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'Invisibility cloak' might enhance efficiency of solar cells

01 October 2015

Optical invisibility cloaks guide sunlight around objects that cast a shadow on the solar panel, such as the contacts used for current extraction.

A special invisibility cloak guides sunlight past the contacts for current removal to the active surface area of the solar cell (graphic: Martin Schumann/KIT)

The efficiency of commercially available photovoltaic cells is about 20 percent. Scientists at Karlsruhe Institute of Technology (KIT) have developed a novel approach to increasing the efficiency by adding optical 'invisibility cloaks' that guide sunlight around objects that cast a shadow on the solar panel, such as the contacts used for current extraction.

Up to one tenth of the surface area of solar cells is covered by so-called contact fingers that extract the current generated. At the locations of these contact fingers, light cannot reach the active area of the solar cell and its efficiency is reduced as a result.

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

Normally, invisibility cloak research is aimed at making objects invisible. For this purpose, light is guided around the object to be hidden. This research project did not focus on hiding the contact fingers visually, but on the deflected light that reaches the active surface area of the solar cell.

The KIT researchers tried two approaches to achieve the cloaking effect, both based on applying a polymer coating to the solar cell. This coating has to possess exactly calculated optical properties: an index of refraction that depends on the location or a special surface shape.

The second approach proved particularly promising, as it can potentially be integrated into mass production of solar cells at low cost. The surface of the cloak layer is grooved along the contact fingers enabling incident light to be refracted away from the contact fingers and reach the active surface area of the solar cell.

The researchers are optimistic that efficiency will be improved by the cloak under real conditions. "When applying such a coating onto a real solar cell, optical losses via the contact fingers are supposed to be reduced and efficiency is assumed to be increased by up to 10 percent," Martin Schumann claims.


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