Superlattices are greener on the other side: How light transforms self-assembled mixed halide perovskite nanocrystals

Perovskite nanocrystal superlattices (NC SLs) are the nearest real-world approximations to monodisperse NC ensembles. NC SLs thus represent ideal model systems for evaluating the optical and structural stability of CsPb(I₁−_xBr_x)₃ NCs at a macroscopic level. Here, photoinduced changes to CsPb(I₁−_xBr_x)₃ NC SLs (0 < x < 1.0) are probed via in situ photoluminescence, X-ray diffraction, and electron microscopy. We find that prolonged (∼10−20 h) ultraviolet−visible irradiation causes irreversible PL blueshifts, photobrightening, and crystal structure contractions. These changes stem from gradual photoinduced I₂ sublimation, which transforms CsPb(I₁−_xBr_x)₃ into CsPbBr₃. Despite eliminating half of the initial halides from individual CsPb(I_0.53Br_0.47)₃ particles, NCs within SLs remarkably preserve their initial crystallinity, cuboidal shapes, edge lengths, and size distributions. This work illustrates compositional control toward generating precisely engineered perovskite NC SLs. It also highlights iodide photo-oxidation as a hurdle that must be overcome if mixed halide perovskite nanomaterials are to be applied beyond fundamental studies.