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The low electronic conductivity of hematite (α-Fe2O3) limits its best performance in many applications. Though highly reducing conditions induce an intrinsic n-type behavior, reaching extremely low oxygen partial pressure [Formula: see text] values is not practical. Alternatively, certain dopants provide hematite with excess electrons at practical [Formula: see text] values. This study employs density functional theory with thermodynamic analysis to compute the concentration of electronic defects in hematite as a function of [Formula: see text], upon doping with 1% of 3d, 4d, and 5d transition metals. Isothermal Kröger–Vink diagrams at 1100 K are plotted to reveal the charge compensation mechanism controlling the electronic carriers in doped hematite and the maximum attainable [Formula: see text] value, which achieves approximately one electron per dopant. A higher [Formula: see text] value is a metric for an effective donor. Ti, Zr, Hf, Nb, Ta, Mo, and W are shown to be effective donors, especially Nb, Ta, and W, which achieve a 1:1 electron/dopant ratio around atmospheric pressure and a maximum electron/dopant ratio greater than one. The latter is a new metric introduced in this study to quantify the doping efficacy of a donor. Moreover, our study shows that W, Ta, and Nb co-doping in specific percentages with any of the other investigated dopants ensures the n-type behavior of the co-doped hematite while opening the possibility of improving other properties via the other dopant. The other dopant can be Ni or Co to enhance the surface catalytic properties or Zn to increase the minority hole carriers. Both properties are desirable in applications such as photoelectrochemical cells.