Heavy indium doping in p-type AgBiSe₂: Synergistic rhombohedral phase stabilization and carrier optimization for enhanced thermoelectrics

P-type AgBiSe₂-based materials exhibit promising thermoelectric performance but suffer from low hole concentration due to inefficient doping. In this work, the thermoelectric performance of p-type AgBiSe₂ has been significantly enhanced through high-concentration indium doping at bismuth sites. Comprehensive structural analyses via Rietveld refinement, differential scanning calorimetry (DSC), and transmission electron microscopy (TEM) confirm that indium doping stabilizes the metastable rhombohedral phase of AgBiSe₂ at room temperature. The optimized AgBi_0.86In_0.14Se₂ exhibits a remarkable enhancement in room-temperature hole concentration, increasing from 2.1 × 10¹⁷ cm⁻³ in pristine AgBiSe₂ to 2.3 × 10¹⁹ cm⁻³. This improvement leads to a notable boost in electrical conductivity from 17.8 S m⁻¹ to 3000 S m⁻¹ and power factor from 0.1 µW cm⁻¹ K⁻² to 4.8 µW cm⁻¹ K⁻². First-principles calculations reveal that indium doping modifies the electronic band structure, narrows the bandgap, and increases the density of states near the valence band edge. As a result, the material achieves a peak zT of 0.52 at 470 K and an average zT of 0.38 across the temperature range of 298–470 K. This work demonstrates a synergistic strategy of phase stabilization, carrier concentration optimization, and band engineering to advance low-temperature thermoelectric materials.