Enhanced Thermoelectric Performance of Nanostructured Cu2SnS3 (CTS) via Ag Doping

The present work aims to investigate the effect of Ag doping on the thermoelectric properties of Cu2SnS3 (CTS). Various Cu2Ag(x)Sn(1-x)S3 (0.05 <= x <= 0.25) samples were synthesized by mechanical alloying followed by spark plasma sintering, and their structural and transport properties were systematically investigated. The x = 0.15 sample presented a similar to 10-fold higher power factor than the undoped CTS. Although, the x = 0.125 sample had a lower power factor than the x = 0.15 sample, owing to its lower thermal conductivity, both the samples showed the highest zT similar to 0.8 at 723 K. This value is comparable to the best results available in the literature for earth-abundant and eco-friendly thermoelectric materials. Interestingly, the thermal conductivity of Cu2Ag(x)Sn(1-x)S3 samples increased with Ag substitution, which was further investigated using the first principles and ab initio molecular dynamics calculations. It was observed that the incorporation of Ag into the system decreases the root mean square displacement of the other cations and anions, reducing the scattering of phonons and thereby increasing the lattice thermal conductivity. Moreover, the calculations on the formation energy have revealed the reason for the structural transformation of CTS and similar diamond-like structures toward high symmetry polymorphs by external doping. The increase in zT is directly related to the optimization of the band gap and the weighted mobility, which have been investigated experimentally and using the first principle method.