Role of Intrinsic Ion Accumulation in the Photocurrent and Photocapacitive Responses of MAPbBr3 Photodetectors.

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.