Electron localization and mobility in layered Cs3Bi2Br9 perovskite: consequences on photocatalytic processes

We study the electronic properties of lead-free layered Cs3Bi2Br9 (CBB) perovskite, which has recently emerged as a promising material for photocatalysis. Our investigation, prompted by optical measurements suggesting self-trapping of excess charges and corroborated by ab initio electronic-structure calculations and molecular dynamics simulations, reveals that photogenerated electrons are assimilated in the material as small polarons, a consequence of sizable structural reorganization of both the inorganic sublattice and of A-site cations. The electron polaron exhibits an energy level at 0.6eV below the conduction band edge, which is consistent with the physical picture ensuing from spectroscopy, and is suitably aligned with respect to redox potentials associated with common photoreduction processes. Estimation of the electron mobility, in the framework of polaron hopping, indicates a remarkable anisotropy, with interlayer movement of electron polarons being at least two orders of magnitude slower than intralayer diffusion. This suggests that heterojunctions, separating holes and electrons, are the most viable architecture to exploit CBB for photocatalysis, and that morphology as well as loading and size of CBB nanostructures are key in determining whether photogenerated electrons might reach the relevant interface or be lost due to recombination.