Small Urbach energies in halide perovskites due to anharmonicity and short-range correlated disorder

Halide perovskites (HaPs), a class of up-and-coming solar absorber materials, feature small Urbach energies and sharp optical absorption edges. These characteristics render the fabrication of efficient HaP based solar-cell devices possible. But they also indicate a low amount of disorder, which is seemingly in contrast with the complex nuclear dynamics and structural effects known for HaPs. Here, we present our findings on spatial correlations in the disorder potential induced for electronic states due to nuclear dynamics in several HaPs. Our theoretical results are based on density functional theory (DFT) calculations and DFT-based molecular dynamics [1]. With this approach, anharmonicity in the nuclear dynamics is accounted for up to all orders in the Taylor expansion of the crystal potential. We find that correlations in the disorder potential are dynamically shortened. The mechanism responsible for this shortening is found to be the nuclear motion of A-site and X-site ions. We conclude that sharp optical absorption edges and small Urbach energies in HaPs are possible due to the dynamical shortening of correlations in the disorder potential.

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