Play the heavy: An effective mass study for α-Fe₂O₃ and corundum oxides

Iron(iii) oxide (α-Fe₂O₃) is a known water splitting catalyst commonly used in photoelectrochemical cells. These cells are severely impaired by poor conductivity in α-Fe₂O₃, and resolving the conductivity issue is therefore crucial. One of the most intrinsic properties of matter, which governs conductivity, is the carrier effective masses. In this work, we investigate the carrier effective masses in α-Fe₂O₃ and other corundum oxides, including Al₂O₃, Cr₂O₃, Ga₂O₃, and In₂O₃ with different theoretical constructs: density functional theory (DFT), DFT+U, hybrid DFT, and G₀W₀. We find DFT sufficiently describes the carrier masses and a quasi-particle theory is only required for accuracies better than 30% for the conduction band effective mass. Additionally, we compare the density of states (DOS) and band effective mass approximations and conclude the DOS effective mass provides poor results whenever the band structure is anisotropic. We find that the charge carriers in Fe₂O₃ "play the heavy" since they have large effective masses that reduce conductivity and device efficiency. Finally, we conclude that the less heavy electron effective masses of other corundum oxides studied relative to Fe₂O₃ could contribute to efficiency improvements in Fe₂O₃ upon Al₂O₃, Ga₂O₃, and In₂O₃ coverage.