Engineering the energy gap near the valence band edge in Mn-incorporated Cu3Ga5Te9 for an enhanced thermoelectric performance

Abstract

Cu₃Ga₅Te₉-based compounds Cu_3−xGa₅Mn_xTe₉ (x = 0–0.2) with Mn substitution for Cu have been synthesized. The engineered energy gap (ΔE_A) between impurity and valence bands is reduced from 44.4 meV at x = 0 to 25.7 meV at x = 0.1, which is directly responsible for the reduction of the potential barrier for thermal excitation of carriers and the increase in carrier concentration. However, the Seebeck coefficient shows an increasing tendency with the increase of determined Hall carrier concentration (n). This anomalous behavior suggests that the Pisarenko plots under assumed effective masses do not fit the current relationship between the Seebeck coefficient and the carrier density. With the combination of enhanced electrical conductivities and reduced thermal conductivities at high temperatures, the maximum thermoelectric (TE) figure of merit (ZT) of 0.81 has been achieved at 804 K with x = 0.1, which is about 1.65 and 2.9 times the value of current and reported intrinsic Cu₃Ga₅Te₉. The remarkable improvement in TE performance proves that we have succeeded in engineering the energy gap near the valence band edge upon Mn incorporation into Cu₃Ga₅Te₉.