Effect of the Amount of Vacancies on the Thermoelectric Properties of Cu–Ga–Te Ternary Compounds

Abstract

The Cu–Ga–Te ternary compounds: Cu₃Ga₅Te₉, Cu₂Ga₄Te₇, CuGa₃Te₅, CuGa₅Te₈, and CuGaTe₂ have zinc-blende or chalcopyrite structure. The compounds except for CuGaTe₂ contain vacancies in the cation site due to the valence mismatch between cation and anion, and the vacancy concentration is different between these compounds. Here we investigated the effect of the amount of vacancies on the thermoelectric (TE) properties of the Cu–Ga–Te ternary compounds. At room temperature, the presence of vacancies reduced the Hall mobility (μ_H) and the lattice thermal conductivity (κ_lat), showing that the vacancies scattered both carriers and phonons. It was found that the decreasing rate of the μ_H was larger than that of the κ_lat. Therefore, the presence of vacancies degraded the TE performance of the Cu–Ga–Te ternary compounds. In other words, CuGaTe₂ without vacancies would show the best TE figure of merit. The room temperature ZT values for Cu₃Ga₅Te₉, Cu₂Ga₄Te₇, CuGa₃Te₅, CuGa₅Te₈, and CuGaTe₂ were 2.0 × 10⁻², 2.4 × 10⁻², 8.9 × 10⁻³, 1.3 × 10⁻², and 5.6 × 10⁻³, respectively.