Perovskite development has been happening quickly, particularly after engineers and chemists recognized their potential for more efficient solar cells a decade ago. The problem with metal halide perovskites is that they are unstable when they interact with water and oxygen, transforming into a different structure that doesn’t work well to create solar power.

The Georgia Tech team uncovered why, using X-ray scattering and spectroscopy to study the chemical interactions between perovskites and the environment. The researchers found the complex interplay of both water and oxygen with the perovskites leads to instability; taking away one of those preserved the perovskites’ energy-capturing crystal structure.

“Before this paper, people thought if you expose them to just water, these materials degrade. If you expose them to just oxygen, these materials degrade. We've decoupled one from the other,” said Correa-Baena, who’s also a Goizueta Early Career Faculty Chair. “If you prevent one or the other from interacting with the perovskites, you mostly prevent the degradation.”

Correa-Baena’s team, which included collaborators at Brookhaven and Argonne national labs and in Italy and Germany, tested their discovery by adding a thin coating of a material called phenethylammonium iodide (PEAI) on a perovskite film. PEAI molecules repel water, and the researchers found that was enough to stabilize the perovskites’ structure and thus their power conversion efficiency.

PEAI does have drawbacks, however: “These molecules are very good at preventing water from interacting with the perovskite, but they're also very bad at thermal stability,” Correa-Baena said.

Once sunlight hits the perovskite cells and they heat up, the PEAI molecules start moving and efficiency drops. So now the team is working on the thermal stability problem.