Fig. 2.

A thermodynamic nucleation model explains the dependence of phase stability on the morphology. a The calculated ΔG_dark per volume (solid line) is negative regardless of the Br content. The dashed lines show the calculated volumetric enthalpy Δh_mix, volumetric entropy term TΔs_mix, and cohesive energy Σc_iW_ir². A relatively large grain size was used here (r = 35 nm). b Under illumination, the calculated free energy ΔG_light becomes partially positive assuming the same grain size. A threshold composition X₀~0.3 divides the mixed-halide perovskites into stable (I-rich) and unstable (Br-rich) regions. c To mimic the experimental conditions of the CsPb(Br_xI_1-x)₃/Cs₄Pb(Br_xI_1-x)₆ composites, a small grain size (r = 7.5 nm) was assumed and the cohesive energy was considered. The calculated ΔG_light turns negative, indicating a stable phase of homogenous mixed-halide perovskites