Yuan Yu

Bismuth sulfide (Bi 2 S 3 ) is a promising thermoelectric material with earth‐abundant, low‐cost, and environment‐friendly constituents. However, it shows poor thermoelectric performance due to its extremely low electrical conductivity derived from the low electron concentration. Here, a high‐performance Bi 2 S 3 ‐based material is reported to benefit from the Fermi level tuning by Ag and Cl co‐doping and defect engineering by introducing dense low‐angle grain boundaries. Both Ag and Cl act as donors in Bi 2 S 3 , upshifting the Fermi level. This increases the electron concentration without degrading the electron mobility, thereby obtaining improved electrical conductivity. The electron localization function (ELF) contour map indicates that interstitial Ag causes electron delocalization, showing higher electron mobility in Bi 2 S 3 . More importantly, dense low‐angle grain boundaries block phonon propagation, yielding an ultralow lattice thermal conductivity of 0.30 W m −1 K −1 . Consequently, a record ZT value of ≈0.9 at 676 K is achieved in the Bi 2 Ag 0.01 S 3 ‐0.5%BiCl 3 sample.