Driving Down Costs to Solar Cells

Wednesday, May 21, 2014 @ 12:05 AM gHale

A new hyperbolic metamaterial has numerous applications in the optical field, including high efficiency solar cells.

While not the only factor in the overall cost of solar power, efficiency gains still play a major role in the steady trend toward solar power as cheap as – and cheaper than – fossil fuels.

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Metamaterials refers to a new class of engineered materials that have precisely designed nanoscale features etched or layered onto their surfaces, said researchers at Purdue University.

In the optical field, metamaterials take advantage of the plasmonic effect. The plasmonic effect refers to clouds of electrons, freed by the presence of light. Given the right surface structure, these electrons can end up manipulated to enable more efficient light harvesting.

Among the metamaterials already in development for plasmonic solar cells, back in 2011 there was a metamaterial with a nanoscale “waffle” design at Stanford. Metamaterials designed with nanopillars or nanorods are also in the works.

One of the obstacles to metamaterials development is the current metals of choice are gold and silver, which are not compatible with the standard manufacturing process for integrated circuits. They are also not particularly efficient conductors of light relative to other options.

For their solution, the Purdue-headed metamaterial team engineered a nanoscale “superlattice” crystalline structure (superlattice refers to a crystal grown in layers) made of the metal titanium nitride and a semiconductor called aluminum scandium nitride.

Titanium nitride has optical properties that resemble gold, but unlike gold it is compatible with the manufacturing standard. Titanium nitride is also durable and stable at high temperatures, and known for its amenability to growing in nanoscale crystalline films.

In the current study, each layer is 5-20 nanometers. The team also demonstrated the layers could get down to a thickness of only two nanometers, which works out to about eight atoms apiece.

This is one of the first successful demonstrations of the metal/semiconductor combo in decades of research, said lead author Bivas Saha of Purdue.

People have tried for more than 50 years to combine metals and semiconductors with atomic-scale precision to build superlattices. However, this is one of the first demonstrations of achieving that step. The fascinating optical properties are a manifestation of extraordinary structural control.

The material that behaves like a metal when light passes through in one direction, but it acts like an insulator when light passes through in a perpendicular direction.

The hyperbolic metamaterial behaves as a metal when light is passing through it in one direction and like a dielectric in the perpendicular direction. This “extreme anisotropy” leads to “hyperbolic dispersion” of light and the ability to extract many more photons from devices than otherwise possible, resulting in high performance.

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