Reversible phase transformation of colloidal 2D lead halide perovskite platelets under photoirradiation

The photostability of lead halide perovskites is an important criterion for their utilization in light harvesting applications. Colloidal two-dimensional mixed halide perovskites offer a convenient approach to probe photoinduced transformations using spectroscopy tools. We employed suspensions of colloidal 2D methylammonium lead halide perovskite platelets with butylammonium and octylammonium as organic spacer cations. Upon irradiation of the mixed halide perovskite (BAOAMAPb2(Br3.5I3.5)) colloidal platelet suspension, we observe a phase transformation, as seen from the formation of bromide rich domains (BAOAMAPb2Br7) and an iodide complex (PbI3–). Unlike in the case of 3D mixed halide films, we did not observe the formation of iodide rich domains as part of the photoinduced phase segregation process. Instead, we observed the dissociation of the BAOAMAPb2I7 to form an iodide complex, PbI3–. Interestingly, this photoinduced phase transformation was reversible, as the products which were confined in the colloidal core restored the original mixed halide phase upon storage in dark. The spectroscopic measurements that characterize excited states of different colloidal methylammonium lead halide perovskites and kinetics of photoinduced transformation and their dark recovery are presented. These new phototransformations of colloidal perovskite platelets provide new insights into the processes that dictate their overall photostability.