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

Abstract

Because n-type Bi<sub>2</sub>Te<sub>3</sub>-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 Bi<sub>2</sub>Te<sub>3</sub>-based materials was improved by reducing thermal conductivity through the formation of a Bi<sub>2</sub>Te<sub>3</sub>-Bi<sub>2</sub>Se<sub>3</sub> solid solution, Bi<sub>2</sub>Te<sub>3-y</sub>Se<sub>y</sub> and optimizing the carrier concentration through doping. As the amount of Se increased in Bi<sub>2</sub>Te<sub>3-y</sub>Se<sub>y</sub>, the carrier concentration decreased, resulting in decreased electrical and thermal conductivities and increased Seebeck coefficients. As a result, Bi<sub>2</sub>Te<sub>2.85</sub>Se<sub>0.15</sub> exhibited ZT = 0.56 at 323 K, and Bi<sub>2</sub>Te<sub>2.4</sub>Se<sub>0.6</sub> exhibited ZT = 0.60 at 423 K. Among the Bi<sub>2</sub>Te<sub>3-y</sub>Se<sub>y</sub> solid solutions, the doping effect was investigated for Bi<sub>2</sub>Te<sub>2.85</sub>Se<sub>0.15</sub> and Bi<sub>2</sub>Te<sub>2.7</sub>Se<sub>0.3</sub>, 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 Bi<sub>2</sub>Te<sub>2.85</sub>Se<sub>0.15</sub>:I<sub>0.005</sub> and ZT = 1.13 at 423 K for Bi<sub>2</sub>Te<sub>2.7</sub>Se<sub>0.3</sub>:I<sub>0.0075</sub>. When the transition elements (Cu and Ag) were doped, the power factor was improved by the increase in Seebeck coefficient, and thereby Bi<sub>2</sub>Te<sub>2.85</sub>Se<sub>0.15</sub>:Ag<sub>0.01</sub> and Bi<sub>2</sub>Te<sub>2.85</sub>Se<sub>0.15</sub>:Ag<sub>0.01</sub> exhibited ZT = 0.76 and ZT = 0.75 at 323 K, respectively, and Bi<sub>2</sub>Te<sub>2.7</sub>Se<sub>0.3</sub>:Cu<sub>0.01</sub> 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 Bi<sub>2</sub>Te<sub>2.85</sub>Se<sub>0.15</sub>, leading to minimal or no improvement in ZT. Next, n-type Bi<sub>2</sub>Te<sub>2.85</sub>Se<sub>0.15</sub>:I<sub>0.005</sub> and Bi<sub>2</sub>Te<sub>2.7</sub>Se<sub>0.3</sub>:I<sub>0.0075</sub>, which exhibited the best thermoelectric performances, were fabricated as bulky compacts, and the uniformity of their thermoelectric properties were evaluated.