Improvement in the Thermoelectric Figure of Merit by La/Ag Cosubstitution in PbTe

Abstract

The thermoelectric properties of La-doped and Ag/La codoped PbTe were investigated in the temperature range of 300 to ∼720 K. All samples crystallize in the NaCl-type structure without noticeable secondary phase and exhibit narrow bandgaps of 0.26−0.30 eV, typical of PbTe. La doping (≤5 at %) in PbTe significantly enhances the room temperature electrical conductivity to >5000 S/cm, indicating that La is an efficient electron donor. Hall coefficient data confirm the significant increase in electron carrier concentration from ∼2 × 1018 cm−3 at 300 K for PbTe to ∼5.3 × 1019 cm−3 for Pb0.99La0.01Te, and ∼1.7 × 1020 cm−3 for Pb0.95La0.05Te. Ag doping in Pb0.99La0.01Te has an opposite effect on the electron carrier concentration (∼2.0 × 1019 cm−3 at 300 K for Ag0.05Pb0.99La0.01Te and ∼1.5 × 1019 cm−3 for Ag0.1Pb0.99La0.01Te), consistent with the role of Ag as electron acceptor. The temperature dependence of mobility is discussed in detail using the physics of electron scattering and possible scattering mechanisms. For Ag0.05Pb0.99La0.01Te, a high power factor of ∼22 μW/(cm K2) at ∼720 K was achieved with the optimal total thermal conductivity of ∼ 1.3 W/(m K) at ∼720 K, giving a maximum figure of merit ZT ≈ 1.2 at ∼720 K.