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 11020 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 doping into the deeper band with higher valley degeneracy, SnTe behaves differently. It is the lighter, upper valence band is shown in this work to result in a higher zT. Therefore decreasing the hole concentration to maximize 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 carrier concentration with Iodine doping, and increasing the carrier concentration with Gd or Te doping. A peak zT value of 0.6 at 700 K was obtained for SnTe0.985I0.015 which optimizes the light, upper valence band, which is about 50% higher than the other peak zT value of 0.4 for GdzSn1-zTe and SnTe1+y which optimize the high valley degeneracy lower valence band. Phys. Chem. Chem. Phys., 2014 a) Seebeck coefficient and b) Hall mobility as a function of Hall carrier concentration at 300 K for SnTe1-xIx and SnTe1+y, GdzSn1-zTe. Solid squares are our experimental results, open squares are Rogers’ reported results9, solid curves are calculated from a two band model. c) A schematic diagram of the near edge band structure in PbTe and SnTe. (Rogers et al reported a band offset of 0.3 eV for SnTe. Our results yielded different fitting parameters, and we found that 0.4 eV was required for best fit.)
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