those responses could have a major impact on thermophotovoltaics and other energy-conversion technologies.

The research team has demonstrated that metamaterials can be designed to emit "blackbody" radiation at efficiencies beyond the natural limits of the material due to temperature. This means better energy conversion efficiency for products such as photovoltaics and possibly waste heat utilization.

A "blackbody" is an ideal material that absorbs all radiation that hits it, regardless of wavelength. It also emits that energy, depending on the temperature of the material. Blackbodies don't exist in nature, which is unfortunate, because they are really efficient, because they achieve a balance. When it comes in, it's electromagnetic radiation, and when it comes out, it's thermal radiation, or "blackbody radiation." That's the ideal case.

Blackbody radiation emitted by metamaterials. Source: Duke University

The Duke team has shown that using metamaterials, artificial materials not found in nature, they can tailor this blackbody radiation in a variety of ways, including some that change the efficiency with which a material naturally emits. In other words, there is a natural limit to how much radiation a given material can emit, and that limit depends on the material's temperature. But the Duke team has shown that its metamaterial can emit radiation more efficiently than the natural properties would allow.

New metamaterials could enable technologies that harness waste heat from industrial processes or other heat-generating sources with unprecedented efficiency. Or they could be used to make thermophotovoltaic cells, which can tailor emitted photons to match the band gap of the semiconductor in the cell, allowing for higher efficiency in energy conversion.