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Abstract
We studied steady state and transient photocurrents in thin film and single-crystal
devices of MAPbBr3, a prototype organic-inorganic hybrid perovskite. We found that
the devices' capacitance is abnormally large, which originates from accumulation of
large densities of Pb2+ and Br- in the active perovskite layer. Under applied bias,
these ions are driven toward the opposite electrodes leading to space-charge fields
close to the metal/perovskite interfaces. The ion accumulation, in turn, causes photocurrent
reversal polarity that depends on the history of the applied bias and excitation photon
energy with respect to the optical gap. Furthermore, the large capacitive response
dominates the transient photocurrent and, therefore, obscures the weaker contribution
from the photocarriers' drift. We show that these properties depend on the ambient
conditions in which the measurements are performed. Understanding these phenomena
may lead to better control over the stability of perovskite photodetectors for visible
light.