Low Urbach energies in anharmonic halide perovskites: A first-principles study on disorder correlations

Structural dynamics in halide perovskites (HaPs) include several interesting phenomena that can be expected to contribute significant amounts of disorder to the material at room temperature. This anticipated disorder is seemingly in contrast with the known small Urbach energies of HaPs allowing for fabrication of efficient solar-cell devices. Using density functional theory (DFT) calculations and DFT-based molecular dynamics, we study structural dynamics and its implications on electronic states in HaPs[1]. Interrelations of anharmonicity in the structural dynamics and their spatial correlations with disorder potentials for the electronic states are investigated and discussed. We observe that particularly A-site and X-site ionic motion results in dynamic confinements of disorder potentials to interatomic distances in various HaPs. These short correlation lengths of the disorder potential lead to small Urbach energies, which are a key parameter for efficient collection of solar light with HaP absorber materials.

[1] C. Gehrmann & D. A. Egger, Nat. Commun. 10, 3141 (2019).