Colloidal mixed halide perovskite nanocrystals (NCs) irreversibly degrade when exposed to ultraviolet−visible irradiation. Here, mixed halide perovskite NC photolysis is tracked via mass spectrometry, electron microscopy, and photoluminescence. The data shows continuous wave ultraviolet−visible irradiation causes the heavier halides within the alloy to sublimate. This ultimately transforms CsPb(I 1−x Br x ) 3 and CsPb(Cl 1−x Br x ) 3 (x ≈ 0.50) NCs into CsPbBr 3 and CsPbCl 3 NCs, respectively. Time-resolved mass spectrometry demonstrates real-time desorption of volatile halide species (e.g., I 2(g) /HI (g) ) during irradiation. Energy-dispersive X-ray spectroscopy confirms near complete expulsion of I − from CsPb(I 1−x Br x ) 3 and Br − from CsPb(Cl 1−x Br x ) 3 NCs. Electron diffraction and cathodoluminescence establish lattice contractions and emission blueshifts consistent with formation of single halide perovskites from parent mixed halide alloys. Finally, increasing photolysis rates at higher temperatures follow an Arrhenius relationship with an effective activation energy of ∼62 kJ mol −1 for CsPb(I 1−x Br x ) 3 NCs (x ≈ 0.50). Altogether, this work provides important insight into the photolysis of colloidal perovskite NC alloys.

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