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.