Role of microstructure in the lattice thermal conductivity and thermoelectric performance of Cu2+yZn1−ySnSxSe4−x alloys

Abstract

Kesterite compounds of the form Cu_2+yZn_1−ySnS_xSe_4−x (CZTSSe) have attracted considerable interest as sustainable thermoelectric materials due to their earth-abundant composition and intrinsically low lattice thermal conductivity. However, several studies on polycrystals report an unexpected trend in which the selenide exhibits higher lattice thermal conductivity than the sulphide, contrary to expectations based on atomic mass and lattice stiffness. In this work, we investigate the origin of this behaviour and shed light on how microstructural, chemical, and structural features affect the thermoelectric performance across the CZTSSe compositional series. First-principles calculations indicate that the intrinsic lattice dynamics favour lower lattice thermal conductivity in Se-rich compositions. In contrast, experimental results show that microstructural effects dominate phonon transport in polycrystalline samples. In particular, larger porosity, smaller grain size, and reduced grain connectivity strongly suppress the lattice thermal conductivity in sulphide-rich compounds, leading to an apparent inversion of the expected CZTS–CZTSe thermal conductivity trend. In addition, electronic transport analysis reveals an increase in weighted mobility above the order-disorder phase transition temperature, observed across all compositions, and resulting in improved performance. This effect is attributed to the band convergence effect related to the increased structural symmetry promoted by Cu–Zn disorder, approaching a pseudo-cubic structure. Despite the lower intrinsic carrier mobility in sulphide-rich compositions, their suppressed lattice thermal conductivity results are comparable to thermoelectric performance across the series, with zT values up to ∼0.5 at 723 K. These results demonstrate that microstructural effects can outweigh intrinsic chemical trends and play a critical role in determining thermal transport and thermoelectric performance in kesterite materials.