On the Origin of Energetic Disorder in Mixed Halides Lead Perovskites

Mixed halide perovskites are emerging as promising candidates for wide-bandgap components in tandem solar cells and color-tunable light-emitting diodes. Yet, halide mixing poses a fundamental question of whether the inhomogeneous halide distribution impacts the intrinsic electronic disorder in these materials. To address this point, density functional theory (DFT)-based molecular dynamics (MD) simulations are performed for pure and mixed halide perovskites, accounting for disorder stemming from inhomogeneous chemical composition associated with the halide component and from finite temperature effects. For pure halide perovskites, finite-temperature band gap fluctuations from the MD simulations are in good agreement with the broadening measured using photoluminescence. Furthermore, these calculations confirm the natively modest inhomogeneous disorder in the electronic structure of these materials. Most notably, such a low degree of electronic disorder is preserved in models mimicking finely intermixed Br/I solid-state solutions. In contrast, models featuring halide segregation show comparably wider band gap fluctuations, with a sizable contribution from inhomogeneous (static) broadening, which is associated, at least in part, with structural distortions stemming from lattice mismatch. Emission linewidth and absorption onset sharpness (Urbach tail) entail information on the electronic disorder in semiconductors. This is discussed for mixed halide perovskites, with halide mixing representing a successful strategy for color-tunability but also an intrinsic source of disorder. Density functional theory-based molecular dynamics simulations unveil the impact of halide inhomogeneity for models mimicking both finely dispersed iodine/bromine solutions and segregated phases.