Lead halide perovskite (HaPs) shows fast technological progress in photovoltaic and other devices due to their attractive optoelectronic properties. However, the stability of perovskite films remains a major roadblock to their practical implementation which creates concern and calls for advanced characterization methods. The intrinsic self-healing (SH) property of halide perovskites, owing to their strong lattice dynamics, offers a potential solution to this issue. SH means that a material can recover from damage autonomously. Till today, though the mechanism(s) of SH in HaPs and the experimental and material parameters and properties for it are still mostly a black box.

We will show how the SH dynamics can be measured by combining Fluorescence Recovery After Photobleaching (FRAP) and PL imaging, for MAPbI₃ (MAPI) and γ-CsPbI₃ (CsPI) [1-2].  FRAP quantifies fluorescence recovery after photobleaching (by a high-intensity laser source) in a particular region of interest (ROI). Here we use FRAP to resolve the diffusion kinetics of SH-related species.  Specifically, we damaged at different power densities for 3 seconds at 8 different ROIs. For both CsPI and MAPI, we observe a combination of fast (few seconds or less) and slow (tens of minutes) kinetics, both at the illuminated spot and in regions of the film surrounding it. In the periphery of the excitation spot, CsPI shows photo-darkening and MAPI shows photo-brightening, immediately following damage. During the self-healing process, of the directly illuminated spot, MAPI peripheral fluorescence returns to its initial level, whereas CsPI exhibits gradual photo-brightening to above the original level.

We attribute these spatio-temporal effects to a combination of fast carrier diffusion following photoexcitation and slower ionic diffusion that kick in at later stages.  The role of carrier migration in emission dynamics following photoexcitation has generally been neglected but may be an important factor in experiments studying the system response under illumination. Assessment of the diffusion coefficients for the slower process from the self-healing rate is commensurate with halide ion diffusion. This is partially supported by Raman spectroscopy of damaged films. Overall, our work provides visual evidence for the spatiotemporal dynamics of photo-damage and SH of HaPs, an understanding of which will aid the development of long-term device applications.