Estimation of the Highest Thermoelectric Performance of the Bi-Doped SnTe at Room Temperature

Abstract

SnTe has drawn much attention due to its Pb-free composition along with tunable electronic and lattice structures. However, its intrinsically high defect concentration and high lattice thermal conductivity (<i>κ₁</i>) have hindered its application in devices. Recently, Bi doping at Sn-sites in Sn_1-<i>x</i>Bi_<i>x</i>Te (<i>x</i> = 0.0 – 0.08) has been demonstrated to be effective in improving the thermoelectric performance (<i>zT</i>) of SnTe. Bi doping was particularly effective in improving the Seebeck coefficient in a wide range of temperature while suppressing its <i>κ₁</i>. However, the effect of Bi doping on electronic band structure of SnTe has not been studied. Here, we applied the Single Parabolic Band (SPB) model to the room temperature electronic transport properties measurements (Seebeck coefficient, electrical conductivity, Hall carrier concentration) and analyzed how electronic band parameters like the density-of-states effective mass (<i>m_d</i> *), non-degenerate mobility (<i>μ₀</i>), weighted mobility (<i>μ_w</i>), and <i>B</i>-factor changes with a changing Bi doping content (<i>x</i>). As the <i>x</i> increases, the <i>m_d</i> * increases while <i>μ₀</i> decreases. As the <i>μ_w</i> depends both on <i>m_d</i> * and <i>μ₀</i>, it peaks at <i>x</i> = 0.02. Lastly, the <i>B</i>-factor is related to the ratio of <i>μ_w</i> to <i>κ₁</i>, due to significantly decreasing <i>κ₁</i> at high <i>x</i>, the <i>B</i>-factor also becomes the highest at <i>x</i> = 0.08. Based on the <i>B</i>-factor of <i>x</i> = 0.08 sample, the highest theoretical <i>zT</i> of 0.31 is predicted using the SPB model. This is approximately 2.2 times higher than the experimental <i>zT</i> (~0.139) reported in literature at 300 K. The SPB model also guides us that the highest theoretical <i>zT</i> of 0.31 can be achieved if its Hall carrier concentration is tuned to 9.06 × 10¹⁸ cm^-3.