High‐Temperature Thermoelectricity in Narrow‐Gap Semiconductor SmS with Strong Electron‐Hole Asymmetry

High‐temperature thermoelectric (TE) materials are common wide‐gap semiconductors that are used in order to prevent the bipolar effect. Here, a potential high‐temperature n‐type TE material SmS with a simple NaCl structure that demonstrates a narrow band gap of ≈0.25 eV is reported. As expected, a temperature‐dependent carrier concertation is observed, which is attributed to the thermal activation of electrons from valence band edge to conduction band. Interestingly, the intrinsic activation does not cause any sign of a bipolar effect. Density functional theory calculations suggest that the phenomenon originates from the strong electron‐hole asymmetry in the electronic structure and the electron‐to‐hole conductivity ratio is as high as 700–900. As a result, the activated minority carriers barely participate in the TE transport and the maximum power factor reaches 1.41 mW K−2 m−1 at 1123 K. By further alloying with Se to reduce lattice thermal conductivity, a peak zT of ≈1.1 is obtained in Sm1.08S0.78Se0.22 at 1123 K, which is among the best n‐type high‐temperature thermoelectrics. This study proves high‐temperature TE materials can be found in narrow‐gap semiconductors, which significantly enriches the scope of possibilities for novel TE materials.