[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.