Thermoelectric properties and chemical potential tuning by Cu-doping in n-type ionic conductors CuxAg2−xSe0.5Te0.5

Here, we prepare polycrystalline CuxAg2−xSe0.5Te0.5 (x = 0.01, 0.05, 0.1) samples using a high temperature melting followed by hot-press sintering and characterize their thermoelectric properties. We demonstrate that the Cu substitution for Ag is achieved by < 10% in Cu content for CuxAg2−xSe0.5Te0.5 and the Cu doping is quite effective for a significant enhancement in n-type carrier density, which is one order of magnitude higher than the pristine Ag2Se0.5Te0.5 (4.10 × 1018 cm−3). Impressively, the enhancement in the electrical conductivity with increasing Cu content is more considerable than the decrease in absolute value of Seebeck coefficient in the superionic conduction state, leading to a relatively high power factors ranging between 1.10 and 1.30 mW m−1 K−2 at a broad temperature range of 400-560 K for Cu0.1Ag1.99Se0.5Te0.5 and thus its highest ZT of 0.85 at 560 K. Furthermore, ZT values approach to >0.7 over a wide temperature range of 460-560 K for x > 0.05. We suggest that the unusual Cu doping effect in Ag2Se0.5Te0.5 should be attributed to the creation of Cu ion conduction besides Ag ion conduction as well as the optimization of its n-type carrier density. Temperature-dependent (a) power factor PF (= S2σ) and (b) ZT values of the CuxAg2−xSe0.5Te0.5 (x = 0.01, 0.05, 0.1) samples.125