Structural, electronic and thermoelectric properties of SnTe with dilute co-doping of Ag and Cu

Abstract

The low bandgap semiconductor, SnTe is receiving significant attention as a thermoelectric material because of its low toxicity and environment-friendly nature. In this study, we report the effect of co-doping dilute concentrations of Ag and Cu ions in SnTe that suppresses the Sn vacancies leading to optimized thermoelectric properties. Samples of nominal chemical composition Sn1.03–2xAgxCuxTe (x = 0, 0.01, 0.02, 0.04) were prepared by the solid-state route. The Rietveld refinement of powder XRD of these compounds showed a fcc (Fm3¯m) structure with no other impurity phases. Diffuse reflectance IR spectroscopy showed an increase in the bandgap upon Ag-Cu co-doping in SnTe, associated with valence band convergence. Electronic band structure calculations confirmed an increase in the bandgap along with a reduction in the energy difference between light and heavy valence bands having maxima at the L and Σ points. Partial density of states (P-DOS) calculations showed that Ag-Cu doping in SnTe does not contribute towards the formation of resonant energy levels. The Seebeck coefficient S of the Sn1.01Ag0.01Cu0.01Te reached a maximum value of ∼ 95 μV/K at 783 K, compared to 86 μV/K of pure SnTe. The power factor increased with the doping concentration, reaching ∼ 10.8 μWK−2cm−1 at 783 K for x = 0.04. The lattice thermal conductivity κL decreased on Ag-Cu co-doping, with κL = 0.44 Wm−1K−1 above 750 K for x = 0.04, which is close to the Cahill model. The combination of higher power factor and reduced lattice thermal conductivity in the Ag-Cu co-doped samples resulted in enhanced ZT = 0.24–0.29 at 773 K.