Background: The recent breakthroughs in the enhancement of zT on PbTe-based materials
have been mostly attributed to the material microstructures incorporated during quench or hot press
processes that cause the reduction of thermal conductivity. However, in the practical applications, the
structural and thermoelectric configurations in the thermoelectric devices need to stay stable at the
operational temperature for long periods of time. Recently, the thermal stability of melt-grown PbTeSe
materials has been demonstrated experimentally. In this report, instead of the thermally unstable
microstructural inhomogeneity/defects, single and multiple impurities were intentionally incorporated
into PbTe crystals during the melt growth to modify their thermoelectric properties.
Methods: Fourteen PbTe crystals were grown from starting melts with various single and multiple
dopants by un-seeded vertical directional solidification. A disc-shaped sample was sliced from each
crystal and various characterizations, including thermal diffusivity, density measurements, electrical
conductivity, Seebeck coefficient and Hall measurement, were performed on the same discs to study
the effects of impurity incorporation on the resultant thermoelectric properties.
Results: The results show that the introduction of impurities reduces the thermal conductivity. The
singly doped Sb-doped PbTe and high Sb-content PbAgSbTe crystals have higher thermal
conductivities than the Bi- and In-doped PbAgSbTe crystals, and the multiple doping PbAgSbTe
crystals have the lowest thermal conductivity. The measured Figure of Merits for thermoelectric
applications, zT, shows that most of the samples have maxima between 1.05 and 1.1 in the 350 to
450°C temperature range, except the Cl-doped PbTe having the highest zT of 1.33 at 420°C and the
multiple doping sample having the lowest zT of 0.6 at 450°C.
Conclusion: Although the introduction of impurities reduces the measured thermal conductivity, it
also reduces the electric carrier conduction by slowing its mobility. From examining the Lorenz
number, L, the ratio of thermal conductivity to the product of electrical conductivity and temperature,
it was shown that the incorporation of multiple impurities has an adversary effect on the Figure of
Merit due to the larger reduction in electrical conductivity than in thermal conductivity. Comparing
with the theoretical calculations on electrical and thermal conductivity, to minimize Lorenz number it
is suggested that (1) the dopant concentration, i.e., the dopant spacing, needs to be optimized to be
comparable to the mean free path of phonons and (2) the concentration of extrinsic hole, i.e., the
acceptor originated either from impurity or from native defect (such as Pb vacancy or interstitial Te)
needs to be minimized to enhance electrical conductivity.