Giant Band Degeneracy via Orbital Engineering 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, the introduction of Sc-d orbitals dramatically increases conduction band degeneracy in Sc2Si2Te6, yielding a significantly improved Seebeck coefficient relative to Sb2Si2Te6. As a result, 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. This work suggests that engineering band degeneracy through compositional variation is an effective strategy for improving the thermoelectric performance of layered materials.