Dual‐Site Doping and Low‐Angle Grain Boundaries Lead to High Thermoelectric Performance in N‐Type Bi2S3

Bismuth sulfide (Bi2S3) 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 Bi2S3‐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 Bi2S3, 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 Bi2S3. 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 Bi2Ag0.01S3‐0.5%BiCl3 sample. The thermoelectric performance of bismuth sulfide (Bi2S3) is greatly improved by the dual doping of Ag and Cl. These dopants move the Fermi level into the conduction band, increasing the carrier concentration and valley degeneracy while maintaining large carrier mobility due to electron delocalization. They also modify the surface energy to create dense low‐angle grain boundaries, significantly suppressing the lattice's thermal conductivity.