  Bandgap tunability of lead mixed-halide perovskites makes them promising
candidates for various applications in optoelectronics since they exhibit sharp
optical absorption onsets despite the presence of disorder from halide
alloying. Here we use localization landscape theory to reveal that the static
disorder due to compositional alloying for iodide:bromide perovskite
contributes at most 3 meV to the Urbach energy. Our modelling reveals that the
reason for this small contribution is due to the small effective masses in
perovskites, resulting in a natural length scale of around 20nm for the
"effective confining potential" for electrons and holes, with short range
potential fluctuations smoothed out. The increase in Urbach energy across the
compositional range agrees well with our optical absorption measurements. We
model systems of sizes up to 80 nm in three dimensions, allowing us to explore
halide segregation, accurately reproducing the experimentally observed
absorption spectra and demonstrating the scope of our method to model
electronic structures on large length scales. Our results suggest that we
should look beyond static contribution and focus on the dynamic temperature
dependent contribution to the Urbach energy.
