Metal halide perovskites (MHPs) possess a remarkable combination of properties that make them highly attractive for a wide range of optoelectronic applications. Their outstanding optical indicators such as high absorption coefficient, tunable bandgap through compositional adjustments, and strong photoluminescence are complemented by the low-cost, solution-based fabrication methods that enable scalable and economically viable production. One of the major challenges with MHPs is their inherent sensitivity to moisture, heat, and UV light, which raises concerns about their long-term stability in practical applications [1], [2]. Understanding the underlying mechanisms of degradation is therefore crucial.

In this context, a combination of in situ and ex situ electron microscopy techniques provide invaluable input. We conducted in situ experiments using both transmission electron microscopy (TEM) and scanning electron microscopy (SEM) at different scales, and compared our findings with ex situ results to corroborate the degradation pathways. In this manner, we could precisely characterize the black-to-yellow phase transition in both CsPbI₃ films and nanocrystals (NCs). Yellow phase CsPbI₃ is formed as hollow microtubes elongated along the {100} plane after NC agglomeration and cannot be transformed back into the black phase upon heating at 350 ⁰C, unlike films. Notably, our results indicate that oxygen does not play a significant role in the degradation process. Instead, one-time exposure to H₂O vapor is sufficient to initiate the detrimental phase transformation. For films, this transformation is further complicated by the formation of side phases, particularly inside pinholes. The combination of H₂O vapor and heat leads to the formation of PbO regions and decomposition of the CsPbI₃ NCs into PbI₂ (P63mc) and CsI.

Obtaining high-resolution details of these crystal phase changes is extremely challenging due to the electron beam sensitivity of MHPs, often leading to Pb/PbX₂-clusters formation or amorphization. Significant efforts have therefore been directed towards developing low-dose imaging protocols, e.g. based on “4D STEM” using event-driven direct electron detectors. Using doses of less than 500 e/Ų, we could characterize the local structure defects of MHPs. Moreover, this approach enables to visualize light elements within the perovskite lattice. As such, we observed for the first time the coexistence of CsPbI₃ and CsPbCl₃ domains, as well as the presence of mixed CsPb(I,Cl)₃ phases in perovskite/chalcohalide heterostructures.

A quantitative interpretation of TEM data is especially critical in the characterization of structural defects. For instance, stacking faults in MHPs frequently result in the formation of Ruddlesden-Popper (RP) phases. By applying statistical parameter estimation theory [3] together with molecular dynamics simulations, we have been able to quantify total column intensities and the probabilities that atomic columns belong to either the RP defect phase or the perovskite phase. This detailed analysis was vital in the study of CsPbI₃ nanocrystals (NCs), where RP-like phases were induced by dopants. The presence of these RP-like phases was correlated with phase stability measurements, offering valuable insights into the relationship between defect formation and the long-term structural stability of the material.

Finally, the influence of nanoparticle shape on the optical properties cannot be overlooked. However, 3D characterization methods such as electron tomography are electron-dose expensive. We have therefore quantified the number of atoms from a single projection and consequently modeled the shapes of perovskite NCs. Such advancements are crucial for understanding and optimizing the performance of perovskite-based devices, where both structural and morphological parameters influence their function.