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

TL;DR

This study reveals that Ag interstitial defects act as electron donors in Ag-excess doped PbTe, explaining the abnormal n-type conductivity and high-temperature behavior of the material.

Contribution

It uncovers the role of Ag interstitials as primary donors and explains the doping mechanism in Ag-excess doped PbTe thermoelectrics.

Findings

Ag interstitial ($Ag_I^+$) is the main defect responsible for n-type conductivity.

Formation energy and diffusion barriers explain high-temperature solubility.

Similar defect behavior observed in Au-doped PbTe.

Abstract

We find that Ag-interstitial ($Ag_I$) acts as an electron donor and plays an important role in Ag-excess doped polycrystalline PbTe thermoelectric materials. When Ag is heavily doped in PbTe, the neutral (Ag-Ag) dimer defect is formed at the Pb-site and the environment becomes Pb-rich/Te-poor condition. Then the positively ionized Ag interstitial ($Ag_I^+$) defect becomes the major defect under Pb-rich condition. Due to the small formation energy and small diffusion barrier of $Ag_I^+$, Ag can be easily dissolved into the PbTe matrix. The temperature behavior of the Ag defect formation energy well explains the $Ag_I^+$ solubility, the electron carrier generation, and the increasing electrical conductivity in Ag-excess doped polycrystalline PbTe at high temperature. This abnormal doping behavior by forming interstitial defects is also found for Au-doped PbTe.