Variation of the zT factor of SnSe with doping: A first-principles study

The effect of doping with elements of the groups 13, 15 and 17 (namely Al, Ga, In, P, As, Sb, Cl, Br and I) on the transport coefficients and thermoelectrical performance of SnSe is studied from a first-principles approach based upon the density functional theory (DFT) and the resolution of the Boltzmann transport equation (BTE). The DFT band structures of doped SnSe are different to that of the pure system, the differences being more marked for dopants of groups 13 and 15. In particular, dopants of group 13 yield flatter bands along some directions and those of the 15 one yield metallic character. Some marked differences with respect to pure SnSe appear in the transport coefficients too, which ultimately give rise to a moderate enhancement of the figure of merit zT at temperatures below 500 K. These differences are discussed not to arise from the changes in the band structures, but in the charge carrier concentrations induced by doping. Thus, the zT factor is found to be much higher at relatively low carrier concentrations, contrarily to the common belief that heavy doping is required to enhance the thermoelectric performance of these systems.