The σ obtained with p-type doping is predicted to be some 2 -8× smaller than with n-type doping. However, for Bi 2 SO 2 and Bi 2 SeO 2 the calculations predict a much larger S = 250 -600 µV K -1 with n = 10 19 -10 21 cm -3 and ∼ 5× larger PFs. The calculated S of p-doped Bi 2 SeO 2 changes sign around 7.5×10 16 cm -3 , indicating intrinsic n-type behaviour, as for Bi 2 TeO 2 , which requires a small extrinsic hole concentration to displace. The dependence of the |S| of p-doped Bi 2 SO 2 on the doping level suggests a similar phenomenon may occur for the sulphide at a lower n. For Bi 2 TeO 2 , a doping level of approx. 10 19 cm -3 is needed to offset the intrinsic electron carriers, but above this n an enhanced S compared to n-type doping results in a 1.5× larger peak S 2 σ ≈ 3 mW m -1 K -2 compared to the ∼ 2 mW m -1 K -2 obtained with n-type doping, albeit at a larger n ≈ 5 × 10 20 cm -3 . The predicted S of Bi 2 SO 2 and Bi 2 SeO 2 at the largest n = 10 21 cm -3 are around ∼ 50% higher than the peak S of Bi 2 TeO 2 , which offsets the lower σ and leads to a considerably larger PF.