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Cloaking Principle Could Boost Solar Cell Performance

Photonics.com
Oct 2015
KARLSRUHE, Germany, Oct. 6, 2015 — Invisibility cloaking may be a long way from reality, but the principle could help improve the performance of solar cells in the near term.

In a series of simulations, researchers at the Karlsruhe Institute of Technology have demonstrated how cloaks made of metamaterials or freeform surfaces could eliminate shadows cast by energy-harvesting components onto the active surfaces of solar cells.

An invisibility cloak (right) guides sunlight past the contacts for current removal to the active surface area of a solar cell.
An invisibility cloak (right) guides sunlight past the contacts for current removal to the active surface area of a solar cell. Courtesy of Martin Schumann/Karlsruhe Institute of Technology.

Contact fingers, which extract electric current, cover up to one-tenth of the surface area of a solar cell. By guiding light around these features, more of the sun's energy could be captured by the solar cell.

"Our model experiments have shown that the cloak layer makes the contact fingers nearly completely invisible," said doctoral student Martin Schumann.

The scientists pursued two approaches, both based on polymer coatings, to achieve the cloaking effect. One approach uses graded-index metamaterials designed using 2D Schwarz-Christoffel conformal maps, and the second uses freeform surfaces designed using 1D coordinate transformations.

In the freeform surface approach, the cloak layer is grooved along the contact fingers. In this way, incident light is refracted away from the contact fingers and toward the active surface of the solar cell.

Freeform surfaces are particularly promising because they could be integrated into mass production processes and can cancel shadows at virtually any angle, the researchers said.

The next step will be to apply cloaking layers to real solar cells. Efficiency is expected to increase by up to 10 percent, Schumann said.

The researchers have already created proof-of-principle demonstrators using direct laser writing of polymer structures on silicon wafers with opaque contacts.

The research was published in Optica (doi: 10.1364/optica.2.000850).


GLOSSARY
metamaterial
A material engineered from artificial matter not found in nature. The artificial makeup and design of metamaterials give them intrinsic properties not common to conventional materials that are exploited as light waves and sound waves interact with them. One of the most active areas of research involving metamaterials currently explores materials with a negative refractive index. In optics, these negative refractive index materials show promise in the fabrication of lenses that can achieve...
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