Improvement of thermoelectric performance of copper-deficient compounds Cu2.5+δIn4.5Te8 (δ = 0–0.15) due to a degenerate impurity band and ultralow lattice thermal conductivity

Abstract

Cu–In–Te ternary chalcogenides have unique crystal and band structures; hence they have received much attention in thermoelectrics. In this work we have observed an enhancement in Hall carrier concentration (n_H) and ultralow lattice thermal conductivity (κ_L) when Cu was added to ternary Cu_2.5+δIn_4.5Te₈ (δ = 0–0.15) compounds. The enhancement in n_H is attributed to a degenerate impurity band at the G point in the valence band maximum (VBM), while the extremely low κ_L results from the increased lattice disorder. We thus obtained the minimum κ_L value of only 0.23 W K⁻¹ m⁻¹ in the sample at δ = 0.1 and 820 K, which is in good agreement with the calculation using the Callaway model. The highest thermoelectric figure of merit ZT is 0.84 for the material at δ = 0.1, which is about 0.38 higher than that of the pristine Cu_2.5In_4.5Te₈.