Enhanced Power Factor and Figure of Merit of Cu₂ZnSnSe₄-Based Thermoelectric Composites by Ag Alloying

The quaternary chalcogenide composites Cu₂ZnSn_1-AgSe₄ (0 ≤ x ≤ 0.075) have been successfully synthesized by high-temperature melting and annealing followed by hot-pressing. The phase structure of the bulk sample has been analyzed by powder X-ray diffraction and Rietveld refinement combined with Raman spectroscopy to confirm Cu₂ZnSnSe₄ as the main phase with ZnSe and Cu₅Zn₈ secondary phases. The thermoelectric properties of all specimens have been investigated in the temperature range of 300-700 K. The replacement of Sn by Ag significantly enhances the electrical transport properties by providing extra charge carriers. The tremendous reduction in electrical resistivity enhances the power factor, and a maximum power factor of 804 μW K^-2 m^-1 is achieved at 673 K for the specimen with 5% Ag content. Furthermore, increased point defects increase phonon scattering, resulting in reduced thermal conductivity. The combined effect of improved power factor and suppressed thermal conductivity provides a good boost to the dimensionless figure of merit. The maximum figure of merit of zT = 0.25 has been achieved at 673 K for Cu₂ZnSn_0.95Ag_0.05Se₄, which is 2.5 times the value of the parent sample.