ICT2014 Abstracts

Much of the field’s current understanding of the outstanding thermoelectric performance of PbTe is based on a temperature dependent shift in the band structure, which allows convergence of the light (L) and heavy (Σ) bands at a temperature of around 450 K. This has been suggested based on historical experimental data that displays saturation in the temperature dependent optical gap. However, our own data shows that the gap continues to increase for all lead chalcogenides to the highest measured temperatures of 673 K. Hence, the band convergence does not occur at the temperatures suggested by historical literature. We used optical absorption in order to probe the temperature dependence of the band gap in the lead chalcogenides: PbX (X=S, Se, Te). For all lead chalcogenides, we measure a dEg/dT of 3.2×10-4 eV/K, for temperatures below 500 K with a decrease in the slope at higher temperatures. Additionally, we performed ab-initio molecular dynamics (AIMD) calculations, averaging over several supercell configurations and taking into account both the lattice thermal expansion and electron-phonon interaction. Specifically in the case of PbTe, the calculations indicate that convergence occurs around 700 K. Furthermore, electron-phonon interaction is shown to be the primary reason for band convergence, accounting for ~70% of the band gap change. Specifically, the phonon interaction is observed to increase the direct (L-L) gap much more than the indirect (L-Σ) gap with temperature. This new evidence of a higher convergence temperature will improve our understanding of the extent to which the Σ band enhances zT, and it can ultimately allow the band structure to be better engineered for optimum thermoelectric performance.