(ii) Large p (or n ) indicates high TE performance in the absence of bipolar conduction. (iii) TE performance of narrow-gap semiconductors and semimetals with R close to 1 (symmetric) su↵er from bipolar conduction e↵ects. When the asymmetry in the electron and hole transport properties is large (| log(R )| >> 0), the materials exhibit superior TE performance because bipolar conduction e↵ects are suppressed.