3D charge and 2D phonon transports leading to high out-of-plane <i>ZT</i> in n-type SnSe crystals

TLDR

Thermoelectric technology harvests waste heat into electricity, and layered SnSe exhibits exceptionally low out‑of‑plane thermal conductivity due to two‑dimensional phonon transport. Bromine doping creates n‑type SnSe with overlapping interlayer charge density enabling three‑dimensional charge transport, while a continuous phase transition raises symmetry and splits two converged conduction bands. These structural and electronic changes yield a maximum out‑of‑plane figure of merit ZT ≈ 2.8 at 773 K, with enhanced carrier mobility and preserved Seebeck coefficient, offering a strategy to improve out‑of‑plane electrical transport in 2D layered materials without compromising thermal performance.

Abstract

Thermoelectric technology enables the harvest of waste heat and its direct conversion into electricity. The conversion efficiency is determined by the materials figure of merit ZT Here we show a maximum ZT of ~2.8 ± 0.5 at 773 kelvin in n-type tin selenide (SnSe) crystals out of plane. The thermal conductivity in layered SnSe crystals is the lowest in the out-of-plane direction [two-dimensional (2D) phonon transport]. We doped SnSe with bromine to make n-type SnSe crystals with the overlapping interlayer charge density (3D charge transport). A continuous phase transition increases the symmetry and diverges two converged conduction bands. These two factors improve carrier mobility, while preserving a large Seebeck coefficient. Our findings can be applied in 2D layered materials and provide a new strategy to enhance out-of-plane electrical transport properties without degrading thermal properties.