Ultrahigh thermoelectric performance in Cu2−ySe0.5S0.5 liquid-like materials

Abstract

Liquid-like thermoelectric materials have recently received heightened attentions due to their exceptional thermal and electrical transport properties. As a typical example, Cu2−ySe has good electrical transport properties while Cu2−yS has extremely low lattice thermal conductivity. Combining these stirring characters into one material is expected to result in excellent thermoelectric performance. In this study, we found that Cu2−ySe and Cu2−yS can form a solid solution in the composition range down to half Se and half S. XRD, SEM and TEM reveal that Cu2−ySe0.5S0.5 possesses a unique hierarchical microstructure composed of mesoscale polymorphs, nanoscale domains and modulations. Besides, the liquid-like copper ions at high temperature not only strongly scatter lattice phonons but also eliminate some of the transverse phonon vibrations. Combining with the extraordinarily low sound speeds, an overall ultralow thermal conductivity is achieved in Cu2−ySe0.5S0.5 with the values similar to that in Cu2S. Furthermore, the electrical transport performance of Cu2−ySe0.5S0.5 is significantly improved through tuning its native Cu vacancies. High electrical power factors similar to or even superior to Cu2−ySe are observed due to the high weighted mobility. All these favorable factors lead to much enhanced quality factor and thus remarkably high thermoelectric performance in Cu2−ySe0.5S0.5, which reaches a ZT of 2.3 at 1000 K, among the highest values in bulk materials.