Modulating structures to decouple thermoelectric transport leads to high performance in polycrystalline SnSe

Abstract

In recent years, the thermoelectric properties of SnSe crystals have been rather impressive, while those of polycrystalline SnSe are not ideal due to the grain boundary scattering, which results in impaired carrier mobility and electrical transport. In this work, we introduce the tetragonal-structure AgInSe₂ into SnSe matrix, which not only modifies the crystal structure symmetry to boost carrier mobility, but also enlarges effective mass by inducing resonant energy levels near the valence bands. The compromise on carrier mobility and effective mass leads to a substantial optimization of the electrical transport in the measured temperature range, realizing a peak power factor (PF) value of ∼7.1 μW cm⁻¹ K⁻² in the 1.5% AgInSe₂ alloyed sample, which exhibits nearly 2 times enhancement compared to the unalloyed sample. Subsequently, Ge alloying was introduced to further suppress phonon transport, leading to significantly suppressed lattice thermal conductivities by constructing microstructural point defects. The successive modulation of crystal, band, and microscopic structures facilitates effective phonon-electron decoupling, favoring a prominent peak ZT of ∼1.6 at 773 K in the 0.75% Ge-alloyed (SnSe)_0.985(AgInSe₂)_0.015 sample. Our study provides a systematic strategy to decouple the phonon-electron transport and enhance thermoelectric performance by modulating various aspects of structures.