Thin-film Solar Cells Freed From Toxic Processing

Photo: First Solar

First Solar's Series 3 Black, with a conversion efficiency of 16.1 percent, is the world's most efficient cadmium telluride photovoltaic module

Cadmium chloride is filthy stuff. Its cadmium ions are extremely toxic, causing heart disease, kidney disorders, and a host of other health problems. One accidental spill of the water-soluble compound can wipe out fish from a river. So it is both unfortunate and ironic that cadmium chloride should be essential for manufacturing a promising source of clean energy: thin-film cadmium telluride solar cells.

Researchers at the University of Liverpool in the United Kingdom have now discovered that the cadmium chloride can be replaced with magnesium chloride, a benign and extremely cheap alternative that could help to cut the cost and environmental impact of thin-film photovoltaics. Magnesium chloride is extracted from seawater, and is used as a low-temperature de-icer for roads or as a coagulant to make tofu. And at roughly US $1 per kilogram in bulk, it is hundreds of times cheaper than cadmium chloride.

The new, poison-free process could help thin-film solar cells challenge the dominance of silicon photovoltaics, which make up roughly 90 percent of the world’s solar market but have some serious drawbacks. Silicon does not absorb sunlight particularly well, so modules require layers of very high purity crystals, each more than 150 micrometers thick. The cost of these silicon slabs is hampering efforts to further reduce the price of solar power.

Thin-film solar cells offer a solution. By using semiconductors that harvest the sun’s rays much more efficiently, they can get similar results with sheets of lower purity material that are only 2 micrometers thick. The difference: a significant reduction in manufacturing costs.

The leading thin-film technology, based on a sandwich of cadmium telluride and cadmium sulfide (CdTe/CdS), makes up between 5 and 7 percent of the solar power market. Although the technology has been around for decades, CdTe cells have been slow to take off—not least because their efficiency at converting light into electricity has lagged behind silicon. But in the past few years, the efficiency of CdTe cells has risen above 20 percent in the lab, trailing silicon by just five percentage points or so. (Commercial modules for both technologies typically have lower efficiencies.)

“Now that the efficiency has improved, CdTe can compete commercially with silicon,” says Jonathan Major, a photovoltaics researcher at the University of Liverpool who developed the new magnesium chloride process.

So what does cadmium chloride, and now magnesium chloride, do for these thin-film photovoltaics?

When light hits the boundary region between CdTe and CdS in the cells, it excites electrons that are drawn into the CdS layer (an n-type semiconductor). As the holes left behind by those electrons fall into the CdTe (p-type) layer, the separation of charge generates a current. But the two layers must be treated with a solution of cadmium chloride or some equivalent to make them function efficiently.

“This process is used by all the [manufacturing] plants,” says Major, and it requires specialized industrial waste processing facilities to handle the material.

The treatment has several effects; one is that the material’s chloride ions help to make a better junction between the two semiconductor layers. And in April, Chen Li at Oak Ridge National Laboratory in Tennessee found that chloride replaces some tellurium in the CdTe layer. “That protects electrons and holes from unwanted recombination,” says Li, which allows current to flow more efficiently.

Major’s team tested a variety of chloride salts as replacements for cadmium chloride, and found that a vapor treatment of magnesium chloride achieved the best results. Their cells reached efficiencies of 13.5 percent, similar to control cells made using the conventional process, and matched up on other factors such as voltage, current density and stability.

Further design improvements, such as thinning the CdS layer, boosted cell efficiencies to 15.7 percent. And rather than needing to use fume hoods and gas masks, as is required during the cadmium chloride process, “We can deposit magnesium chloride using an airbrush, spraying it on the back of the solar cell,” says Major.

Their results were presented at the Euroscience Open Forum meeting in Copenhagen this week, and are published in Nature today.

Curiously, the presence of magnesium in the mix seemed to have no effect—perhaps because conventional CdTe cells already contain a little magnesium, as it diffuses out of the glass covering the cell.

Major says he has already been in touch with the leading manufacturer of CdTe solar cells: First Solar, based in Tempe, Ariz. It furnished the world’s largest solar photovoltaic power facility, Arizona’s Agua Caliente Solar Project, which opened in April and has an installed capacity of 290 megawatts.

“The cadmium chloride treatment is to date a critical part of the CdTe solar cell manufacturing sequence,” says Raffi Garabedian, chief technology officer at First Solar. “We apply a full and robust set of environmental, health, and safety controls in order to guarantee that we have no adverse impacts as a result of our manufacturing operation.”

Garabedian adds that "Despite the cost of these controls, the cadmium chloride treatment step is not an major cost driver in our manufacturing process.” But that’s not what Major heard: “Talking to them privately," says Major, "they said that cadmium chloride was the second biggest expense in their process.”

Whatever the cost implications, replacing toxic cadmium chloride is clearly a sensible move. “We have been, and continue to study various alternatives to the cadmium chloride treatment step,” Garabedian acknowledges. But for now, the company is keeping quiet on whether tofu-thickener really can offer a boost to its business.

UPDATED 27 June 2014: Clarified that chloride replaces some tellurium in the CdTe, not specifically at the junction.

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