Chirality Yields Colossal Photocurrent

The Weyl semimetal tantalum arsenide has a colossal bulk photovoltaic effect – an intrinsic, or non-linear, generation of current from light more than ten times larger than ever previously achieved. (Source: K. Burch, Boston Col.)

A recently discovered Weyl semimetal delivers the largest intrinsic conversion of light to electricity of any material, discovered now an inter­national team lead by a group of Boston College researchers. The discovery is based on a unique aspect of the material where electrons can be separated by their chira­lity. The findings may offer a new route to efficient generation of electricity from light, as well as for thermal or chemical sensing.

“We discovered that the Weyl semimetal tantalum arsenide, has a colossal bulk photo­voltaic effect – an intrinsic, or non-linear, generation of current from light more than ten times larger than ever previously achieved,” said Kenneth Burch. “Further­more this is in the mid-infrared regime, which means this material can also be used for chemical or thermal sensing, as well as waste heat recovery,” Burch added.

Typically, light is converted to elec­tricity by creating a built-in electric field in a semi­conductor, Burch said. “This is achieved through chemical modu­lation, and results in a fundamental upper limit to the potential efficiency – known as the Shockley-Queisser limit.” The alter­native approach taken by the team explored exploiting the handedness of the electrons in the material to intrin­sically generate direct current through the nonlinear mixing of the waves of light, Burch said.

This approach has typically been too small to be useful. But researchers recently realized it is closely connected to the topo­logical properties of the electrons. That prompted predictions that the unique, DNA-like behavior of electrons in Weyl semi­metals could produce enormous nonlinear effects. “We focused on answering whether Weyl semimetals live up to the predictions of large, intrinsic nonlinear responses to generate current,” said Burch. He added that the team was surprised at the magni­tude of the electronic effect, which was provoked by a new fabrication approach.

“The size of the effect was far larger than we dreamed,” said Burch. “A previous group from MIT found their response was dominated by thermal, or extrinsic, terms, our use of the focused ion beam fabri­cated devices and symmetry allowed us to uncover the colossal bulk photo­voltaic effect at room tempera­ture.” Burch said the team is working to determine the “sweet spot” for the effect, speci­fically what is the ideal device confi­guration and wavelength of light. (Source: Boston College)

Reference: G. B. Osterhoudt et al.: Colossal mid-infrared bulk photovoltaic effect in a type-I Weyl semimetal, Nat. Mat., online 4 March 2019; DOI: 10.1038/s41563-019-0297-4

Link: Dept. of Physics, Boston College, Chestnut Hill, USA

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