  Mixed-halide based hybrid perovskite semiconductors have attracted tremendous
attention as a promising candidate for high efficient photovoltaic and
light-emitting devices. However, these advanced perovskite materials may
undergo phase segregation under light illumination due to halide ion migration,
affecting their optoelectronic properties. Here, we report photo-excitation
induced phase segregation in triple-cation mixed-halide perovskite films that
yields to red-shift in photoluminescence response. We demonstrate that
photo-excitation induced halide ion migration leads to the formation of
smaller-bandgap iodide-rich and larger-bandgap bromide-rich domains in the
perovskite film, where the phase segregation rate is found to follow the
excitation power-density as a power law. Results confirm that charge carrier
lifetime increases with redshift in photoluminescence due to the trapping of
photo-excited carriers in the segregated smaller-bandgap iodide-rich domains.
Interestingly, we found that these photo-induced changes are fully reversible
and thermally activated when the excitation power is turned off. A significant
difference in activation energies for halide ion migration is observed during
phase segregation and recovery process under darkness. Additionally, we have
investigated the emission linewidth broadening as a function of temperature
which is governed by the exciton-optical phonon coupling. The mechanism of
photo-induced phase segregation is interpreted based on excitonphonon coupling
strength in both mixed and demixed (segregated) states of perovskite film.
