Study of Thermoelectric Transport Properties of Doped and Vacancy Induced BiCuSeO

Abstract

About 70% of the heat generated for energy production is wasted and thermoelectric generators can capture this waste heat and convert it into useful electricity. The stability at high temperature in the ambient atmosphere is much needed for the thermoelectric materials for the generator applications, as a result, oxides are the materials of choice. BiCuSeO is a thermoelectric material having cheap, earth-abundant elements and stable at high temperatures. It has a high Seebeck coefficient low thermal conductivity but high electrical resistivity due to its low hole concentration. The effect of Bi vacancies coupled with O vacancies on the thermoelectric properties of BiCuSeO was studied. The Bi vacancies highly reduce the Seebeck coefficient which is partially recovered by the O vacancy. Both the electrical resistivity and Seebeck coefficient decreased with increasing vacancy content. As a result of the decreased electrical resistivity and moderate Seebeck coefficient, the highest power factor of 0.41 mW/m-K2 at 773 K was obtained for the Bi0.92CuSeO0.98 and Bi0.92CuSeO0.98 samples. The zT in the samples with vacancies could not be improved because of the higher thermal conductivity compared to the pristine sample. The transition metals Mn and Zn was doped at the Bi site to decrease the electrical resistivity in addition to that local magnetic moments of Mn can improve the Seebeck coefficient. Since both the Mn and Zn have similar chemical and atomic properties with Cu, the formation energies of the dopants were calculated to determine the probable doping site. The similar formation energies of doping Zn at Bi site and Cu site enable Zn to dope both at the Cu and the Bi site while Mn is more probable to dope at Bi site. The Seebeck coefficient and electrical resistivity monotonically decrease with increasing doping fraction which indicates that Zn is primarily doped at the Bi site and Mn+2 is doped at the Bi site. Both the dopants exhibit small polaron hopping conduction and localization of charge. The highest power factor of 0.35 mW/m-K2 and highest zT of 0.48 was achieved for Zn-doped samples while that of the Mn-doped samples was 0.28 mW/m-K2 and 0.4 respectively. Pb and Na were dually doped at the Bi site of BiCuSeO to improve the carrier concentration without deteriorating the mobility. The increase in the carrier concentration due to Pb doping decreases both the Seebeck coefficient and the electrical resistivity of the samples. As a result of low thermal conductivity and high power factor of 0.53 mW/m-K2, the highest power factor of 0.63 was obtained for the Bi0.905 Na0.015Pb0.08CuSeO sample at 773 K.