Are Cu₂Te‐Based Compounds Excellent Thermoelectric Materials?

Abstract 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 Bi₂X₃‐, 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 Cu₂X‐based compounds in which Cu₂Te has much lower TE figure of merit (zT) than Cu₂S and Cu₂Se. It is demonstrated that the Cu₂Te‐based compounds are also excellent TE materials if Cu deficiency is sufficiently suppressed. By introducing Ag₂Te into Cu₂Te, the carrier concentration is substantially reduced to significantly improve the zT with a record‐high value of 1.8, 323% improvement over Cu₂Te and outperforms any other Cu₂Te‐based materials. The single parabolic band model is used to further prove that all Cu₂X‐based compounds are excellent TE materials. Such finding makes Cu₂X‐based compounds the only type of material composed of three sequent main group elements that all possess very high zT s above 1.5.