Optimization of thermoelectric efficiency in SnTe: The case for the light band

p-Type PbTe is an outstanding high temperature thermoelectric material with zT of 2 at high temperatures due to its complex band structure which leads to high valley degeneracy. Lead-free SnTe has a similar electronic band structure, which suggests that it may also be a good thermoelectric material. However, stoichiometric SnTe is a strongly p-type semiconductor with a carrier concentration of about 1 × 10²⁰ cm^-3, which corresponds to a minimum Seebeck coefficient and zT. While in the case of p-PbTe (and n-type La₃Te₄) one would normally achieve higher zT by using high carrier density in order to populate the secondary band with higher valley degeneracy, SnTe behaves differently. It has a very light, upper valence band which is shown in this work to provide higher zT than doping towards the heavier second band. Therefore, decreasing the hole concentration to maximize the performance of the light band results in higher zT than doping into the high degeneracy heavy band. Here we tune the electrical transport properties of SnTe by decreasing the carrier concentration with iodine doping, and increasing the carrier concentration with Gd doping or by making the samples Te deficient. A peak zT value of 0.6 at 700 K was obtained for SnTe_0.985I_0.015 which optimizes the light, upper valence band, which is about 50% higher than the other peak zT value of 0.4 for Gd_zSn_1-zTe and SnTe_1+y which utilize the high valley degeneracy secondary valence band.