Photoexcited Carrier Dynamics in Mixed Halide Perovskites: A Morphological Perspective

Abstract

In this work, we probe the mechanisms of excitation and subsequent recombination of electron-hole pairs in the mixed bandgap perovskite crystal CH₃NH₃Pb(I_1-xBr_x)₃, x=0-0.33, using ultrafast spectroscopies. The perovskite grain size can be tuned to reflect the size of intrinsic iodide-rich nuclei, which depend strongly on coordination of PbI₂ with the deposition solvent prior to spin-casting. These iodide-rich nuclei, visualized for the first time in SEM and STEM-EDS, serve as low-bandgap recombination centers within the mixed crystal. Picosecond time-resolved photoluminescence (tr-PL) reveals that higher bromide compositions and smaller intrinsic nuclei maximize carrier lifetime. Introduction of bromide also affects absorbance: as bromide composition increases, the bulk bandgap increases, shifting the absorbance band edge into the visible range. While femtosecond transient absorption spectroscopy (TAS) reveals that lifetimes of carrier trapping and charge injection are independent of bromide content, the lifetime of electron thermalization shortens with added bromide, indicating that bromide introduction improves phonon transport as well as carrier transport. With an understanding of intrinsic compositional variations in mixed halide perovskites, the unique carrier transport properties of these material may be realized in future solar cells and light-emitting diodes.