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 Ag 2 S 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 ‐Ag 2 S films can be obtained and underwent a phase transition from α ‐Ag 2 S monoclinic, which had a significant effect on their electrical and thermal properties. The grain size of Ag 2 S 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 Ag 2 S. The carrier concentration of body‐centered cubic phase β ‐Ag 2 S is about three orders of magnitude higher than that of monoclinic phase α ‐Ag 2 S, and the mobility is also 2–3 times that of the latter. Especially, after the phase transition, the conductivity of β ‐Ag 2 S rises exponentially from the zero conductivity of α ‐Ag 2 S and increases with the increase of temperature. The Ag 2 S 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 Ag 2 S‐based thermoelectric materials reported so far.