The σ σ σ , S S S, and κ κ κ el are 3×3 tensors in which the diagonal components correspond to transport along the principal x, y and z directions. TEGs are typically based on thin films or pressed/sintered pellets composed of randomly-oriented crystal grains, and we therefore focus mainly on the scalar averages of the diagonal elements, which we denote S, σ and κ el to distinguish them from the tensor quantities. Fig. compares the averaged thermoelectric power factor (PF) S 2 σ and electronic thermal conductivity κ el of the four systems, for both p-and n-type doping. At a fixed T = 800 K, close to the high-temperature phase transitions of the Pnma phases, the PFs of Pnma GeS and GeSe phases is negligible up to n ≈ 10 18 cm -3 and then increases rapidly with the doping level (Fig. ). Comparison with the Seebeck coefficient and conductivity shows that this is due to an exponential-like increase in σ at large n, which compensates a monotonic fall in the S from the peak values around n = 10 17 cm -3 (Fig, ). Interestingly, the calculations predict that n-type doping leads to comparable or larger S but 2 -3× larger σ than p-type doping, resulting in a much larger PF.