In-situ elemental reaction-regulated Ag2S films enable the best thermoelectric performances

Abstract

Silver sulfide thin film, with excellent thermoelectric properties, is few reported due to the complex and time-consuming high-temperature or high-pressure synthesis process. Here, a fast ionic conductor n-type Ag₂S film with good crystallinity and uniform density is prepared by sputtering metal Ag films of different thicknesses on glass and then reacting in S precursor solution at low temperature. At 450 K, β-Ag₂S films can be obtained and underwent a phase transition from α-Ag₂S monoclinic, which had a significant effect on their electrical and thermal properties. The grain size of Ag₂S films increases with the increase of film thickness. Before and after the phase transition, the carrier concentration and mobility cause obvious changes in the electrical properties of Ag₂S. The carrier concentration of body-centered cubic phase β-Ag₂S is about three orders of magnitude higher than that of monoclinic phase α-Ag₂S, and the mobility is also 2–3 times that of the latter. Especially, after the phase transition, the conductivity of β-Ag₂S rises exponentially from the zero conductivity of α-Ag₂S and increases with the increase of temperature. The Ag₂S film shows the highest figure of merit of 0.83 ± 0.30 at 600 K from the sample with ∼1600 nm thickness, which is the highest record among Ag₂S-based thermoelectric materials reported so far.