Isotropic Thermoelectric Performance of Layer-Structured n-Type Bi₂Te_2.7Se_0.3 by Cu Doping

The lamellar structure of (Bi,Sb)₂(Te,Se)₃ alloys makes it difficult to achieve isotropic thermoelectric properties in the directions along and perpendicular to the c-axis, especially for n-type samples. In this work, by introducing Cu in polycrystalline n-type CuBi₂Te_2.7Se_0.3 and applying the traditional synthesis process of high-energy ball milling and hot pressing, substantial enhancement of the thermoelectric figure of merit zT is obtained in both in-plane and out-of-plane directions. The intercalated Cu not only provides electron transport media for mobility improvement but also reduces the lattice thermal conductivity owing to the strain fluctuation. Typically, the van der Waals gap in the out-of-plane direction leads to relatively slower mobility and lower lattice thermal conductivity. Taking into account the same average density-of-state effective mass (m_avg^* ∼ 1.5m_e) predicted based on a single parabolic model, the obtained quality factor β is comparable in both directions. As a result, a peak zT ∼ 1.05 at 420 K and the average zT approaching to 1.0 in the temperature range 300-500 K are obtained in both directions for the Cu_0. 02Bi₂Te_2.7Se_0.3 sample. The simple synthesis process and isotropic thermoelectric properties in this work make n-type Bi₂Te₃ more convenient for potential production and application.