Study: To boost solar cell efficiency, curb 'hot electrons'

"These tin-based perovskites could be a game changer," researcher Maria Antonietta Loi said.
By Brooks Hays  |  Jan. 16, 2018 at 11:39 AM
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Jan. 16 (UPI) -- New research suggests the key to improving the efficiency of solar cells is the reduction of so-called hot electrons.

When photos with energy levels greater than the solar cell's band gap are absorbed by the semiconductor, hot electrons are produced. The extra energy is quickly converted into thermal energy and is lost.

Researchers at the University of Groningen in the Netherlands have developed a way to keep hot electrons around for a while longer, so that their energy might be converted into electricity.

A semiconductor's bandgap refers the range of energy at which absorbed photons are converted into free electrons, which form an electric current that can be funneled out and used.

When photons with too much energy hit the solar cell, they cause the photovoltaic material to vibrate, generating thermal energy.

"This energy loss puts a limit to the maximum efficiency of solar cells," Maria Antonietta Loi, a professor of photophysics and optoelectronics at Groningen, said in a news release.

Loi has developed a unique solar cell material that solves the problem of energy loss. The material is made of organic-inorganic hybrid perovskites. All perovskites boast the basic chemical formula of ABX3. The structure features an octahedron of anions, the X, which binds an outer cube of larger A atoms to smaller B atoms in the center.

Most perovskites feature lead, which is toxic, but Loi and her colleagues designed a perovskite material featuring tin.

"When we studied this material further, we observed something strange," Loi explained. "The hot electrons gave off their energy after several nanoseconds instead of some hundred femtoseconds. Finding such long-lived hot electrons is what everybody in this field is hoping for."

The technology effectively expands the material's bandgap. The longer the excess energy lingers, the more likely it is to get converted into electricity. The breakthrough could inspire solar cells capable of producing much higher voltages, as well as provide an efficiency boost of between 33 to 66 percent.

"These tin-based perovskites could be a game changer, and could ultimately make a big contribution to providing clean and sustainable energy in the future," Loi said.

She and her research partners shared their discovery this week in the journal Nature Communications.

The researchers are now working to better understand why the addition of tin effectively slows down the energy dissipation of hot electrons.

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