In a decade since they were first reported, lead halide perovskite solar cells are enticingly close to market given their high power conversion efficiency and low cost. But their lead content remains a concern. Now researchers have quantified lead leakage from damaged perovskite modules under various simulated weather conditions. They report in Nature Energy that encapsulating the modules with a self-healing epoxy polymer can dramatically reduce lead leakage.

Encapsulation helps solar cells withstand impacts from hail and debris, and keeps dirt and moisture from penetrating and degrading the cells. Silicon solar cells, for example, are typically encased inside layers of glass and ethylene vinyl acetate. Perovskite solar cells typically have a glass cover and are encapsulated with materials such as ultraviolet-cured resin, ethylene vinyl acetate, polyolefin, or butyl rubber.

Hail, snow, and wind can still damage solar modules, though. And damage to perovskite solar cells is especially concerning because lead in these materials can dissolve in water and solvents leaching into the environment.

Yabing Qi of the Okinawa Institute of Science and Technology and his colleagues decided to study how encapsulation affects lead leakage. By dropping a metal ball to simulate hail impact, they shattered perovskite solar modules made with three different encapsulation methods. One module had no top cover; the second had a glass substrate with a commercial copolymer resin film on top; and the third had a glass cover with an epoxy resin film on top. The researchers chose the epoxy resin (diglycidyl ether bisphenol A:n- octylamine:m-xylylenediamine = 4:2:1) because it is known to soften and heal itself above its glass transition temperature of 42°C.

The researchers place the damaged modules in a funnel and drop acidic water on it to mimic rain. The leaked lead species flow out of the modules with the water, which the researchers collect and place in an inductively-coupled plasma mass spectrometry setup to measure the lead concentration.

For a more realistic assessment, the research team used four different weather conditions: rain and clouds; continuous rain, rain followed by sunny weather and more rain; and finally sunny weather followed by rain. Sunshine heating drastically reduces the lead leakage from the epoxy resin encapsulated modules by a factor of 375 compared to modules encapsulated with a glass cover and an ultraviolet-cured resin. Under extreme continuous rain conditions, lead leakage from all the modules goes up, but even then the epoxy resin encapsulation has a fifth of the leakage of the other methods.

This is “an important study,” says Michael McGehee, a professor of chemical and biological engineering at the University of Colorado-Boulder. “It is good to see that it is possible to make a package that can contain lead even after being hit by hail.” But while the self-healing of epoxy resin has an upside, it might affect the integrity in other ways. “I do not think people want the encapsulation layer to soften up under typical operating conditions,” he says. “Having the encapsulation layer be moderately hard helps hold the two sheets of glass together.” It would be important for the researchers to demonstrate that the resin’s mechanical properties are not compromised when it is softened.

Read the abstract in Nature Energy.