For comparison, we also plot the measured zT of Bi-doped Y 2 Te 3 (Y 2 Te 2.7 Bi 0.3 ) 10 in Figure . Although not explicitly measured, we predict that the electron concentrations of the samples reported by Zhu et al. are between 10 19 to 10 20 cm -3 as evidenced by the electrical conductivity (Figure ) and Seebeck coefficient (Figure ). However, the optimal electron concentration of Y 2 Te 3 is 1-2×10 20 cm -3 (Figure ), which means that Bi-doping is slightly suboptimal and results in a lower zT than I-doping (Figure ).