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

摘要

High-temperature thermoelectric (TE) materials are common wide-gapsemiconductors that are used in order to prevent the bipolar effect. Here,a potential high-temperature n-type TE material SmS with a simple NaClstructure that demonstrates a narrow band gap of ≈0.25 eV is reported. Asexpected, a temperature-dependent carrier concertation is observed, whichis attributed to the thermal activation of electrons from valence band edgeto conduction band. Interestingly, the intrinsic activation does not cause anysign of a bipolar effect. Density functional theory calculations suggest thatthe phenomenon originates from the strong electron-hole asymmetry in theelectronic structure and the electron-to-hole conductivity ratio is as high as700–900. As a result, the activated minority carriers barely participate in theTE 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 Sm_(1.08)S_(0.78)Se_(0.22) at 1123 K, which is among the best n-typehigh-temperature thermoelectrics. This study proves high-temperature TEmaterials can be found in narrow-gap semiconductors, which significantlyenriches the scope of possibilities for novel TE materials.