Temperature-Dependent Structural Variation and Cu Substitution in Thermoelectric Silver Selenide

Ag2Se is a narrow band gap semiconductor with glass-like thermal conductivity which makes it attractive for room temperature thermoelectric applications. However, its structural stability is strongly affected by compositional changes, which have led to difficulty in tuning the transport properties in the past. Here, we investigated the temperature-dependent structural stability of stoichiometric Ag2Se and its minute anion excess counterpart, Ag2SeS0.01, from room temperature up to the transition temperature (where low-temperature orthorhombic phase changes to high-temperature cubic phase at ∼407 K). The metastable structure, which is formed to compensate for the instability of the main orthorhombic structure in Ag2Se, was found to exist in the entire temperature range studied. However, no metastable structure was formed in the Ag2SeS0.01 sample, proving that minute anion excess is capable of stabilizing the orthorhombic structure throughout the temperature range of interest (300–400 K). The effect of Cu substitution on the thermoelectric properties of Ag2SeS0.01 was also investigated, and it was found that 1% Cu substitution ((Ag0.99Cu0.01)2SeS0.01) enhances carrier mobility and carrier concentration, compared to that for Ag2SeS0.01 and Ag2Se. As a result, up to 18% enhancement in S2/ρ (∼2.6 mW m–1 K–2 at 300 K and ∼2.8 mW m–1 K–2 at 375 K) was achieved compared to that of the non-Cu-substituted samples. The ZT (0.7 at 300 K), however, could not be improved compared to that of Ag2SeS0.01 (0.8 at 300 K) but remained higher than that of Ag2Se (0.5 at 300 K), due to the increase in electronic contribution to thermal conductivity.