Realization of valence band convergence for high thermoelectric performance p-type PbS

Abstract

PbS is an attractive thermoelectric material with low cost, high thermostability, and large abundance in the Earth’s crust. However, its largest energy difference between the light and heavy valence bands (ΔEv) leads to a huge challenge in achieving valence band convergence (VBC), resulting in the lowest power factor (PF) compared to PbSe and PbTe. This low PF impedes the output power density of thermoelectric devices. In this work, the Ag dopant can reduce ΔEv from 0.34 eV to 0.20 eV, achieving VBC in p-type PbS for the first time, significantly improving the Seebeck coefficient. Additionally, Pb0.98Ag0.02S exhibits semiconductor-like electrical conductivity caused by the grain boundaries scattering below 473 K. With coarse-graining, the mobility improves substantially from 4.6 cm2 V−1 s−1 to 241 cm2 V−1 s−1 at room temperature. Consequently, the PF of Pb0.98Ag0.02S increases from 0.3 μW cm−1 K−2 to 12.2 μW cm−1 K−2 with the figure of merit ZT value rising from 0.0027 to 0.10 at 300 K. More importantly, Pb0.98Ag0.02S crystal maintains a high PF from 300 K to 823 K, giving a record-high average PF of 13.0 μW cm−1 K−2 and an increased ZT. This work certifies Ag as an effective p-type dopant for weakening electron transport coupling for PbS-based thermoelectric materials.