Defects responsible for abnormal n-type conductivity in Ag-excess doped PbTe thermoelectrics.

Density functional calculations have been performed to investigate the role of Ag defects in PbTe thermoelectric materials. Ag-defects can be either donor, acceptor, or isovalent neutral defect. When Ag is heavily doped in PbTe, the neutral (Ag-Ag) dimer defect at Pb-site is formed and the environment changes to the Pb-rich/Te-poor condition. Under Pb-rich condition, the ionized Ag-interstitial defect (Ag_I⁺) becomes the major donor. The formation energy of Ag_I⁺ is smaller than other native and Ag-related defects. Also it is found that Ag_I⁺ is an effective dopant. There is no additional impurity state near the band gap and the conduction band minimum. The charge state of Ag_I⁺ defect is maintained even when the Fermi level is located above the conduction band minimum. The diffusion constant of Ag_I⁺ is calculated based on the temperature dependent Fermi level, formation energy, and migration energy. When T > 550 K, the diffusion length of Ag within a few minutes is comparable to the grain size of the polycrystalline PbTe, implying that Ag is dissolved into PbTe and this donor defect is distributed over the whole lattice in Ag-excess doped polycrystalline PbTe. The predicted solubility of Ag_I⁺ well explains the increased electron carrier concentration and electrical conductivity reported in Ag-excess doped polycrystalline PbTe at T= 450-750 K [Pei et ah, Adv. Energy Mater. 1, 291 (2011)]. In addition, we suggest that this abnormal doping behavior is also found for Au-doped PbTe. [ABSTRACT FROM AUTHOR]