Photoluminescence Study of the Photoinduced Phase Separation in Mixed-Halide Hybrid Perovskite CHNHPb(BrI) Crystals Synthesized via a Solvothermal Method.

We systematically synthesized mixed-halide hybrid perovskite CHNHPb(BrI) (0 ≤ x ≤ 1) crystals in the full composition range by a solvothermal method. The as-synthesized crystals retained cuboid shapes, and the crystalline structure transitioned from the tetragonal phase to the cubic phase with an increasing Br-ion content. The photoluminescence (PL) of CHNHPb(BrI) crystals exhibited a continuous variation from red (768 nm) to green (549 nm) with increasing the volume ratio of HBr (V%), corresponding to a variation in the bandgap from 1.61 eV to 2.26 eV. Moreover, the bandgap of the crystals changed nonlinearly as a quadratic function of x with a bowing parameter of 0.53 eV. Notably, the CHNHPb(BrI) (0.4 ≤ x ≤ 0.6) crystals exhibited obvious phase separation by prolonged illumination. The cause for the phase separation was attributed to the formation of small clusters enriched in lower-band-gap, iodide-rich and higher-band-gap, bromide-rich domains, which induced localized strain to promote halide phase separation. We also clarified the relationship between the PL features and the band structures of the crystals.