Cu_xS superionic compounds: Electronic structure and thermoelectric performance enhancement

In the present work, we have investigated the effect of Cu content (x) on the phase structure and thermoelectric transport properties of Cu_xS (x = 1.92, 1.94, 1.98) bulks fabricated by combining mechanical alloying and spark plasma sintering. From density functional theory calculations of the valence band energy levels, we hypothesized that favourable valence band alignments accelerate electrical transport and can be achieved by optimizing the molar ratio of Cu_1.96S to Cu₂S. Experimentally, the sample with x = 1.92 showed an improved electrical conductivity 538.7 Scm⁻¹ at 623 K and retained a good Seebeck coefficient, synergistically leading to a broad, high power factor ranging from 105.9 to 900.3 μWm⁻¹K⁻². The highly-disturbed Cu ions give Cu_xS compounds an intrinsically low lattice thermal conductivity (∼0.5 Wm⁻¹K⁻¹), which endows a high ZT of 1.23 as x = 1.94 to this otherwise simple semiconductor. The results indicate that low-cost and non-toxic Cu_xS superionic compounds are promising thermoelectric materials.