While organometal halide perovskites are promising for a variety of optoelectronic applications, the morphological and compositional defects introduced by solution processing techniques have hindered efforts at understanding their fundamental properties. To provide a detailed picture of the intrinsic carrier transport properties of methylammonium lead iodide without contributions from defects such as grain boundaries, we utilized pump-probe microscopy to measure diffusion in individual crystalline domains of a thin film. Direct imaging of carrier transport in 25 individual domains yields diffusivities between 0.74 and 1.77 cm s, demonstrating single-crystal-like, long-range transport characteristics in a thin film architecture. We also examine the effects of excitation density on carrier diffusivity, finding that transport is nearly independent of photoexcited carrier density between 6 × 10 cm and 4 × 10 cm. Transport modeling of the observed density independence suggests that strong carrier-phonon scattering coupled with a large static relative permittivity is responsible for the unusual transport characteristics of methylammonium perovskite.

While significant research efforts directed toward characterizing the excited-state dynamics of lead halide perovskites have enabled promising advances in photovoltaic, light-emitting diode, and laser technologies, a detailed correlation between composition and functionality in this promising class of materials remains unestablished. We use pump−probe microscopy to characterize both transport and relaxation dynamics in individual crystals of CsPbI 2 Br, a mixed halide, all-inorganic analogue to the well-studied organic−inorganic hybrid perovskites. In contrast to the methylammonium lead tri-iodide perovskite, we find excited-state dynamics that decay primarily via first-order and Auger mechanisms. By global fitting of power-dependent kinetics collected from individual domains, we find a range of Auger rate constants between 3.3 × 10 −30 and 1.5 × 10 −28 cm 6 /s, with negligible contributions from secondorder (bimolecular) processes. Direct imaging of the excited-state spatial evolution reveals an average diffusion constant of 0.27 cm 2 /s, a value that is nearly an order of magnitude smaller than that of single-crystal, organic−inorganic analogues.

Excitation-fluence-dependent charge carrier transport and decay dynamics in individual lead bromide perovskite microcrystals were studied using time-resolved pump−probe microscopy. Measurements show that, in contrast to MAPbI 3 , diffusion in both CsPbBr 3 and MAPbBr 3 decreases for carrier densities that exceed ∼10 18 cm −3 . Fits to the carrier−carrier scattering rate in CsPbBr 3 and MAPbBr 3 indicate that the Coulomb interaction between mobile charges is mediated by an effective dielectric of ε = 11.53 ± 2.23 for CsPbBr 3 and ε = 13.01 ± 1.76 for MAPbBr 3 , values which are a factor of 2−10 lower than reported values of the static dielectric. Excited state decay kinetics, again collected on individual microcrystalline domains, are consistent with a lower effective dielectric for bromide perovskites relative to iodide perovskites. Nonlinear carrier recombination (2nd order and Auger processes) in CsPbBr 3 and MAPbBr 3 scale more strongly with excitation density than in MAPbI 3 . Together, these results indicate that low-frequency contributions to the static dielectric, such as low-frequency phonon modes and ionic mobility, have reduced effectiveness screening the rapidly moving mobile charges at room temperature.

While carrier mobility is a practical and commonly cited measure of transport, it conflates the effects of two more fundamental material properties: the effective mass and mean scattering time of charge carriers. This Letter describes the correlation of two ultrafast imaging techniques to disentangle the effect of each on carrier transport in lead halide perovskites. Two materials are compared: methylammonium lead tri-iodide (MAPbI) and cesium lead bromide diiodide (CsPbBrI). By correlating photoinduced changes to the refractive index with a direct measure of carrier diffusion, both the carrier optical mass and mean scattering time are uniquely determined on microscopic length scales. These results show that the factor of 4 lower mobility of CsPbBrI is due not to differing optical masses of charge carriers, which are measured to be similar in CsPbBrI and MAPbI, but rather to a difference in mean carrier scattering time. The scope and limitations of the approach are discussed.

With sub-micron spatial resolution and femtosecond temporal resolution, pump probe microscopy provides a powerful spectroscopic probe of complex electronic environments in bulk and nanoscale materials. However, the electronic structure of many materials systems are governed by compositional and morphological heterogeneities on length scales that lie below the diffraction limit. We have recently demonstrated Structured Pump Probe Microscopy (SPPM), which employs a patterned pump excitation field to provide spectroscopic interrogation of sub-diffraction limited sample volumes. Herein, we develop the imaging theory of SPPM in two dimensions to accompany the previously published experimental methodology. We show that regardless of pump and probe wavelengths, a nearly two-fold reduction in spectroscopic probe volume can be achieved. We also examine the limitations of the approach, with a detailed discussion of ringing in the point spread function that can reduce imaging performance.