Stacking polymorphism is a common characteristic of van der Waals layered materials which can substantially modify their physical properties. Here, based on first-principles calculations combined with electron and phonon transport theories, we systematically investigate the thermodynamic stability, electronic structure, lattice dynamics, and thermoelectric performance of ${\mathrm{Sc}}_2{\mathrm{Si}}_2{\mathrm{Te}}_6$ with three high-symmetry stacking sequences, namely, AA, AB, and ABC. We find that the AA- and AB-stacked structures are nearly degenerate in energy with the experimentally reported ABC phase and that the maximum sliding barrier among these stacking sequences is only about 10 meV/atom, thereby accounting for the stacking faults observed experimentally. These three stacking sequences exhibit distinct electronic structures, with the conduction band minimum being highly sensitive to the stacking sequence. As a consequence, the conduction band degeneracies are 12, 2, and 8 for the ABC, AA, and AB stackings, respectively, leading to markedly different electronic transport properties near the band edge. The lattice thermal conductivity is governed primarily by three-phonon (3ph) scattering, whereas four-phonon scattering provides an additional reduction, particularly in the ABC stacking. Among the three structures, the AB stacking exhibits the lowest lattice thermal conductivity owing to its stronger 3ph scattering and lower phonon group velocity. As a result, the maximum thermoelectric figure of merit $ZT$ is achieved in the AB structure, followed closely by the ABC structure, whereas the AA structure shows a substantially reduced value. These results demonstrate that the stacking sequence exerts a non-negligible influence on the thermoelectric performance of ${\mathrm{Sc}}_2{\mathrm{Si}}_2{\mathrm{Te}}_6$.

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• Electrical conductivity
• Electronic structure
• Lattice thermal conductivity
• Seebeck effect
• Thermoelectrics
• Layered semiconductors
• van der Waals systems
• Density functional calculations