Spatiotemporal Dynamics of Photo-excited Electrons and Holes in Organometal Trihalide Perovskites by Microwave Impedance Microscopy*

The outstanding performance of organometal trihalide solar cells benefits from the exceptional photo-physical properties of both electrons and holes in the material. Here, we directly probe the spatial diffusion and temporal dynamics of photo-generated free carriers in Cs-doped FAPbI₃ thin films by nanoscale microwave impedance microscopy. The sample exhibits two relaxation times on the order of 1 μs and 10 μs, which are uniform across grains and grain boundaries. Using charge transport layers to selectively quench one type of carriers, we are able to attribute the short and long lifetimes to that of electrons and holes, respectively. Both types of carriers display a long diffusion length of 3 ~ 5 μm, indicating that the difference in lifetimes is compensated by their distinct mobility. The spatiotemporal microwave imaging provides the most direct method to accurately determine photo-physical properties, which are crucial for the development of perovskite solar cells.