Advancing thermoelectrics by suppressing deep-level defects in Pb-doped AgCrSe ₂ alloys

AgCrSe ₂-based compounds have attracted much attention as an environmentally friendly thermoelectric material in recent years due to the intriguing liquid-like properties. However, the ultra-low carrier concentration and the high Ag _Cr deep-level defects limit the overall thermoelectric performance. Here, we successfully introduced Pb into Ag-deficient Ag _0.97CrSe ₂ alloys to tune the carrier concentration across a broad temperature range. The Pb ²⁺ as an acceptor dopant preferentially occupies Cr sites, boosting the hole carrier concentration to 1.77 × 10 ¹⁹ cm ⁻³ at room temperature. Furthermore, the Pb strongly inhibits the creation of intrinsic Ag _Cr defects, weakens the increased thermal excited ionization with the increasing temperature and slowed the rising trend of the carrier concentration. The designed carrier concentration matches the theoretically predicted optimized one over the entire temperature range, leading to a remarkable enhancement in power factor, especially the maximum power factor of ∼ 500 μW⋅m ⁻¹⋅K ⁻² at 750 K is superior to most previous results. Additionally, the abundant point defects promote phonon scattering, thus reducing the lattice thermal conductivity. As a result, the maximum figure of merit zT (∼ 0.51 at 750 K) is achieved in Ag _0.97Cr _0.995Pb _0.005Se ₂. This work confirms the feasibility of manipulating deep-level defects to achieve temperature-dependent optimal carrier concentration and provides a valuable guidance for other thermoelectric materials.