Nanoscale mapping of humid degradation-induced local mechanical property variation in CH₃NH₃PbI₃ polycrystalline film by scanning probe microscopy

We reported on the humid degradation-induced local elastic modulus (Young's modulus) variation in CH₃NH₃PbI₃ (MAPbI₃) polycrystalline film investigated by scanning probe microscopy (SPM) in humid ambient. The SPM equipped with PeakForce Quantitative Nanomechanical Measurement (PeakForce QNM) has ability to probe the near-surface elastic modulus on the specific position of material, examining the correlation between the local mechanical property and the microstructure of film through the nanoscale mapping of elastic modulus. Based on the observed variation of elastic modulus of film with aging time, we proposed a four stage degradation process, due to the development of MAPbI₃·H₂O and PbI₂ phases, occurred on the MAPbI₃ film when it aged in ambient. Evidently, the grain boundary plays a critical role for degradation process in a polycrystalline film, leading to a distinct result to that in the MAPbI₃ single crystal. When the MAPbI₃·H₂O developed at the grain boundary, it perturbed from surface to form the low modulus humps. Our study result also suggested that grain boundary is the last region of film surface to form PbI₂ under a humid ambient. These analyses provided key insights into the microstructural effects on the degradation process of a MAPbI₃ polycrystalline film.