Crystal symmetry enables high thermoelectric performance of rhombohedral GeSe(MnCdTe2)x

Abstract

High symmetry favors high power factor by virtue of the balanced Seebeck coefficient and carrier mobility, nonetheless, the role of crystal symmetry in enhancing the material’s thermoelectric performance is abstruse. Here, we employ the interplay between crystal symmetry and native point defects towards high zT of IV-VI semiconductor GeSe, which is scarce in thermoelectric study. Pristine orthorhombic GeSe has a low zT ~ 0.05 due to the high formation energy of Ge vacancy and thus the low carrier concentration (~ 1016 cm−3). Alloying GeSe with MnCdTe2 stabilizes higher-symmetry rhombohedral structure at ambient conditions, thereby effectively lowering the formation energy of Ge vacancy and raising the carrier concentration by four orders of magnitude. Meanwhile, compared to orthorhombic GeSe, the rhombohedral Ge1-yBiySe(MnCdTe2)x own higher valley degeneracy and smaller band effective mass, rendering the decent Seebeck coefficient and larger carrier mobility, respectively. Moreover, the generated multiscale microstructures in rhombohedral Ge0.96Bi0.04Se(MnCdTe2)0.10, including atomic-scale native Ge vacancies and substitution point defects, nanoscale domain structures, and micron-sized secondary phases effectively depress the lattice thermal conductivity. As a result, a state-of-the-art zT ~ 1.0 at 723 K is achieved in Ge0.96Bi0.04Se(MnCdTe2)0.10. These results attest to the efficacy of the interplay between crystal symmetry and native point defects towards high performance GeSe-based and other thermoelectric materials.