Thermopower enhancement in Pb1−xMnxTe alloys and its effect on thermoelectric efficiency

The Seebeck coefficient of p-type PbTe can be enhanced at 300 K, either due to the addition of Tl-resonant states or by manipulation of the multiple valence bands by alloying with isovalent compounds, such as MgTe. PbTe alloyed with MnTe shows a similar thermopower enhancement that could be due to either mechanism. Here we investigate the characteristics that distinguish the resonant state mechanism from that due to multiple valence bands and their effect on the thermoelectric figure of merit, zT. Ultimately, we find that the transport properties of PbTe alloyed with MnTe can be explained by alloy scattering and multiple band model that result in a zT as high as 1.6 at 700 K, and additionally a ∼30% enhancement of the average zT. Thermoelectric materials, which can convert a heat flow into an electrical current, and vice versa, have potential for applications in cooling and in power generation. Yanzhong Pei, Jeffrey Snyder and co-workers from Tongji University in Shanghai and the California Institute of Technology have now enhanced the efficiency of a state-of-the-art thermoelectric material, lead telluride, by alloying it with manganese telluride (MnTe), and proposed a mechanism to account for this improvement. The properties of lead telluride can be improved by doping and alloying it with other species. This tuning in composition can reduce its thermal conduction or improve its electronic properties – the latter either by introducing resonant states or by utilizing multiple band conduction. Through comparisons with other doped PbTe materials, Pei, Snyder and co-workers suggest that the MnTe-induced performance enhancement arises from multiple band conduction. They also show that these band effects are more effective than the formation of resonant states. Increasing the Seebeck coefficient has long been pursued for increasing thermoelectric efficiency, but other changes in transport properties may compensate this effect and ultimately lead to no improvement in figure of merit. The Seebeck coefficient can be enhanced by either the addition of resonant states or the involvement of multiple band conduction. This work demonstrates the beneficial effect of multiple band conduction for highefficiency thermoelectrics.