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

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.