Carrier optimization and reduced thermal conductivity leading to enhanced thermoelectric performance in (Mg, S) co-doped AgSbTe₂

Optimizing materials' electrical and thermal properties takes time and effort to achieve high thermoelectric performance. However, the appropriate dopants based on the solution can mitigate the difficulties and increase the overall thermoelectric performance. We have investigated the influence of (Mg, S) doping on the thermoelectric properties of AgSb_1−xMg_xTe_2−yS_y (x = 0.02, 0.03 and 0.04; y = 0.05, 0.1 and 0.15). AgSbTe₂ likely possesses a cationic disorder due to its significant cationic radii mismatch and cationic charge imbalance between Ag¹⁺ (115 p.m.) and Sb³⁺ (76 p.m.), leading to poor electronic transport properties. We demonstrate that decreasing the cationic charge imbalance (Mg²⁺ (72 p.m.), Zn²⁺ (74 p.m.) and Ca²⁺ (100 p.m.)) or ionic radii (Y³⁺ (96 p.m.)) can substantially decrease the cationic disorder in AgSbTe₂. Magnesium doping considerably increased the hole carrier concentration from ∼4.38 × 10¹⁹ cm⁻³ to ∼9.17 × 10¹⁹ cm⁻³. It increased the band gap from 0.35 eV to 0.39 eV, which boosts the density of states effective mass (m_d*) and suppresses the bipolar contribution. Co-doping (Mg, S) in AgSbTe₂ enhances the power factor from ∼8.58 to ∼17.18 μW cm⁻¹ K⁻² at 600 K, resulting from the enhanced carrier concentration, weighted mobility and quality factor, and valence band convergence. The sulfur substitution at the tellurium site significantly suppresses the bipolar contribution by increasing the bandgap/reducing minority charge carriers, which is better than Se-doping. We predicted a high thermoelectric conversion efficiency of ≈13.6% for the temperature gradient of 300 K, which shows excellent potential for mid-temperature thermoelectric device applications. Our co-doping strategy using eco-friendly and earth-abundant elements synergistically optimizes the power factor and thermal conductivity of AgSbTe₂, leading to an improved figure of merit of about ∼1.96 at 600 K.