N-Type Thermoelectric AgBiPbS₃ 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, AgBiPbS₃, which adopts a Fm3̅m structure with a narrow band gap of ∼0.32 eV. Despite the homogeneous distribution of elements at the scale of micrometer, Ag₂S nanoprecipitates with dimensions of several nanometers were detected throughout the matrix. AgBiPbS₃ 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 Ag₂S and the effective scattering of phonons at nanoprecipitate boundaries. Moreover, compared to AgBiS₂, AgBiPbS₃ demonstrates a significantly improved weighted mobility of >16 cm² 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 AgBiPbS₃ 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 AgBiPbS₃, 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 AgBiPbS₃, highlighting the potential of AgBiPbS₃ as an n-type thermoelectric sulfide.