Influence of bromide content on iodide migration in inverted MAPb(I_{1-: X}Br_x)₃ perovskite solar cells

The effect of a systematic increase in the bromide content on mixed anion methyl ammonium lead halide, MAPb(I_1-xBr_x)₃, perovskite solar cells is investigated. We show that at a critical bromide concentration (7.5%) we supress the slow impedance response from the cells. We link the changing impedance spectrum to a large increase in the activation energy for iodide motion. These results are corroborated by muon spin relaxation measurements, where we show that at the concentration of bromide typically used in high performance perovskite solar cells (17%) there is no sign of iodide motion in powders. Finally, we show JV curve hysteresis as a function of bromide content. The scan rate at which the maximum hysteresis index is observed does not change as the % Br is increased, leading us to conclude that the low frequency impedance response and the JV curve hysteresis are not caused by the same mobile ions.