Role of Point Defects in Enhancing the Conductivity of BiVO4

Bismuth vanadate is a promising photoanode for solar-to-fuel photocatalytic applications, and it has been extensively studied in recent years. However, the microscopic mechanism underlying the observed changes in electronic conductivity due to oxygen vacancies and nitrogen dopants remains unclear. Here, we combine electronic structure calculations at the hybrid density functional theory (DFT) level with constrained DFT, and we elucidate the role of defects in enhancing the transport properties of the material. We show that at low temperature, oxygen vacancies give rise to deep levels within the fundamental gap of BVO; however even as deep levels, oxygen vacancies can act as effective n-dopants and polaronic charge carriers, due to their favorable position in energy relative to polarons in the pristine bulk. In addition, we show that N atoms can be easily introduced in n-doped BiVO4 and that the presence of substitutional nitrogen affects the formation energy of polarons, effectively contributing to an increase of the carrier mobility in the material. Our results reconcile apparently conflicting experiments and they may be generalized to other transition metal oxides, thus providing a foundation for polaronic defect engineering in photoanodes for water photocatalysis.