Typically, carrier concentrations on the order of 10 19 -10 21 cm -3 are needed to fully optimize the performance of most thermoelectric materials. Therefore, we explore plausible extrinsic dopants to further enhance the electron concentration in n-type Y 2 Te 3 . We find that halogens (Cl, Br, I) can increase the electron concentration under the most Y-rich and halogen-poor conditions, when Y 2 Te 3 is in three-phase equilibrium with YTe and YX 3 (X = Cl, Br, or I). As shown in Figure , halogens preferentially substitute on the Te site and create 1-electron shallow donor states. Halogen doping does not introduce mid-gap states, and the generated electrons are therefore available as freely conducting electrons. We predict that the maximum achievable free electron concentrations are 1.1 × 10 20 cm -3 (Cl doping), 1.5 × 10 20 cm -3 (Br doping), and 2.0 × 10 20 cm -3 (I doping). The trend in the electron concentrations can be understood in terms of the halogen solubilities -the lower the formation energy of substitutional X Te defects, the higher is the solubility and concentration of generated electrons. An additional benefit of halogen doping is that the conduction band edge is composed of Y-d states (Figure ); accordingly, substitution on the Te site does not significantly perturb the conduction band dispersion, and electron mobility will not be severely affected by doping.