From the charge transport analysis earlier, Sb 2 O 5 was found to have good electron mobility, but to achieve high conductivity comparable to conventional TCOs, high carrier (and thus doping) concentrations are required. However, at high doping concentrations, the impact of extrinsic dopants on the host lattice and carrier scattering can severely diminish the carrier mobilities. Therefore, to optimise the electrical conductivity, the objective is to achieve electron-doping while minimising disruption of the CBM states (in order to retain decent carrier mobilities). As shown in Figure , Sb 5s states mainly comprise the CBM, followed by O 2p. Considering O is the minor contributor to the CBM, doping on the O site is preferred since it is likely to have a smaller impact on electron mobilities. In addition, the relatively small V O but large V Sb formation energies shown in the intrinsic transition level diagram (Figure ) also suggest that substituting on O will be easier, as removing O from the lattice requires less energy than removing Sb. Fluorine (F) was selected as a potential dopant due to its similar size to oxygen. In other TCOs such as SnO 2 , F has also often been incorporated as an O-site dopant to enhance performance. Defects with concentrations lower than 1 × 10 10 cm -3 are omitted from the legend, plotted using doped and py-sc-fermi. Figure shows the transition level diagram for the low energy extrinsic point defects in F-doped Sb 2 O 5 under Sb-rich/O-deficient conditions in which the limiting secondary phase is Sb 2 F 7 . F i is found to be a high energy defect with a formation energy of around 3.7 eV in its negative charge state at the CBM, precluding its formation in any significant concentration within the material and thus having a negligible impact on conductivity. By comparing F i with the intrinsic O i interstitial, it can be seen that F i has higher formation energy for most (n-type) Fermi levels. Usually, the smaller ionic radius of F would suggest F i formation to be more favorable, especially in ionic compounds. eV at the CBM), reflecting the trend in oxygen vacancy formation energies (Figure ).