Enhancement of thermoelectric properties of Ag2Te semiconductors through In-doping induced resonant levels and multi-valley degeneracy†

Energy band engineering is an effective approach to improve thermoelectric performance by addressing the trade-off between carrier concentration and the Seebeck coefficient. However, despite its potential as a promising thermoelectric material near room temperature, Ag₂Te has been scarcely studied for tuning its properties through band engineering. In this work, we achieved a synergistic optimization of carrier concentration and Seebeck coefficient in Ag_2−xIn_xTe (x = 0–0.007) thermoelectric materials by modulating the energy band structure. Indium (In) doping increased the carrier concentration and introduced resonant levels and multi-valley degeneracy in the energy band, significantly enhancing the power factor of Ag_2−xIn_xTe samples. The Ag_1.995In_0.005Te sample achieved a maximal power factor of 1.88 × 10⁻³ W m⁻¹ K⁻² at 300 K, which is 3 times higher than that of pristine Ag₂Te (0.46 × 10⁻³ W m⁻¹ K⁻²). Additionally, In-doping introduced point defects and softened chemical bonds reducing the lattice thermal conductivity to 0.2 W m⁻¹ K⁻¹ at room temperature, representing a reduction of approximately 71.4% compared to undoped Ag₂Te (0.7 W m⁻¹ K⁻¹). As a result, the Ag_1.996In_0.004Te sample achieved a maximum dimensionless figure of merit (ZT) value of 1.0 at 400 K and an average ZT (ZT_avg) of 0.67 from 300 to 400 K. These findings provide a pathway for designing Ag₂Te-based near-room-temperature thermoelectric materials for commercial applications.