Invisibility Cloak Hikes Solar Cell Efficiency

Wednesday, September 30, 2015 @ 04:09 PM gHale

Success of the energy turnaround will depend decisively on the extended use of renewable energy sources.

However, their efficiency is much smaller than that of conventional energy sources. The efficiency of commercially available photovoltaic cells, for instance, is about 20 percent. Having said that, an unconventional approach can increase the efficiency of the panels, said scientists of Karlsruhe Institute of Technology (KIT).

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Optical invisibility cloaks can guide sunlight around objects that cast a shadow on the solar panel.

Energy efficiency of solar panels has to hike significantly not only for the energy turnaround, but for enhancing economic efficiency. Modules mounted on roofs convert just one fifth of the light into electricity, which means about 80 percent of the solar energy is lost.

There are plenty of reasons of these high losses. Up to one tenth of the surface area of solar cells, for instance, remains covered by 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 efficiency of the cell decreases.

“Our model experiments have shown that the cloak layer makes the contact fingers nearly completely invisible,” said doctoral student Martin Schumann of the KIT Institute of Applied Physics, who conducted the experiments and simulations. Physicists of KIT around project head Carsten Rockstuhl, together with partners from Aachen, Freiburg, Halle, Jena, and Jülich, modified the optical invisibility cloak designed at KIT for guiding the incident light around the contact fingers of the solar cell.

Normally, invisibility cloak research aims at making objects invisible. For this purpose, light guides around the object that needs hiding. 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 thanks to the invisibility cloak.

To achieve the cloaking effect, the scientists pursued two approaches.

Both approached end up based on applying a polymer coating onto the solar cell. This coating has to possess exactly calculated optical properties, i.e. an index of refraction that depends on the location or a special surface shape. The second concept is promising, as it can potentially integrate into mass production of solar cells at low costs. The surface of the cloak layer ends up grooved along the contact fingers. In this way, incident light refracts away from the contact fingers and finally reaches the active surface area of the solar cell.

By means of a model experiment and detailed simulations, the researchers found the two concepts can hide the contact fingers. In the next step, the researchers planned to apply the cloaking layer onto a solar cell in order to determine the efficiency increase. The physicists are optimistic that efficiency will improve 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,” Schumann said.