Modulating the valence of Ga and the deep level impurity for high thermoelectric performance of n-type Pb0.98Ga0.02Te1-xSex compounds

It has been found that the doped Ga has a distinct role in PbTe and PbSe compounds though they possess the same crystal structure. Therefore, to understand how the thermoelectric performance evolves in the Ga-doped PbTe1-xSex solid solution is crucial. Herein, we report that a maximum ZT value of 1.57 at 721 K and a high average ZT value of 1.13 in the temperature range of 300–843 K are attained for Pb0.98Ga0.02Te0.96Se0.04 compound. The experimental studies show that doping Ga into PbTe introduces a shallow level impurity state Ga3+ and a deep level associated with the Ga + state simultaneously. Interestingly, upon Se-alloying all of the Ga + are oxidized into Ga3+ at low temperatures, which increases the room-temperature carrier concentration from 1.16 × 1019 cm−3 to 2.17 × 1019 cm−3. This leads to an improved power factor of 3.04 mW m−1 K−2 for Pb0.98Ga0.02Te0.96Se0.04 sample at 482 K. Additionally, Se-alloying further enhances the point defect phonon scattering and lowers the lattice thermal conductivity to 0.46 W m−1 K−1 for Pb0.98Ga0.02Te0.96Se0.04 sample at 690 K. As a result, the ZT value is increased by about 112% compared with the maximum ZT of 0.74 for pristine PbTe. This work provides a new avenue for tuning the doping behavior of cation site dopant via alloying on anion site to further improve the ZT value in thermoelectric materials.