Material Descriptors to Predict Thermoelectric Performance of Narrow-gap Semiconductors and Semimetals

Thermoelectric (TE) cooling is an environment-friendly alternative to vapor compression cooling. Narrow-gap semiconductors and semimetals have garnered interest for Peltier cooling, especially following the discovery of Mg3Bi2-based materials. New TE materials with high coefficients of performance are needed to further advance this technology. Computations have enabled the discovery and design of new TE materials. Large-scale computational searches often rely on material descriptors, which are based on the single-band model that does not account for bipolar conduction effects. In this work, we derive three material descriptors to assess the TE performance of narrow-gap semiconductors and semimetals -- band gap, n- and p-type TE quality factors, and an asymmetry parameter. These computationally-accessible descriptors are derived from Boltzmann transport theory applied to a two-band model. We show that a large asymmetry parameter is critical to achieving high TE performance through suppression of bipolar conduction. We validate the predictive power of the descriptors by correctly identifying Mg3Bi2 as a high-performing room-temperature TE material. By applying these descriptors to a broader set of 650 Zintl phases, we identify four candidate materials for room-temperature TEs, namely, SrSb2, Zn3As2, NaZnSb, and NaCdSb. The proposed material descriptors will enable fast targeted search of narrow-gap semiconductors and semimetals for low-temperature TEs.