Electron localization and mobility in layered Cs3Bi2Br9 perovskite: consequences of photocatalytic water splitting

The precise description of charge localization is paramount to defining the proper charge transport mechanism, the reactivity and long-term stability of novel materials to be employed in third-generation photovoltaics and photocatalysis. In this context, first-principles atomistic simulations represent a powerful tool for unravelling charge localization in semiconductors and the underlying correlation with the opto-electronic properties and device efficiencies.

We here present the results obtained by employing a plethora of computational techniques to study polaron formation, energetics and migration in the novel vacancy-ordered layered Cs₃Bi₂Br₉ (CBB) material, a novel water-stable metal halide perovskite that has been recently found to be particularly promising as a photocatode for the photocatalytic water splitting reaction.

Our calculations reveal that small electron polarons form in CBB upon photo-excitations, with electron localization induced by sizable distortions of both Bi-Br bonds and displacement of the A-site cations, with a wide spread of the polaron energy levels, due to the soft nature of the CBB lattice. By combining molecular dynamics simulations at the hybrid functional level with the thermodynamic integration technique, we are able to evaluate the polaron energy level in striking agreement with the experiment and we observe its favourable alignment with respect to water redox levels, which is consistent with measurements. However, our analysis of polaron hopping rates based on Marcus–Emin–Holstein–Austin–Mott theory reveals a strong anisotropy, with intra-layer charge mobility being up to four orders of magnitude lower than intra-layer mobility. The consequences of this finding of the use of CBB in heterojunctions for photocatalytic water splitting are discussed in view of the experimental results.