Multi-valley evolution and corresponding effects on thermoelectric transport in low-symmetry heterostructure GeS/GeSe

Abstract

Layered germanium chalcogenides are promising thermoelectric (TE) materials, yet it remains difficult to achieve synergistic optimization of electrical and thermal transport in single-component materials. The complex multi-valley characteristics in low-symmetry van der Waals heterostructure GeS/GeSe provide a new strategy for optimizing TE transport. Therefore, this work systematically explores the evolution of multi-valley characteristics in bilayer GeS, bilayer GeSe, and GeS/GeSe, revealing its impact on TE transport properties. Since there exist 2 (5) valleys near the valence band maximum (conduction band minimum), the optimal power factor S2σ for p-type and n-type GeS/GeSe reaches 88.5 and 149.6 μW cm−1 K−2. For phonon transport, due to the distinct differences of projected phonon density of states overlap in the low-frequency region, which is closely related to the cooperativity of phonon vibrational modes, the average phonon relaxation times τq¯ differ substantially among the three structures. At 300 K, the average lattice thermal conductivities κL¯ of bilayer GeS, bilayer GeSe, and GeS/GeSe are 3.72, 8.57, and 7.52 W m−1 K−1, respectively, which are relatively low among similar semiconductors. Overall, GeS/GeSe possesses superior TE performance due to the presence of multi-valley degeneracy and low κL. This work clarifies the complex multi-valley distribution and evolution mechanism, uncovers the intrinsic link between multi-valley degeneracy and TE performance, and offers theoretical guidance for optimizing TE properties via low-symmetry structure engineering and multi-valley modulation.