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.

a b s t r a c tWe compare UV transient grating (TG) experiments of aqueous tryptophan with transient absorption (TA) and fluorescence up-conversion measurements. The TG and TA signals show a bi-exponential rise with sub-ps and ps time constants, which are consistent with the fluorescence studies. Using experimental data, we provide an equation for the homodyne-detected TG signal, taking into account the sub-100 fs internal conversion of tryptophan after excitation. In addition, we measure a sub-100 fs homogeneous electronic dephasing time for tryptophan in water by the photon echo (PE) technique. These measurements provide a consistent picture of excited state dynamics of aqueous tryptophan that may serve as a basis for coherent 2D-UV spectroscopy of biosystems.

No abstract