  We report the application of femtosecond four-wave mixing (FWM) to the study
of carrier transport in solution-processed CH3NH3PbI3. The diffusion
coefficient was extracted through direct detection of the lateral diffusion of
carriers utilizing the transient grating technique, coupled with simultaneous
measurement of decay kinetics exploiting the versatility of the boxcar
excitation beam geometry. The observation of exponential decay of the transient
grating versus interpulse delay indicates diffusive transport with negligible
trapping within the first nanosecond following excitation. The in-plane
transport geometry in our experiments enabled the diffusion length to be
compared directly with the grain size, indicating that carriers move across
multiple grain boundaries prior to recombination. Our experiments illustrate
the broad utility of FWM spectroscopy for rapid characterization of macroscopic
film transport properties.
