Role of Impurity Defects in Thermoelectric Material Property Degradation of Mg₂(Si,Sn) Modules

Mg₂(Si,Sn) materials are prominent in mid-temperature range thermoelectric applications. Despite their potential, challenges arise due to material degradation during the joining process with metallic electrodes. In this study, we used hybrid-density functional calculations to understand the influence of point impurities in Mg₂Si and Mg₂Sn thermoelectric materials. Charged defect formation energy calculations revealed that several elemental impurities, such as Ag, serve as electron trap centers, especially in n-type Bi-doped Mg₂(Si,Sn). Interestingly, both the formation energy and diffusion barrier for these impurities are remarkably low. This suggests that point impurities can easily diffuse from the metal electrode deep into the thermoelectric materials. The Seebeck microprobe measurements further validate our "defect diffusion and charge compensation model". In particular, the Seebeck coefficient near the contact shows a significant increase, due to charge compensation resulting from the diffusion of these impurities.