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Plastic semiconductor makes solar cells more efficient

Quantum ‘dark state’ helps capture wasted energy

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It sounds paradoxical, but reducing the amount of energy captured in a silicon solar cell can make it more efficient, according to University of Texas researchers.

The research by chemist Xiaoyang Zhu addresses the problem of “hot electrons” – electrons created in a silicon solar cell that are too energetic to be useful and are released as heat.

Zhu claims that successfully capturing hot electrons could more than double the efficiency of solar panels, from today’s theoretical maximum of 31 percent to as much as 66 percent. More realistically, he says that a practical and affordable system could be built offering 44 percent efficiency.

Instead of allowing the hot electrons to be wasted, Zhu has found that a photon impinging on the solar panel produces what he calls a dark quantum “shadow state” from which a pair of electrons can be captured.

This is achieved using a layer of an organic plastic semiconductor called pentacene. When a photon strikes the pentacene, it produces an exciton – an excited electron-hole pair (“holes” are an oddity in the world of semiconductors; think of “a place where an electron should be but isn’t” for now, because a long explanation would take ages).

The exciton produced in this fashion has a quantum coupling to the dark “shadow state”, called a multiexciton, the source of two electrons that aren’t too hot to capture. And because pentacene is not a particularly exotic material, Zhu’s breakthrough should be cheap to implement as well. ®

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