Many chemicals are used to make different types of photovoltaic cells. Here are some of the most common chemicals associated with solar panel manufacturing and their impacts.
Crystalline silicon (c-Si)
Lots of energy is used to generate the high temperatures necessary to make this material—silica ores must be melted at more than 4,000 degrees Fahrenheit before the purified silicon is allowed to crystallize. It’s the crystalline structure that gives c-Si the semiconductor properties that make it so valuable for PV cells and for computer chips, too.
Sawing c-Si into the thin wafers used in panels creates a significant amount of waste silicon dust, up to 50 percent of which is lost in the air and water used to rinse the wafers. The process of making crystalline silicon from silicon is also inefficient; as much as 80 percent of the raw silicon is lost in the process.
The manufacturing process also releases silicon tetrachloride, an extremely toxic substance that reacts violently with water, causes skin burns, and is a respiratory, skin and eye irritant. And then there’s sulfur hexafluoride—an extremely potent greenhouse gas, 23,000 times worse than CO2—which is used to clean the reactors used in silicon production. In developed countries, these extreme molecules are usually captured and reused in a closed-loop process, but that’s not always the case elsewhere. There are reports from China, for example, of silicon tetrachloride pollution from new PV cell factories that are springing up in response to the global demand for solar electricity.
Cadmium telluride (CdTe) thin film
Cadmium is a toxic, cancer-causing heavy metal. It’s nasty stuff, but expensive, too—and that means double the incentives to keep it under control and out of the air and water. Cadmium rinsed away during the production of CdTe films could potentially pollute water systems, but it is generally reclaimed and reused in other steps of the thin-film manufacturing process. Still, about one percent of the cadmium used as input is disposed of as waste. And there’s a risk that cadmium could be released from thin-film panels installed in homes in the event of a fire.
Copper indium selenide (CIS) and copper indium gallium (di)selenide (CIGS)
Hydrogen selenide, which is used in the manufacture of CIS, is toxic and dangerous even at very low concentrations. Manufacturers are developing new processes that avoid its use, or substitute it with solid selenium, which largely obviates this hazard. Another concern is that selenium dioxide can sometimes form at high temperatures during the manufacturing of these cells. It is a dangerous air pollutant, and can pose health risks to workers.