Redox Chemistry Dominates the Degradation and Decomposition of Metal Halide Perovskite Optoelectronic Devices

We report a comprehensive study of the chemistry of perovskite optoelectronic device degradation and show that redox reactions are fundamental to the degradation process for CH₃NH₃PbI₃, CsPbI₃, and CsPbBr₃ perovskites with Ag, Al, Yb, or Cr contacts. Using in situ X-ray diffraction measurements, we study the chemistry of CH₃NH₃PbI₃ perovskite devices equipped with Al electrodes; we find that Al⁰ rapidly reduces Pb²⁺ to Pb⁰, converting CH₃NH₃PbI₃ first to (CH₃NH₃)₄PbI₆·2H₂O and then to CH₃NH₃I. In situ scanning electron microscopy measurements show that moisture enables continued reaction of the Al and perovskite layers by facilitating ion diffusion, before serving as a decomposition reagent for the perovskite film. Redox reactions follow what is expected based on standard electrochemical potentials for Al, Cr, and Yb; for Ag, the redox chemistry is enabled by the presence of iodide. We emphasize that critical chemical reactions can stem from intrinsic interfacial interactions between the layers in a device and not necessarily from external agents; degradation studies must consider the device as an entity, rather than focusing only on the stability of perovskite films.