The self-consistent Fermi level under O-poor conditions was predicted using the py-scfermi package. An annealing/growth temperature of 600 • C was assumed, matching that of Kim et al.'s work on Sb 2 O 5 nanowires. To understand the defect behavior at operating (room) temperature, the 'frozen defect approach' was applied where the total concentration of each defect is fixed to the equilibrium concentration at the annealing temperature. Upon cooling, the total concentration of each defect remains unchanged, whereas the relative populations of each defect charge state are allowed to re-equilibrate. The self-consistent Fermi level is then re-calculated using the operating temperature (300 K). Figure shows the self-consistent Fermi level in undoped Sb 2 O 5 sits at around 0.53 eV lower than the CBM at room temperature, corresponding to an essentially negligible electron concentration around 10 8 cm -3 , and showing undoped Sb 2 O 5 to be highly insulating as expected.