The calculated electronic band structure and DoS of Bi 2 TeO 2 obtained using the HSE06 hybrid functional is shown in Fig. , and corresponding plots for Bi 2 SO 2 and Bi 2 SeO 2 are provided in Figs S8/S9 in Section S2.1 of the Supporting Information. The electronic-structure calculations predict all three oxychalgoenides to be indirectgap semiconductors with bandgaps E g of 1.46, 1.1 and 0.33 eV for Bi 2 SO 2 , Bi 2 SeO 2 and Bi 2 TeO 2 respectively. In Bi 2 SO 2 and Bi 2 SeO 2 the valence-band maximum and conduction-band minimum (VBM/CBM) lie at the k = N and Γ wavevectors, respectively, with a secondary VBM along the M → S path. Increasing the chalcogen mass results in a "convergence" of a secondary CBM at k = X, such that the VBM and CBM of Bi 2 TeO 2 are located along k = M → S and at k = X. These predicted band gaps agree well with the reported E g = 1.5 eV for Bi 2 SO 2 and 0.23 eV for Bi 2 TeO 2 . Our predicted bandgap for Bi 2 SeO 2 is however significantly lower than the E g = 1.77 eV reported in Ref. . We suspect this may be an issue with the data fitting in the optical measurements, since the value drops significantly to ≈ 0.9 eV with a very small 2 at.% Te doping, which is much closer to our predicted value. In keeping with this, our calculated value is also in good agreement with the E g = 1.3 eV obtained in a previous theoretical study. Having confirmed the accuracy of our calculated bandgaps, we next proceeded to calculate the electrical transport properties using semi-classical Boltzmann transport theory and approximate models for the electron relaxation times. The transport tensors S, σ and κ el are determined for a series of specified extrinsic carrier concentrations ("doping levels") n and temperatures. The intrinsic charge-carrier concentration of Bi 2 SeO 2 is reported to be n ≈ 10 15 cm -3 at room temperature, whereas Te-doped Bi 2 SeO 2 and pristine Bi 2 TeO 2 attain values on the order of 10 18 cm -3 in experiments, and even higher n ≈ 10 19 and 10 20 cm -3 have been reported for Bi 1.9 Ta 0.1 SeO 2 and Bi 2 Se 1 -x O 2 Cl x respectively. On the other hand, n as high as 10 21 cm -3 have been reported for p-type LaCuOSe and BiCuOSe in experiments. We therefore consider a range of n = 10 16 -10 21 cm -3 , given that studies with the lower carrier concentrations report poor electrical properties. While most experiments focus on n-type Bi 2 ChO 2 , since other oxychalcogenides are often p-type doped we consider both types of doping here. We consider a temperature range of 300-900 K, up to the reported decomposition temperature of Bi 2 TeO 2 . The diagonal xx = yy and zz components correspond to the transport along the independent a = b and c directions in the tetragonal crystal structure. Since most experiments are performed on films or pressed pellets composed of randomly-oriented crystal grains, as opposed to single crystals, we focus mainly on the scalar averages (2xx + zz)/3 = (xx + yy + zz)/3 and provide a brief discussion of the anisotropy. Fig. shows the averaged electrical properties σ, S, S 2 σ (PF) and κ el as a function of carrier concentration at a fixed T = 800 K, typical of the higher temperatures probed in measurements, and as a function of temperature for a fixed n = 10 20 cm -3 corresponding to the largest n reported in experiments. At 800 K, the σ of n-doped Bi 2 SO 2 and Bi 2 TeO 2 are predicted to be comparable and very low for n < 10 19 cm -3 , setting a practical minimum doping level. The narrow bandgap of Bi 2 TeO 2 results in a small intrinsic n-type conductivity on the order of 100 S cm -1 below n ≈ 10 19 cm -3 , which is comparable to the measured σ = 75 S cm -1 at n = 1.06 × 10 18 cm -3 and T = 700 K. At larger n, the conductivity is predicted to increase sharply, as for the other oxychalcogenides, and Bi 2 TeO 2 is predicted to support the largest σ of the three systems.