Humidity-Induced Degradation Processes of Halide Perovskites Unveiled by Correlative Analytical Electron Microscopy.

Improving the stability of lead halide perovskite solar cells (PSCs) for industrialization is currently a major challenge. It is shown that moisture induces changes in global PSC performance, altering the nature of the absorber through phase transition or segregation. Understanding how the material evolves in a wet environment is crucial for optimizing device performance and stability. Here, the chemical and structural evolution of state-of-the-art hybrid perovskite thin-film Cs0.05 (MA0.15 FA0.85 )0.95 Pb(I0.84 Br0.16 )3 (CsMAFA) is investigated after aging under controlled humidity with analytical characterization techniques. The analysis is performed at different scales through Photoluminescence, X-ray Diffraction Spectroscopy, Cathodoluminescence, Selected Area Electron Diffraction, and Energy Dispersive X-ray Spectroscopy. From the analysis of the degradation products from the perovskite layer and by the correlation of their optical and chemical properties at a microscopic level, different phases such as lead-iodide (PbI2 ), inorganic mixed halide CsPb(I0.9 Br0.1 )3 and lead-rich CsPb2 (I0.74 Br0.26 )5 perovskite are evidenced. These phases demonstrate a high degree of crystallinity that induces unique geometrical shapes and drastically affects the optoelectronic properties of the thin film. By identifying the precise nature of these specific species, the multi-scale approach provides insights into the degradation mechanisms of hybrid perovskite materials, which can be used to improve PSC stability.

On the use of SEM correlative tools for in situ mechanical tests.

In situ SEM mechanical tests are key to study crystal plasticity. In particular, imaging and diffraction (EBSD) allow microstructure and surface kinematics to be monitored all along the test. However, to get a full benefit from different modalities, it is necessary to register all images and crystallographic orientation maps from EBSD into the same frame. Different correlative approaches tracking either Pt surface markings, crystal orientations or grain boundaries, allow such registrations to be performed and displacement as well as rotation fields to be measured, a primary information for crystal plasticity identification. However, the different contrasts that are captured in different modalities and unavoidable stage motions also give rise to artifacts that are to be corrected to register the different information onto the same material points. The same image correlation tools reveal very powerful to correct such artifacts. Illustrated by an in situ uniaxial tensile test performed on a bainitic-ferritic steel sample, recent advances in image correlation techniques are reviewed and shown to provide a comprehensive picture of local strain and rotation maps.

Measuring topographies from conventional SEM acquisitions.

The present study extends the stereoscopic imaging principle for estimating the surface topography to two orientations, namely, normal to the electron beam axis and inclined at 70° as suited for EBSD analyses. In spite of the large angle difference, it is shown that the topography can be accurately determined using regularized global Digital Image Correlation. The surface topography is compared to another estimate issued from a 3D FIB-SEM procedure where the sample surface is first covered by a Pt layer, and its initial topography is progressively revealed from successive FIB-milling. These two methods are successfully compared on a 6% strained steel specimen in an in situ mechanical test. This analysis is supplemented by a third approach estimating the change of topography from crystal rotations as measured from successive EBSD images. This last technique ignores plastic deformation, and thus only holds in an elastic regime. For the studied example, despite the large plastic flow, it is shown that crystal rotation already accounts for a significant part of the deformation-induced topography.