Enhancing the Thermoelectric Power Factor of Ag-Bi-Te Alloy Films via Post-Annealing Treatment

Ag-Bi-Te thin films were successfully synthesized using a simple and cost-effective thermal evaporation route and post-annealing at 400°C for different durations (0, 30, 60, and 90 min) to tailor their structural, morphological, and thermoelectric properties. X-ray diffraction confirmed the formation of a polycrystalline Ag-Bi-Te alloy with preferred orientation along the (012) plane. The crystallinity and grain size were significantly influenced by annealing duration, reaching a maximum for the 60 min annealed sample. Scanning electron microscopy (SEM) micrographs revealed a smooth and compact morphology for the as-grown film, evolving into large, well-defined grains upon annealing, with the maximum grain growth observed after 60 min. Extending the annealing time to 90 min led to slight grain coalescence and microstructural degradation, likely due to Te volatilization and defect reformation. Electrical and thermoelectric measurements demonstrated that post-annealing strongly affects the charge transport mechanism. The Seebeck coefficient and electrical conductivity exhibited opposite trends to carrier concentration and mobility, reaching optimal values at 60 min of annealing. The sample annealed for 60 min showed the highest Seebeck coefficient (148 µV K⁻¹ at 300 K), electrical conductivity (170 S cm⁻¹), and mobility (31.7 cm² V⁻¹ s⁻¹), resulting in a maximum power factor of 3.73 × 10⁻⁴ W m⁻¹ K⁻² at 350 K. This enhancement is attributed to improved crystallinity, reduced defect density, and energy-filtering effects at grain boundaries. However, excessive annealing beyond 60 min caused a degradation in all parameters due to structural instability and non-stoichiometric variations. This study demonstrates that controlled post-annealing at 400°C for 60 min provides an optimal balance between structural ordering and carrier transport, significantly enhancing the thermoelectric power factor of Ag-Bi-Te alloy films. The results highlight the crucial role of thermal processing in tuning microstructural and electronic properties for high-performance, low-cost thermoelectric devices.