How selenium alloying gives CdTe thin film a boost

A new study conducted by an international team of scientists from leading institutions in the U.K. and the United States provides an explanation as to how the addition of selenide can greatly improve the efficiency of cadmium telluride solar cells. Explaining the performance boost will provide researchers with a route to explore further efficiency improvements, pushing the material’s cost even lower.

Comparing selenium distribution in the material (left) with the material's luminescence, which is clearly brighter in areas with more selenium.

Image: Loughborough University

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Several studies have confirmed that the addition of selenium to cadmium telluride (CdTe) solar cell material can provide a significant efficiency boost. Leading CdTe solar panels currently on the market have around 17% conversion efficiency, and alloying selenium into the material has increased this to the current world record of 22.1%, set by First Solar in 2016.

But the mechanism causing this efficiency boost was not well understood, limiting its further progress. Now, an international team of researchers led by Loughborough University in the U.K. has been able to observe the material’s luminescence properties and provide an explanation for the phenomenon. Their results are published in the paper Understanding the role of selenium in defect passivation for highly efficient selenium-alloyed cadmium telluride solar cells, published in the journal Nature Energy.

Experiments using cathodoluminescence and secondary ion mass spectrometry showed that the addition of selenium has the effect of passivating defects in the bulk crystal of the CdTe layer — meaning that electrons generated when sunlight hits the material are less likely to become trapped in the material and recombine.

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The researchers compared the light emitted from areas with little to no selenium with areas in the material where selenium was more concentrated. “We mapped the luminescence emitted from a selenium-containing solar cell at a resolution of around 1/10,000th of a millimetre and compared it to a similarly high-resolution map of the selenium concentration taken on the exact same area of the cell,” explains Tom Fiducia, a Ph.D student at Loughborough University’s Centre for Renewable Energy Systems Technology and the lead author of the paper. “”It is strikingly obvious when you see the data that selenium-rich regions luminesce much more brightly than the pure cadmium telluride, and the effect is remarkably strong.”

Understanding the mechanism behind the increased efficiency will allow scientists to look at optimizing it for even greater gains. “This could be simply by increasing the amount of selenium in the devices, or altering its distributions within the cell,” explains Fiducia. “I hope that the results can be useful to other researchers and ultimately make a positive impact.”

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