Enhancing thermoelectric performance in non-stoichiometric Bi2Sy

The thermoelectric properties of Bi2Sy with non-stoichiometric composition were investigated, where the sulfur concentration, y was varied between 3.00 and 2.50. Changes in the sulfur concentration resulted in the observation of systematic behaviors of the thermoelectric properties, with a decrease in resistivity and an increase in the Seebeck coefficient being observed with a decrease in sulfur concentration. It was also observed that the thermal conductivity was largely independent of the sulfur concentration. As a result, the non-dimensional figure of merit, ZT was enhanced with decreasing y, and a ZT greater than 0.1 was achieved in the low sulfur concentration region of y~2.50. An enhancement of the figure of merit with decreasing sulfur concentration was interpreted to be due to an increase in the carrier number, through the creation of sulfur vacancies. However, qualitative consistency in the thermoelectric properties has not been always achieved among different studies where samples have been prepared according to different methods. It is, therefore, highly desirable to remedy this through further studies of the intrinsic thermoelectric properties in the Bi2S3 material.