  The outstanding photovoltaic performances of lead halide perovskite (LHP)
thin film solar cells are due in particular to the large diffusion length of
photocarriers. The mechanism behind this property and its dependence on the
various anions and cations in the LHP material composition are still an active
topic of debate. Here, we apply ultrabroadband terahertz spectroscopy with a
time resolution < 200 fs to probe the early carrier mobility dynamics of
photoexcited perovskite samples of different chemical compositions. An increase
in the carrier mobility with time constants in the range of 0.3 to 0.7 ps is
observed for different LHPs, in which the signal amplitude is larger at shorter
excitation wavelengths. This feature is assigned to the relaxation of hot
carriers from low-curvature regions of the band structure to the bottom of the
valley, where their effective mass accounts for the maximum mobility. The
dependence of the mobility dynamics on the initial photogenerated carrier
population shows that carrier cooling competes with a dynamic screening effect
associated with polaron formation and relaxation. A kinetic model that
considers all competing processes is proposed. Simulations based on this model
allow us to fit the experimental data well and evidence a composition
dependence of the dynamic screening lifetime.
