Two American physicists have developed an imaging technique that makes it possible to directly observe light-emitting excitons as they diffuse in a new material that is being explored for its extraordinary electronic properties.

The new material, called rubrene, is one of a new generation of single-crystal organic semiconductors – which could open the way to a cheaper solar energy.

Excitons, which are created by light, play a central role in the harvesting of solar energy using plastic solar cells.

The pair, both from Lehigh University in the states, Ivan Biaggio, professor of physics, and Pavel Irkhin, a PhD candidate, have for what they claim is the first time used an advanced imaging technique to witness the long-range diffusion of energy-carrying excitons in an organic crystal.

An understanding of exciton diffusion is considered to be critical for plastic solar cell technology, in which the intake of light creates excitons instead of directly inducing a current, as it does in the most commonly used silicon systems.

After they are created in plastic solar cells, excitons diffuse toward specially designed interfaces where they drive electrons into an external circuit, creating the flow of electrons we know as electric current.

This diffusion process is one of the technical challenges limiting the efficiency of plastic solar cells.

“This is the first time that excitons have been directly viewed in a molecular material at room temperature,” said professor Biaggio.

“We believe the technique we have demonstrated will be exploited by other researchers to develop a better understanding of exciton diffusion and the bottleneck it forms in plastic solar cells.

“When will we have cheap and efficient plastic solar cells? It is the goal of researchers around the world to improve exciton diffusion lengths until they become as large as the light absorption–that’s the point when sunlight is most efficiently collected and converted into energy.”

An article by Irkhin and Biaggio, titled “Direct Imaging of Anisotropic Exciton Diffusion and Triplet Diffusion Length in Rubrene Single Crystals,” was published in the journal Physical Review Letters.

The work was supported by a Faculty Innovation Grant from Lehigh, which provides resources to develop novel ideas and demonstrate new approaches to important research questions.

Luke Walsh

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