Prolonged Phase Segregation\nof Mixed-Halide Perovskite\nNanocrystals in the Dark

A critical issue hindering the potential applications\nof semiconductor\nmixed-halide perovskites is the phase segregation effect, wherein\nlocalized regions enriched with one type of halide anions would be\nformed upon continuous photogeneration of the excited-state charge\ncarriers. These unexpected phases are capable of remixing again in\nthe dark under the entropic driving force, the process of which is\nnow being exclusively studied after the mixed-halide perovskites have\narrived at the final stage of complete phase segregation. Here, we\nshow that after the removal of laser excitation from a solid film\nof the mixed-halide perovskite nanocrystals (NCs) with partial phase\nsegregation, the iodide- and bromide-rich regions can continuously\ngrow in the dark for a prolonged time period of several minutes. We\npropose that this dark phase segregation is sustained by the local\nelectric fields from the surface-trapped charge carriers, whose slow\ndissipation out of the mixed-halide perovskite NCs causes a delayed\noccurrence of the reversal phase remixing process.