Halide Heterogeneity Affects Local Charge Carrier Dynamics in Mixed-Ion Lead Perovskite Thin Films

The mechanism and elemental composition that form the basis for the improved optical and electronic properties in mixed-ion lead halide perovskite solar cells are still not well understood compared to standard methylammonium lead triiodide perovskite devices. Here, we use synchrotron-based X-ray fluorescence to map the composition of perovskite thin films. To get insight into the elemental distribution during film growth, we fabricate films with three different thicknesses. To create a link between the composition and electronic properties, we perform Kelvin probe force microscopy and time-resolved photoluminescence spectroscopy. We find that the elemental composition is highly dependent on the film thickness, in particular, the I/Pb ratio is altered for single grains based on the film thickness. The difference in the I/Pb ratio reveals to be the root cause for the underlying difference in the film lifetime and defect density influencing charge carrier dynamics. Our results provide an in-depth analysis approach combining micro- and nanoscale techniques to shed light onto the fundamental processes, which help to further engineer perovskite thin films and improve device efficiencies.