  A critical issue hindering the potential applications of semiconductor
mixed-halide perovskites is the phase segregation effect, wherein localized
regions enriched with one type of halide anions would be formed upon continuous
photogeneration of the excited-state charge carriers. These unexpected phases
are capable of remixing again in the dark under the entropic driving force, the
process of which are now being exclusively studied after mixed-halide
perovskites have arrived at the final stage of complete phase segregation. Here
we show that after the removal of laser excitation from a solid film of
mixed-halide perovskite nanocrystals with partial phase segregation, the
iodide- and bromide-rich regions can continuously grow in the dark for a
prolonged time period of several minutes. We propose that this dark phase
segregation is sustained by the local electric fields associated with the
surface-trapped charge carriers, whose slow dissipation out of mixed-halide
perovskite nanocrystals causes a delayed occurrence of the reversal phase
remixing process.
