Major limitation in the performance of solar cells happens within the photovoltaic material itself: When photons strike the molecules of a solar cell, they transfer their energy, producing quasi-particles called excitons — an energized state of molecules. That energized state can hop from one molecule to the next until it’s transferred to electrons in a wire, which can light up a bulb or turn a motor.

But as the excitons hop through the material, they are prone to getting stuck in minuscule defects, or traps — causing them to release their energy as wasted light.

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Now a team of researchers at MIT and Harvard University has found a way of rendering excitons immune to these traps, possibly improving photovoltaic devices’ efficiency. The work is described in a paper in the journal Nature Materials.

Their approach is based on recent research on exotic electronic states known as topological insulators, in which the bulk of a material is an electrical insulator — that is, it does not allow electrons to move freely — while its surface is a good conductor.

The MIT-Harvard team used this underlying principle, called topological protection, but applied it to excitons instead of electrons, explains lead author Joel Yuen, a postdoc in MIT’s Center for Excitonics, part of the Research Laboratory of Electronics. Topological protection, he says, “has been a very popular idea in the physics and materials communities in the last few years,” and has been successfully applied to both electronic and photonic materials.

For more on this article: http://www.sciencedaily.com/releases/2014/09/140921145108.htm

Source: Science Daily/Torronto

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