Structure-Dependent Thermoelectric Properties of GeSe_1-Te (0 ≤ x ≤ 0.5)

GeSe was theoretically predicted to have thermoelectric (TE) performance as high as SnSe. However, the relatively high TE performance was not achieved experimentally in doped GeSe samples with an original orthorhombic structure but observed in Ag(Sb,Bi)(Se,Te)₂ alloyed samples that crystalize in either a rhombohedral or cubic structure. Herein, to clarify the crystal structure-dependent properties, the electrical and thermal transport properties of GeSe_1-Te (0 ≤ x ≤ 0.5), where orthorhombic, hexagonal, and rhombohedral phases are stable at room temperature for different Te content, have been studied, without any intentional manipulation on carrier concentration. It is found that the three phases show intrinsically different hole concentrations: ∼10¹⁶ cm^-3 for the orthorhombic phase but as high as 10²¹ cm^-3 for the hexagonal and rhombohedral phases. Ge-rich status in the orthorhombic phase and Ge-poor status in hexagonal and rhombohedral phases may be responsible for the huge difference in hole concentrations. The rhombohedral phase shows a much higher Seebeck coefficient than the hexagonal phase with similar hole concentration, indicating that the profile of valance band maximum for the rhombohedral structure is more favorable for high TE performance than the hexagonal phase in GeSe_1-Te. The highest zT of 0.69 has been obtained in GeSe_0.55Te_0.45 at 778 K, at which temperature the rhombohedral phase has already transformed to a cubic phase; however, a zT value of 1.74 at 628 K is predicted by the quality factor analysis for rhombohedral GeSe_0.55Te_0.45 if optimum hole concentration can be achieved.