Iodine (I) expulsion at photoirradiated mixed halide perovskite interface. Should I stay or should I go?

Visible light irradiation of a mixed halide perovskite film in contact with a solvent (dichloromethane, DCM) in which the film otherwise is stable leads to selective expulsion of iodide (I) from the film with a concurrent shift in the band edge to lower wavelengths. We have now employed mixed halide perovskites to uncover the influence of A-site cation [methylammonium (MA) and cesium (Cs)] on the mobility of iodide ions under photoirradiation. In the absence of solvent contact, the mixed halide perovskite films undergo photoinduced segregation with a rate constant that decreases with increasing Cs content. Interestingly, the iodide expulsion rate in DCM is strongly dependent on the rate of photoinduced segregation. At Cs atomic concentrations greater than 50%, the films become stable as the iodide expulsion is largely suppressed. The role of the A-site cation in dictating the mobility of halide ions is discussed.