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What Makes for Better CdTe Solar Cells

Mystery of why CdTe solar cells efficiency improves with cadmium-chloride treatment explained

2 min read
What Makes for Better CdTe Solar Cells
Image: Oak Ridge National Laboratory

Cadmium-telluride (CdTe) solar cell materials have had a bumpy ride ever since they were first introduced as an alternative to silicon-based photovoltaics. They were never quite as efficient at converting sunlight into electricity as silicon.

As a result, back in 2002, British Petroleum, which at the time was billing itself as the world’s biggest solar company, terminated U.S. production of CdTe and amorphous silicon cells. However, the fortunes of CdTe started to turnaround back in 2010 when General Electric announced its plans to enter the business. Since then, GE has been announcing ever-increasing conversion efficiency numbers, with a 19.6-percent conversion efficiency reached last year. First Solar Inc. recently broke this record by achieving a conversion efficiency of 20.4 percent.

As good as these numbers are, they still fall short of the 18 to 21 percent conversion efficiency of conventional silicon. (Recently, Panasonic announced that it had achieved a conversion efficiency of 25.6% for its silicon-based solar cells, a new record.)

To see if the latest conversion efficiency numbers of CdTe solar cells could be improved upon, and to find out what was behind the escalating numbers of the recent past, researchers at the Department of Energy’s Oak Ridge National Laboratory along with colleagues from the University of Toledo and DOE’s National Renewable Energy Laboratory used electron microscopy to peer into cadmium-telluride solar cell materials to see what made them tick.

Specifically, the researchers wanted to examine CdTe solar cell materials that had been treated with cadmium-chloride, which had been improving the efficiency numbers of the cadmium-based solar cells since the 1980s, though no one knows why.

“We knew that chlorine was responsible for this magical effect, but we needed to find out where it went in the material’s structure,” said ORNL’s Chen Li in a press release. “Only by understanding the structure can we understand what’s wrong in this solar cell—why the efficiency is not high enough, and how can we push it further.”

In research published in the journal Physical Review Letters (“Grain-Boundary-Enhanced Carrier Collection in CdTe Solar Cells”), the research team discovered atom-scale grain boundaries were involved in the enhanced performance. Grain boundaries are essentially tiny defects, which, in the case of solar cells, typically result in reduced efficiency numbers.

Using electron microscopy, the researchers saw that chlorine atoms were replacing tellurium atoms within these grain boundaries. The substitution was creating local electric fields at the grain boundaries that were improving the photovoltaic performance rather than worsening it.

The researchers believe that this understanding could lead future research into CdTe solar cells that could push their conversion efficiency closer to their theoretical maximum of 32 percent.

“We think that if all the grain boundaries in a thin film material could be aligned in same direction, it could improve cell efficiency even further,” Li added.

The Conversation (0)
This photograph shows a car with the words “We Drive Solar” on the door, connected to a charging station. A windmill can be seen in the background.

The Dutch city of Utrecht is embracing vehicle-to-grid technology, an example of which is shown here—an EV connected to a bidirectional charger. The historic Rijn en Zon windmill provides a fitting background for this scene.

We Drive Solar

Hundreds of charging stations for electric vehicles dot Utrecht’s urban landscape in the Netherlands like little electric mushrooms. Unlike those you may have grown accustomed to seeing, many of these stations don’t just charge electric cars—they can also send power from vehicle batteries to the local utility grid for use by homes and businesses.

Debates over the feasibility and value of such vehicle-to-grid technology go back decades. Those arguments are not yet settled. But big automakers like Volkswagen, Nissan, and Hyundai have moved to produce the kinds of cars that can use such bidirectional chargers—alongside similar vehicle-to-home technology, whereby your car can power your house, say, during a blackout, as promoted by Ford with its new F-150 Lightning. Given the rapid uptake of electric vehicles, many people are thinking hard about how to make the best use of all that rolling battery power.

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