Uniformity of Thermoelectric Properties of N-type Bi<sub>2</sub>Te<sub>3-y</sub>Se<sub>y</sub> Bulky Compacts

Because n-type Bi₂Te₃-based materials exhibit lower thermoelectric performance than p-type materials and because their thermoelectric properties are sensitively changed by the composition and carrier concentration, optimizing these aspects in n-type materials is necessary to improve the thermoelectric figure of merit (ZT). In this study, the thermoelectric performance of n-type Bi₂Te₃-based materials was improved by reducing thermal conductivity through the formation of a Bi₂Te₃-Bi₂Se₃ solid solution, Bi₂Te_3-ySe_y and optimizing the carrier concentration through doping. As the amount of Se increased in Bi₂Te_3-ySe_y, the carrier concentration decreased, resulting in decreased electrical and thermal conductivities and increased Seebeck coefficients. As a result, Bi₂Te_2.85Se_0.15 exhibited ZT = 0.56 at 323 K, and Bi₂Te_2.4Se_0.6 exhibited ZT = 0.60 at 423 K. Among the Bi₂Te_3-ySe_y solid solutions, the doping effect was investigated for Bi₂Te_2.85Se_0.15 and Bi₂Te_2.7Se_0.3, which recorded excellent thermoelectric performance at low temperatures. When halogen element (I) was doped, the power factor improved owing to the increase in carrier concentration, and the thermal conductivity decreased. As a result, the ZT values were greatly enhanced to ZT = 0.90 at 423 K for Bi₂Te_2.85Se_0.15:I_0.005 and ZT = 1.13 at 423 K for Bi₂Te_2.7Se_0.3:I_0.0075. When the transition elements (Cu and Ag) were doped, the power factor was improved by the increase in Seebeck coefficient, and thereby Bi₂Te_2.85Se_0.15:Ag_0.01 and Bi₂Te_2.85Se_0.15:Ag_0.01 exhibited ZT = 0.76 and ZT = 0.75 at 323 K, respectively, and Bi₂Te_2.7Se_0.3:Cu_0.01 exhibited ZT = 0.73 at 423 K. Conversely, doping with other transition elements (Ni and Zn), as well as group-III (Al and In) and group-IV (Ge and Sn) elements, resulted in power factors and thermal conductivities that were similar to or slightly less than those of undoped Bi₂Te_2.85Se_0.15, leading to minimal or no improvement in ZT. Next, n-type Bi₂Te_2.85Se_0.15:I_0.005 and Bi₂Te_2.7Se_0.3:I_0.0075, which exhibited the best thermoelectric performances, were fabricated as bulky compacts, and the uniformity of their thermoelectric properties were evaluated.