Are Cu₂Te‐Based Compounds Excellent Thermoelectric Materials?

Most of the state-of-the-art thermoelectric (TE) materials exhibit high crystal symmetry, multiple valleys near the Fermi level, heavy constituent elements with small electronegativity differences, or complex crystal structure. Typically, such general features have been well observed in those well-known TE materials such as Bi₂ X₃ -, SnX-, and PbX-based compounds (X = S, Se, and Te). The performance is usually high in the materials with heavy constituent elements such as Te and Se, but it is low for light constituent elements such as S. However, there is a great abnormality in Cu₂ X-based compounds in which Cu₂ Te has much lower TE figure of merit (zT) than Cu₂ S and Cu₂ Se. It is demonstrated that the Cu₂ Te-based compounds are also excellent TE materials if Cu deficiency is sufficiently suppressed. By introducing Ag₂ Te into Cu₂ Te, the carrier concentration is substantially reduced to significantly improve the zT with a record-high value of 1.8, 323% improvement over Cu₂ Te and outperforms any other Cu₂ Te-based materials. The single parabolic band model is used to further prove that all Cu₂ X-based compounds are excellent TE materials. Such finding makes Cu₂ X-based compounds the only type of material composed of three sequent main group elements that all possess very high zT s above 1.5.