Visualizing the impact of chloride addition on the microscopic carrier dynamics of MAPbI3 thin films using femtosecond transient absorption microscopy

The spatial heterogeneity of carrier dynamics in mixed halide perovskite CH₃NH₃PbI_3-xCl_x thin films with a range of different chloride additions is mapped using femtosecond transient absorption microscopy (TAM). The comparison of TAM images of fibrous and granular polycrystalline CH₃NH₃PbI_3-xCl_x films indicates that the impact of chloride addition on the local heterogeneity of carrier dynamics is highly dependent on the film preparation method and the resulting morphology. In addition to signals of pristine CH₃NH₃PbI₃, CH₃NH₃PbI_3-xCl_x films with a fibrous structure show long-lived excited state absorption (ESA) signals in localized, microscopic regions. The ESA signal exhibits transient absorption with a rise time of about 5 ps after the excitation pulse, indicating that these distinct micrograins have preferential carrier trapping properties. The chemical composition of these micrograins does not differ detectably from their surroundings. In contrast, in CH₃NH₃PbI_3-xCl_x films with a granular structure, Cl addition does not seem to affect the charge carrier dynamics. These results provide insight into the localized effects of halide mixing and on the resulting photophysical properties of mixed halide perovskite materials on the micrometer length scale.