Enhancement of the Thermoelectric Performance of Cu₂GeSe₃ via Isoelectronic (Ag, S)-co-substitution

Recently, ternary Cu-based Cu-IV-Se (IV = Sb, Ge, and Sn) compounds have received extensive attention in the thermoelectric field. Compared with Cu-Sb-Se and Cu-Sn-Se, Cu-Ge-Se compounds have been less studied due to its poor Seebeck coefficient and high thermal conductivity. Here, the Cu2GeSe3 material with high electrical conductivity was first prepared, and then, its effective mass was increased by doping with S, which led to the Seebeck coefficient of the doped sample being 1.93 times higher than that of pristine Cu2GeSe3 at room temperature. Moreover, alloying Ag at the Cu site in the Cu2GeSe2.96S0.04 sample could further cause a 5.16 times increase in the Seebeck coefficient at room temperature, and the lattice thermal conductivity was remarkably decreased because of the introduction of the dislocations in the Cu2GeSe3 sample. Finally, benefitted from the high Seebeck coefficient and low thermal conductivity, a record high ZT = 0.9 at 723 K was obtained for the Cu1.85Ag0.15GeSe2.96S0.04 sample, which increased 345% in comparison with the pristine Cu2GeSe3, and it is among the highest reported values for Cu2GeSe3-based thermoelectric.