Unveiling Nanoscale Heterogeneity of Strain in MAPbI ₃ Perovskite Films Using Bragg Coherent X‐Ray Diffraction Imaging

Abstract Cation alloying and substrate morphology control have proved to be effective in controlling strains in perovskite films by macroscale characterizations. However, the nanoscale characterizations of strains are still limited, which hinder the comprehensive understanding of the strain regulation. Here, the strain regulation of MAPbI ₃ (MA = CH ₃ NH ₂ ) is done by Cs (Cesium) alloying and introduction of a nano‐structured substrate to the perovskite films. Laboratory X‐ray diffraction analysis shows that Cs alloying introduces compressive strain, whereas providing a nano‐structured substrate introduces tensile strain. Bragg coherent X‐ray diffraction imaging further demonstrates that nanoscale homogeneity of the strain in pure MAPbI ₃ would be destroyed through 3 at% Cs alloying, as the strain varies from compressive to tensile. Both compressive and tensile domains exist in the perovskite crystals at the same time. The application of a nano‐structured substrate is found to cause the nanoscale heterogeneity of strains in the MAPbI ₃ films. The strain homogeneity caused by combining both 3% Cs alloying and providing a nano‐structured substrate is found to enhance the structural stability of perovskite films. The results provide 3D nanoscale monitoring of strains for the purpose of strain regulation, which contributes to further understanding of the strains in perovskite materials.