Rubrene's red crystals: the next semiconductor for cheaper solar?

Rubrene crystals may lead to cheaper, more efficient organic solar cells. Could they give silicon cells a run for their money? Researchers at Rutgers University think so.
Written by Melissa Mahony, Contributor on

Organic solar cells are typically less expensive to make than their silicon-based counterparts. The potential versatility of these lightweight and flexible plastic cells is also impressive. Their problem? Low efficiency.

But in a flash of red, physicists from Rutgers University see a bright future for these cells. Their work with rubrene, an organic crystal used in glow sticks (I've always wondered...), shows that this red material can conduct the sun's energy much more efficiently than previous carbon-based semiconductors.

When a photon strikes a material’s surface, it excites an electron to a higher energy state, creating an exciton in the process. The exciton, which is an electron and a positively charged "electron hole," moves through the material, generating voltage when its components separate at a semiconductor barrier. The farther the exciton travels, the more likely it is to reach these barriers.

Currently, excitons usually travel less than 20 nanometers in organic solar cells, with the cells losing about 99 percent of the absorbed sunlight, according to the researchers. Their study, published this week inNature Materials, shows excitons going a thousand times as far on their trek through very pure rubrene.

Physicist Vitaly Podzorov in a statement:

This is the first time we observed excitons migrating a few microns. Once the exciton diffusion distance becomes comparable to the light absorption length, you can collect most of the sunlight for energy conversion.

The excitons traveled between 2 and 8 micrometers in the rubrene. To us, that's not very far, a micrometer (or micron) is one-millionth of a meter. But the distance covered is similar to that within the inorganic semiconductors, silicon and gallium arsenide. So if rubrene can prove its photo-conducting stuff outside of the lab, that little bit might bring organic solar cells, and cheaper solar power, a long way.

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Images: Flickr_kidjay and Rutgers University

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