Boosting Small Polaron Hopping Mobility of Bismuth Vanadate by Doping from First-Principles Calculations

Bismuth vanadate (BiVO₄) is one of the most promising photoanode materials in water-splitting photoelectrochemical cell. Its extremely low carrier mobility fundamentally limits its effeciency, while recent experimental work shows atomic doping can improve its carrier mobility, which shines light on the possibility of overcoming this limitation. However, its mechanism at the atomic level remains unclear. In this work, we explored the small polaron hopping mechanism in pristine and Mo doped BiVO₄. We compared several first-principles methods to obtain polaron hopping barriers, and investigated the mobility as a function of dopant-polaron distances and dopant concentrations. We found that atomic doping in the stable monoclinic phase leads to a phase transition to the tetragonal phase, which may reduce the barrier. Also, the electrostatic attraction between the polaron and the dopant combined with the energies raised from the lattice expansion on both sites may further reduce the barrier. The understanding of the mechanism provides us the guidance on the rational design of dopants for a better carrier conductivity in BiVO₄ and other similar polaronic metal oxides.