Cadmium chloride is definitely not healthy to be around. Its cadmium ions are extremely toxic, and can cause heart disease, kidney disorders, and many other health problems. It is ironic that such a toxic substance is essential for the manufacturing of clean energy: thin-film cadmium telluride solar cells. University of Liverpool researchers have discovered a way to work around this however. They have found that the cadmium chloride can be replaced with magnesium chloride, a safe and inexpensive alternative that could help to decrease the cost and environmental impact of thin-film photovoltaics. At approximately $0.50 per pound, magnesium chloride is hundreds of times cheaper than cadmium chloride.
This new poison-free process could allow thin-film solar cells to challenge the dominance of silicon photovoltaics, which currently account for approximately 90 percent of the world’s solar market. There are some major drawbacks with silicon photovoltaics. They do not particularly absorb light well, so modules require layers of very high purity crystals, each more than 150 micrometers thick. The price of these silicon slabs is hindering the efforts to reduce the price of solar power. Thin-film solar cells may be a solution. By using semiconductors that absorb the sun’s rays more efficiently, similar results can be obtained with sheets of lower purity material that are only 2 micrometers thick. This results in drastically lower manufacturing costs.
The leading thin-film technology, which is a sandwich of cadmium telluride and cadmium sulfide (CdTe/CdS), makes up between 5-7 percent of the solar power market. While the technology is nothing new, CdTe cells have been slow to take off. However, their efficiency has risen above 20 percent in the lab in the last few years, now only trailing silicon by approximately 5 percent.
“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. 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. The two layers must be treated with a solution of cadmium chloride or an 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 being that the material’s chloride ions help to make a better junction between the two semiconductor layers. Also, 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 several chloride salts as replacements for cadmium chloride, and found that a vapor treatment of magnesium chloride achieved the best results. Their cells were able to achieve efficiency levels of 13.5 percent, similar to control cells made using the conventional process. They were also able to match on other factors, such as voltage, current density, and stability. Other design improvements, such as thinning the CdS layer, increased cell efficiency to 15.7 percent. While fume hoods and gas masks are required during the cadmium chloride process, magnesium chloride can be deposited using an airbrush.
Major has already been in touch with the leading manufacturer of CdTe solar cells: First Solar, located in Tempe, Arizona. First Solar manufactured the world’s largest solar photovoltaic power facility, Arizona’s Agua Caliente Solar Project, which 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.” That however is not what Major was told. “Talking to them privately,” says Major, “they said that cadmium chloride was the second biggest expense in their process.”
Regardless of cost implications, replacing toxic cadmium chloride is clearly a sensible move, as we may see more magnesium chloride used in the future.