Why does selenium increase solar panel efficiency?

A PhD student from Loughborough University has helped shed light (no pun intended) on a solar panel puzzle that could lead to more efficient devices being developed.

Most of the world’s solar power is currently produced by solar panels — also known as photovoltaic panels — that are made of silicon. Recently, new solar panels have been created that are made from a semiconducting material called cadmium telluride (CdTe).

CdTe panels have been found to produce electricity at lower costs than silicon panels and there has been a dramatic gain in efficiency brought about by adding an element called selenium to the cadmium telluride. As a result, electricity from CdTe solar farms is being produced more cheaply than it is from fossil fuels.

Until now, it was not well understood why selenium increases efficiency. But thanks to an international team of researchers, including PhD research student Tom Fiducia from Loughborough University’s Centre for Renewable Energy Systems Technology (CREST), the puzzle has been solved. Their work has been published in the journal Nature Energy.

Working with solar experts from CREST, Durham University, the University of Oxford and Colorado State University, Fiducia has revealed that selenium works by overcoming the effect of harmful, atomic-scale defects in CdTe panels. This explains the increase of efficiency as electrons (subatomic particles that carry electricity), which are generated when sunlight hits the solar panel, are less likely to be trapped and lost at the defects. This increases the amount of power extracted.

Fiducia says the team discovered this “unexpected” behaviour by measuring how much light is emitted from selenium-containing panels. As selenium is not evenly distributed across the panels, they compared the ‘luminescence’ emitted from areas where there was little to no selenium present and areas where the selenium was very concentrated.

“While it seems counterintuitive, good solar cell material that is defect-free is very efficient at emitting light, and so luminesces strongly,” said Fiducia, who served as lead author on the paper.

“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.

“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.”

Left: Map of the selenium distribution in the solar cell material. The brighter turquoise/white regions are where there are higher concentrations of selenium. Right: A corresponding map of the luminescence emitted from the material. It can clearly be seen that there is brighter luminescence where there is more selenium.

Fiducia hopes the findings will lead to improved solar panels and increased usage across the globe, stating, “Now that the selenium-induced efficiency improvement is better understood, it may be possible to use this knowledge to increase the efficiency of cadmium telluride solar panels even further.

“For instance, this could be by simply increasing the amount of selenium in the devices or altering its distributions within the cell.

“If efficiency can be increased, this would further decrease electricity prices and have a direct positive impact on regions that adopt the technology.”

Professor Michael Walls, the academic overseeing Tom’s PhD research, added, “Now we know how the selenium improves the solar cell efficiency, it should be possible to improve the power output still further.

“It’s a great example of an international team working together contributing their expertise and facilities and developing a fundamental understanding of how devices really work.”

Top image credit: ©stock.adobe.com/au/Thinapob

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