Dark discovery could double solar cell efficiency

LONDON – The efficiency of solar cells could be doubled bythe discovery of a quantum "shadow state" that allows two – ratherthan one – high-energy electrons to be produced by optoelectronic energyconversion. The latest research on the mechanisms of solar energy conversionwas led by chemistry Professor Xiaoyang Zhu at The University of Texas atAustin and was published in Science magazine.

Zhu and his team have discovered that it's possible to double the number ofelectrons harvested from one photon of sunlight using pentacene, an organicplastic semiconductor material.

"Plastic semiconductor solar cell production has great advantages, one ofwhich is low cost," said Zhu, in a statement. "Combined with the vastcapabilities for molecular design and synthesis, our discovery opens the doorto an exciting new approach for solar energy conversion, leading to much higherefficiencies."

The maximum theoretical efficiency of the silicon solar cell in use today isapproximately 31 percent. This is because much of the photonic energy hittingthe cell is not that at wavelengths that can be turned into usable electricity.That energy is instead lost as heat. Capturing the thermally excited hot electronicenergy could potentially increase the efficiency of solar-to-electric powerconversion to as high as 66 percent, according to the research team.

Zhu and his team previously demonstrated that those hot electrons could becaptured using semiconductor nanocrystals. They published that research inScience in 2010, but Zhu says the actual implementation of a viable technologybased on that research is very challenging. The primary one being that itrequires focused sunlight rather than ambient light that typically hits a solarpanel.

Zhu and his team have found an alternative. They discovered that in thesemiconductor pentacene a photon produces a dark quantum "shadowstate" from which two electrons can then be efficiently captured togenerate more energy.

The absorption of a photon creates an excited electron-hole pair, called anexciton. The exciton is coupled quantum mechanically to a dark"shadow-state" called a multi-exciton. It so happens that themulti-exciton can be an efficient source of two electrons via transfer to anelectron acceptor material such as fullerene, the ball-shaped 60-atom allotropeof carbon, which was used in the study. Exploiting this could raise pentacenesolar cell efficiency to 44 percent without the need for a focused solar beam,according to the researchers.

The research team was led by Wai-lun Chan, a postdoctoral fellow in Zhu'sgroup, with the help of postdoctoral fellows Manuel Ligges, Askat Jailaubekov,Loren Kaake and Luis Miaja-Avila.

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