High performance thermoelectrics from earth-abundant materials: Enhanced figure of merit in PbS by second phase nanostructures

Lead sulfide, a compound consisting of elements with high natural abundance, can be converted into an excellent thermoelectric material. We report extensive doping studies, which show that the power factor maximum for pure n-type PbS can be raised substantially to ∼12 μW cm ^-1 K ^-2 at >723 K using 1.0 mol % PbCl ₂ as the electron donor dopant. We also report that the lattice thermal conductivity of PbS can be greatly reduced by adding selected metal sulfide phases. The thermal conductivity at 723 K can be reduced by ∼50%, 52%, 30%, and 42% through introduction of up to 5.0 mol % Bi ₂S ₃, Sb ₂S ₃, SrS, and CaS, respectively. These phases form as nanoscale precipitates in the PbS matrix, as confirmed by transmission electron microscopy (TEM), and the experimental results show that they cause huge phonon scattering. As a consequence of this nanostructuring, ZT values as high as 0.8 and 0.78 at 723 K can be obtained for nominal bulk PbS material. When processed with spark plasma sintering, PbS samples with 1.0 mol % Bi ₂S ₃ dispersion phase and doped with 1.0 mol % PbCl ₂ show even lower levels of lattice thermal conductivity and further enhanced ZT values of 1.1 at 923 K. The promising thermoelectric properties promote PbS as a robust alternative to PbTe and other thermoelectric materials.