Band Degeneracy and Anisotropy Enhances Thermoelectric Performance from Sb₂Si₂Te₆ to Sc₂Si₂Te₆

The complex interrelationships among thermoelectric parameters mean that a priori design of high-performing materials is difficult. However, band engineering can allow the power factor to be optimized through enhancement of the Seebeck coefficient. Herein, using layered Sb₂Si₂Te₆ and Sc₂Si₂Te₆ as model systems, we comprehensively investigate and compare their thermoelectric properties by employing density functional theory combined with semiclassical Boltzmann transport theory. Our simulations reveal that Sb₂Si₂Te₆ exhibits superior electrical conductivity compared to Sc₂Si₂Te₆ due to lower scattering rates and more pronounced band dispersion. Remarkably, despite Sb₂Si₂Te₆ exhibiting a lower lattice thermal conductivity and superior electrical conductivity, Sc₂Si₂Te₆ is predicted to achieve an extraordinary dimensionless figure of merit (ZT) of 3.51 at 1000 K, which significantly surpasses the predicted maximum ZT of 2.76 for Sb₂Si₂Te₆ at 900 K. We find the origin of this behavior to be a combined increase in band (valley) degeneracy and anisotropy upon switching the conduction band orbital character from Sb p to Sc d, yielding a significantly improved Seebeck coefficient. This work suggests that enhancing band degeneracy and anisotropy (complexity) through compositional variation is an effective strategy for improving the thermoelectric performance of layered materials.