Thermoelectric Properties of (Bi1‑xSbx)2S3 with Orthorhombic Structure

Thermoelectric properties of the substitution system (Bi_1‑xSb_x)₂S₃ have been investigated, where binary Bi₂S₃ and Sb₂S₃ are narrow-gap semiconductors. It is confirmed that metallic conduction, originating from mobile electrons due to production of sulfur vacancies, is observed in Bi₂S₃ over a wide temperature range below room temperature. In Sb₂S₃, mobile carriers are not created and insulating behavior is observed because of the considerably wide bandgap. Change of the carrier number by substitution of antimony contributes strongly to the thermoelectric properties (resistivity and Seebeck coefficient). As a result, the nondimensional figure of merit, ZT, decreases monotonically with increasing antimony content. The maximum value of ZT is obtained in Bi₂S₃ as ZT ≈ 0.1 at room temperature. It is pointed out that control of the carrier number, which is achieved by production of sulfur vacancies, is important to achieve high thermoelectric performance in the (Bi_1‑xSb_x)₂S₃ system. It is possible that the thermoelectric efficiency could be improved by control of the carrier concentration in the bismuth-rich region, including pure binary Bi₂S₃.