zT factors in Ag- and Na-doped SnSe: Chemical potentials, relaxation times and predictions for other dopant species

The chemical potential μ and relaxation time τ of SnSe doped with Ag and Na have been investigated by a methodology that combines first-principles calculations with solutions of the Boltzmann transport equation under the relaxation time approximation. The best comparison with experimental data available is obtained for temperature-dependent μ and τ in both cases. The chemical potential relative to the Fermi energy is found to be independent on the particular doping species, and similar to that of pure SnSe. The relaxation time shows the same trend in all cases at temperatures below 550 K, which suggests that the same scattering mechanism is active in the pure and doped systems. As for the thermal conductivity, the analysis of the experimental results reveals that it is the same for both dopants below roughly 550 K as well, but higher than that of pure SnSe. From these findings, estimations of zT values for other dopants, namely Ge and S, are made. The results indicate that doping with S yields very poor figures of merit while doping with Ge could yield better thermoelectrical performance than that reported for Na-doped SnSe.