Weakness in perovskite crystals uncovered

Methylammonium lead halide perovskites (CH₃NH₃PbI₃) show great promise for solar-cell materials because they are more efficient at converting sunlight into electricity than current commercial materials are. Perovskite efficiencies can reach 22%, while those of commercial materials are about 15%. However, these perovskites suffer from a serious drawback: They degrade rapidly upon exposure to oxygen and light.

A group including M. Saiful Islam at the University of Bath and Saif A. Haque of Imperial College London now reports experimental and theoretical evidence for the mechanism behind this degradation (Nat. Commun. 2017, DOI: 10.1038/ncomms15218). Their findings have led them to a way to protect these promising materials.

The team previously determined that when light excites these crystals, it creates electrons, which can react with O₂ to make reactive superoxide species. These superoxides then wreak havoc on the perovskite crystal structure, breaking it down to PbI₂, methylamine, and water.

In the new work, the group uses computational simulations and experiments to show that O₂ diffuses into iodide vacancies in perovskite crystal structures. These spaces were the most vulnerable and facilitated the superoxide degradation process.

With this mechanism in mind, the team tried to stabilize the material by coating it with a thin layer of iodide salts, which reduced the number of iodide vacancies. The protection was striking. Without the coating, the material began degrading within hours and became useless within days. However, crystals blanketed with the iodide salts remained stable for three weeks.