All-inorganic Cs-based halide perovskites have attracted considerable attention as promising materials for durable perovskite solar cells because of their potentially improved thermal stability compared with organic–inorganic hybrid perovskites. The performance and reliability of perovskite solar cells are closely related to their microstructure, including the grain structure and defects of the perovskite layer, the morphology of individual functional layers, and the characteristics of their interfaces. Therefore, detailed cross-sectional characterization of the internal structure of each layer and the interfaces between layers is important for understanding the structural characteristics of these materials and devices1), 2).    
However, the accuracy of cross-sectional analysis strongly depends on the quality of specimen preparation. Mechanical cleaving is a simple and widely used method for cross-section preparation; however, it often produces uneven fracture surfaces, which can hinder detailed observation of individual layers and interfaces. Argon ion milling is an effective technique for producing smooth cross sections, but ion-beam-induced damage may affect the observed microstructure in perovskite materials. Therefore, a preparation method capable of achieving both a smooth cross section and minimal processing damage is required for reliable microstructural characterization.
In this study, cryogenic argon ion milling was applied to the preparation of cross sections of Cs-based perovskite solar cells, and its effectiveness was evaluated. The cross sections prepared by mechanical cleaving, Ar ion milling at 8 kV under room-temperature conditions, and Ar ion milling at 4 kV under liquid-nitrogen cooling were compared, as shown in Figure (a).
The mechanically cleaved cross section exhibited significant surface roughness. In contrast, room-temperature Ar ion milling partially improved cross-sectional smoothness; however, swelling of the perovskite layer was observed, indicating processing damage caused by ion irradiation. Under the cryogenic milling conditions employed in this study, deformation of the perovskite layer was reduced while maintaining a smooth cross-sectional surface, providing a low-damage cross section suitable for SEM observation.

In addition to cross-sectional preparation, this study demonstrates the use of a high-resolution Schottky SEM equipped with multiple detectors to acquire different signal types from the same field of view. A schematic cross-sectional illustration showing the arrangement of the SEM objective lens and the respective detectors is presented in Figure (b). By distinguishing complementary signals, different contrast characteristics associated with surface morphology, material composition, and other electron–sample interactions within a common observation area. Correlating these signals helps provide a more comprehensive understanding of the structural and material characteristics of the device than relying on a single image alone. The combination of cryogenic argon ion milling and multi-signal SEM observation thus provides a practical approach to preparing and characterizing cross sections of complex multilayer photovoltaic devices.
This work presents an integrated workflow combining cryogenic cross-sectional preparation with multimodal SEM characterization of Cs-based perovskite solar cells. By addressing the challenges associated with mechanically cleaved surfaces and enabling the complementary analysis of multiple signals from the same region, the approach supports detailed investigation of layer morphology, interfaces, and material-related contrast. It is expected to be useful for cross-sectional characterization of perovskite photovoltaic devices and other multilayer materials for which specimen preparation quality and the correlation of different imaging signals are important.