Thermoelectric quaternary sulfide Cu2+ x Zn1− x SnS4 (x = 0–0.3): Effects of Cu substitution for Zn

Highlights: • Thermoelectric properties were investigated for Cu_2+xZn_1−xSnS₄ (x = 0–0.3) kesterite. • The substitution of Cu for Zn led to the significant enhancement of power factor. • A drastic increase in the power factor was observed at temperatures above 450–550 K. • Effects of the Cu substitution on the electronic properties were discussed. • Dimensionless figure of merit (ZT) reached 0.36 at 673 K for the x = 0.3 sample. Kesterite Cu₂ZnSnS₄ has attracted increasing attention owing to its high potential as a thermoelectric material and a unique order–disorder-type structural transformation at high temperatures. For both characteristics, their composition dependences should be investigated to gain insight into the way for improving the thermoelectric performance. Therefore, we synthesized the wide composition range of Cu_2+xZn_1−xSnS₄ (x = 0, 0.1, 0.2, and 0.3) of kesterite samples and assessed their thermoelectric properties up to 673 K. The polycrystalline samples were prepared by the direct reaction of the constituent elements followed by spark plasma sintering. The sintered samples were dense and single phased. The substitution of Cu for Zn increased the hole carrier concentration, and thereby the electrical conductivity increased. For all samples, a simultaneous increase in the electrical conductivity and the Seebeck coefficient was observed at 450–550 K, which can be attributed to the structural transformation. The resultant increase in the thermoelectric power factor became more pronounced at x ≥ 0.1. These effects of the substitution increased the power factor at 673 K to 643 µW K⁻² m⁻¹ at x = 0.3. The thermal conductivity was changed only slightly by the substitution because the reduction in the lattice component was counterbalanced by an increase in the electronic component. As a result, thanks to the increased power factor, the dimensionless thermoelectric figure of merit ZT was enhanced to 0.36 at 673 K for the x = 0.3 sample.