Unveiling Polaronic Effects on Carrier Transport in BiOBr, BiOI, and BiOBr–BiOI Heterostructures

Bismuth oxyhalides (BiOX; X = Br, I) are semiconductors with attractive electronic and optical properties. However, charge transport in these ionic compounds is predominantly dominated by polaron formation, which is still insufficiently understood. Herein, we employ density functional perturbation theory (DFPT) to investigate polaronic effects on carrier transport in BiOBr, BiOI, and their BiOBr–BiOI heterostructure. We compute a comprehensive set of transport-relevant properties, including band structures, effective masses, dielectric constants, phonon dispersions, electron–phonon coupling strengths, and both polaronic and total carrier mobilities. The results show that all systems exhibit anisotropically large polaron behavior arising from intermediate coupling between carriers and longitudinal optical phonon modes. Electron polarons show high mobility along the [100] and [010] directions, while hole polarons preferentially move along the layered [001] axis. In the BiOBr–BiOI heterostructure, both electron and hole mobilities are relatively reduced due to stronger electron–phonon coupling induced by interfacial dipole fields. By incorporating various carrier scattering mechanisms, including acoustic deformation potential, ionized impurities, and large polarons, we calculate carrier mobilities in close agreement with available experimental data of BiOI. Our study provides detailed insight into polaron-assisted charge transport in bismuth oxyhalides and their heterostructures, offering guidance for the design of efficient optoelectronic and energy devices.