The Effects of Doping on Small Polaron Formation and Transport in Hematite (Fe₂O₃)

Hematite is an earth abundant, low-cost semiconductor with a suitable band gap (~2.1 eV) and good visible light absorption, making it an ideal candidate for use as a photoanode in photoelectrochemical (PEC) devices. However, similar to other transition metal oxides, it suffers from a severely limited carrier mobility due to the formation of small polarons. Doping this system with Sn and oxygen vacancies, has shown to lead to dramatic improvements in the photoconductivity of these materials. Despite this achievement, the role of these dopants is still unknown from the atomistic level, making it difficult to design rational doped systems for further improvement in photoconduction. Through first-principles calculations we investigate small polaron formation and transport in pristine and doped Hematite. We found that certain defect complexes may be more stable in bulk Fe₂O₃ and have superior properties than the individual defects, with higher carrier concentrations, although they typically lead to increased polaron hopping activation energy, resulting in lower or comparable carrier mobility to pristine system. This work provides fundamental understanding of doping effects on improving carrier conductions in polaronic transition metal oxides.