Enhanced Thermoelectric Performance of Quaternary Cu_{2-2 x}Ag_{2 x}Se_{1- x}S _x Liquid-like Chalcogenides

Liquid-like binary Cu_2-δX (X = S, Se, and Te) chalcogenides and their ternary solid solutions have gained notable attention in thermoelectrics due to their interesting and abnormal thermal and electrical transport properties. However, previous studies mainly focus on a single element alloying at either an anion or cation site whereas the investigation on cation/anion co-alloying is very rare so far. Here, a series of quaternary Cu_{2-2 x}Ag_{2 x}Se_{1- x}S _x ( x = 0.01, 0.03, 0.05, 0.1, 0.15) liquid-like copper chalcogenide materials have been fabricated and the effects of Ag/S co-alloying on the thermoelectric properties of Cu₂Se have been systematically studied. It is found that all compounds are mixed phases at room temperature but single cubic phase at high temperatures. The introduction of Ag and S in Cu₂Se brings about a large mass fluctuation rather than strain field fluctuation that effectively suppresses the lattice thermal conductivity. Furthermore, on increasing the Ag and S contents, the high electrical conductivity of pristine Cu₂Se is well tuned to the optimal range derived from the single parabolic band model analysis. Consequently, a peak zT of 1.6 at 900 K is achieved in Cu_1.8Ag_0.2Se_0.9S_0.1, which is about 33% higher than that of binary Cu₂Se.