  Electric fields arising from the distribution of charge in metal halide
perovskite solar cells are critical for understanding the many weird and
wonderful optoelectronic properties displayed by these devices. Mobile ionic
defects are thought to accumulate at interfaces to screen electric fields
within the bulk of the perovskite semiconductor on application of external
bias, but tools are needed to directly probe the dynamics of the electric field
in this process. Here we show that electroabsorption measurements allow the
electric field within the active layer to be tracked as a function of frequency
or time. The magnitude of the electroabsorption signal, corresponding to the
strength of the electric field in the perovskite layer, falls off for
externally applied low frequency voltages or at long times following voltage
steps. Our observations are consistent with drift-diffusion simulations,
impedance spectroscopy, and transient photocurrent measurements. They indicate
charge screening/redistribution on time-scales ranging from 10 ms to 100 s
depending on the device interlayer material, perovskite composition, dominant
charged defect, and illumination conditions. The method can be performed on
typical solar cell structures and has potential to become a routine
characterization tool for optimizing hybrid perovskite devices.
