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.

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Practical Power Beaming Gets Real

A century later, Nikola Tesla’s dream comes true

8 min read
This nighttime outdoor image, with city lights in the background, shows a narrow beam of light shining on a circular receiver that is positioned on the top of a pole.

A power-beaming system developed by PowerLight Technologies conveyed hundreds of watts of power during a 2019 demonstration at the Port of Seattle.

PowerLight Technologies
Yellow

Wires have a lot going for them when it comes to moving electric power around, but they have their drawbacks too. Who, after all, hasn’t tired of having to plug in and unplug their phone and other rechargeable gizmos? It’s a nuisance.

Wires also challenge electric utilities: These companies must take pains to boost the voltage they apply to their transmission cables to very high values to avoid dissipating most of the power along the way. And when it comes to powering public transportation, including electric trains and trams, wires need to be used in tandem with rolling or sliding contacts, which are troublesome to maintain, can spark, and in some settings will generate problematic contaminants.

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