Using semi-classical Boltzmann transport theory, we deduce three important material descriptors for TE performance: (1) band gap, (2) nand p-type TE quality factors, and (3) asymmetry of the quality factors. In wider-gap semiconductors, a large TE quality factor (n-or p-type) translates to higher TE performance, thereby recovering the standard result for materials with negligible bipolar conduction. However, for narrow-gap semiconductors and semimetals, high TE performance critically depends on the asymmetry ratio between the individual TE quality factors. This is a direct consequence of the asymmetry in the transport functions (Figure ), which suppresses the e↵ects of bipolar conduction. Finally, smaller or negative band gaps lead to lower TE performance due to increased bipolar conduction. We then apply the proposed material descriptors to search for new TE materials within a set of narrow-gap and semimetallic Zintl phases. We compute the parameters in the material descriptors with density functional theory (DFT) and semi-empirical models for transport. In addition to the Zintl phases, we also calculated the material descriptors for Mg 3 Bi 2 , which is a wellknown semimetallic TE material used as a benchmark material in this study. We show that Mg 3 Bi 2 is indeed a high-performing TE material near room temperature, which validates the predictive power of the descriptors. Through this search, we identify four narrow-gap/semi-metallic Zintl phases (SrSb 2 , Zn 3 As 2 , NaZnSb, NaCdSb) with promising room-temperature TE performance.