Nanomaterial to drive new generation of solar cells

(Nanowerk News) Australian physicists have discovered radical new properties in a nanomaterial which opens new possibilities for the fabrication of highly efficient thermophotovoltaic cells, which could one day harvest heat in the dark and turn it into electricity.
The research team from the Australian National University (ARC Centre of Excellence CUDOS) and the University of California Berkeley demonstrated a new artificial material, or metamaterial, that glows in an unusual way when heated.
The findings could drive a revolution in the development of cells which convert radiated heat into electricity, known as thermophotovoltaic cells.
“Thermophotovoltaic cells have the potential to be much more efficient than solar cells,” said Dr Sergey Kruk from the ANU Research School of Physics and Engineering.
“Our metamaterial overcomes several obstacles and could help to unlock the potential of thermophotovoltaic cells.”
Thermophotovoltaic cells have been predicted to be at least two times more efficient that conventional solar cells. They do not need direct sunlight to generate electricity, and instead can harvest heat from their surroundings in the form of infrared radiation. Besides that, they can also be combined with a burner to produce on-demand power, or can recycle heat radiated by hot engines.
The team created the metamaterial, made of tiny nanoscopic structures of gold and magnesium fluoride that radiates heat in specific directions. The metamaterial can also be tweaked to give off radiation in specific spectral range, in contrast to standard materials that emit their heat as a broad range of infrared wavelengths. This makes it ideal for use as an emitter within a thermophotovoltaic cell.
The project was started with the theoretical discovery of Dr. Kruk who predicted new fascinating properties of a novel metamaterial. The ANU team then worked with scientists at the University of California Berkeley, who had a unique expertise to manufacture the material to required specifications.
“To fabricate this material the Berkeley team were operating at the cutting edge of technological possibilities,” Dr Kruk said.
“The size of individual building block of the metamaterial is so small that we could fit more than twelve thousands of them on a cross-section of a human hair.”
The research is published in Nature Communications ("Magnetic hyperbolic optical metamaterials").
The key to the metamaterial’s remarkable behaviour is its novel physical property: magnetic hyperbolic dispersion. In optical physics the interactions of light with materials are described with dispersion equations. The dispersion can be visualized as a three- dimensional surface in virtual mathematical space. For natural materials, such as glass or crystals the dispersion surfaces have simple forms, spherical or ellipsoidal. But the dispersion of the new metamaterial is drastically different and takes hyperbolic form. It arises from remarkably strong interactions of the material with magnetic component of light. This unusual property is the key to remarkable properties of the material.
The full potential of the new type of artificial thermal materials is employed if the emitter and the receiver are closely spaced with just a nanoscopic gap between them. In this condition radiative heat transfer between them can be more than ten times more efficient than between conventional materials.
Source: Cudos