N-Type Thermoelectric AgBiPbS 3 with Nanoprecipitates and Low Thermal Conductivity.

Thermoelectric sulfide materials are of particular interest due to the earth-abundant and cost-effective nature of sulfur. Here, we report a new n-type degenerate semiconductor sulfide, AgBiPbS 3 , which adopts a Fm 3̅ m structure with a narrow band gap of ∼0.32 eV. Despite the homogeneous distribution of elements at the scale of micrometer, Ag 2 S nanoprecipitates with dimensions of several nanometers were detected throughout the matrix. AgBiPbS 3 exhibits a low room-temperature lattice thermal conductivity of 0.88 W m -1 K -1 , owing to the intrinsic low lattice thermal conductivity of Ag 2 S and the effective scattering of phonons at nanoprecipitate boundaries. Moreover, compared to AgBiS 2 , AgBiPbS 3 demonstrates a significantly improved weighted mobility of >16 cm 2 V -1 s -1 at 300 K, leading to an enhanced PF of 1.6 μW cm -1 K -2 at 300 K. The superior electrical transport in AgBiPbS 3 can be attributed to the high valley degeneracy of the L point (the conduction band minimum), which is contributed by the Pb s and Pb p orbitals. Further, Ga doping is found to be effective in modulating the Fermi levels of AgBiPbS 3 , leading to further enhancement of PF with a PF ave of 2.7 μW cm -1 K -2 in the temperature range of 300-823 K. Consequently, a relatively high ZT ave of 0.22 and a peak ZT of ∼0.4 at 823 K have been achieved in 3% Ga-doped AgBiPbS 3 , highlighting the potential of AgBiPbS 3 as an n-type thermoelectric sulfide.