Enhanced Carrier Mobility and Diffusion Length in Formamidinium-Rich Hybrid Perovskites: Effects of Grain-Size and...

Carrier mobility, recombination rates and diffusion length directly govern the efficiency of hybrid lead-halide perovskites. Yet, their behavior across different carrier concentrations and the effects of microstructure remain poorly understood. Using time-resolved photoluminescence and optical pump-THz probe spectroscopy, we quantify mobility, carrier recombination rates and diffusion length for polycrystalline films of methylammonium (MA)- and formamidinium (FA)-rich lead-halide perovskites, across carrier concentrations ranging from ∼10¹⁵ to ∼10¹⁹ cm^-3. For example, at a carrier concentration of ∼10¹⁸ cm^-3, FA_0.95MA_0.05Pb(I_0.95Br_0.05)₃ exhibits a mobility of 127 ± 9 cm² V^-1 s^-1 and a diffusion length of 392 ± 85 nm, compared to 69 ± 1 cm² V^-1 s^-1 and 139 ± 1 nm for MAPbI₃. These differences in mobility and diffusion length persist across different fluences, and are captured by a fluence-dependent rate model that accounts for both carrier generation and recombination at different material depths. From scanning electron microscopy and THz time-domain spectroscopy measurements, we attribute the increased mobility and diffusion length for the FA-rich perovskite mainly to a larger average grain size, after considering possible Fröhlich-type interactions between carriers and THz-active phonon modes. Our work establishes a mechanistic link between material microstructure and ultrafast carrier dynamics, informing crucial design principles for perovskite-based photovoltaic and optoelectronic applications.