Domain Size, Temperature, and Time Dependence of Photodegradation in MAPbI₃Probed by Raman Spectroscopy

Organic-inorganic hybrid perovskites display remarkable photovoltaic properties, but instability arises from the material's surface and photoexcitation. The photodegradation process of methylammonium lead triiodide (MAPbI3) investigated with above bandgap excitation Raman spectroscopy displays two stages of structural change in polycrystalline films of varying domain sizes and a single crystal. Subtle surface changes occur below an excitation density threshold; PbI2 formation scales linearly above this threshold in thin films. However, the rate of change with excitation density increases with more grain surfaces. Whereas with a moderate level of heating into the cubic phase, accelerated photodegradation occurs in a single crystal. From experimental kinetic modeling, PbI2 nuclei on the surface grow until they meet in the polycrystalline films, proceeding one-dimensionally inward from the surface. These results suggest that the surface acts as a location for photogenerated carrier accumulation, leading to degradation and PbI2 formation.