Mixed-halide hybrid perovskite semiconductors have attracted tremendous attention as a promising candidate for efficient photovoltaic and lightemitting devices. However, these perovskite materials may undergo phase segregation under light illumination, thus affecting their optoelectronic properties. Here, photoexcitation induced phase segregation in triplecation mixed-halide perovskite films that yields to red-shift in the photoluminescence response is reported. It is demonstrated that photo excitation induced halide 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 due to the trapping of photoexcited carriers in the segregated smaller bandgap iodide-rich domains. Interestingly, these photoinduced 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. Additionally, the emission linewidth broadening is investigated as a function of temperature which is governed by the exciton-optical phonon coupling. The mechanism of photoinduced phase segregation is interpreted based on exciton-phonon coupling strength in both mixed and demixed (segregated) states of perovskite films.

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