Tellurium-free thermoelectrics: Improved thermoelectric performance of n-type Bi2Se3 having multiscale hierarchical architecture

Abstract

We report an improved thermoelectric performance of n-type Bi2Se3 bulk alloys synthesized by vacuum melt method followed by vacuum hot-pressing. In the samples so prepared, the synergetic combination of ultra low thermal conductivity (∼0.7W/mK), high Seebeck coefficient (∼−168μV/K), and low electrical resistivity (∼15μΩ-m) has been observed to successfully lead to a high figure-of-merit (ZT) of ∼0.96 at 370K. A detailed characterization of the samples reveals a presence of multiscale hierarchical defect structures i.e. atomic scale disorder arising from a multitude of factors such as large anharmonicity of Bi-Se bond due to electrostatic repulsion between the lone pair of Bi and charge of Se, nanoscale grains and dislocations trapped between mesoscale grains/grain boundaries accompanied by intrinsic layered structure of Bi2Se3. This compact layered grain structure in its consequence offers a high charge carrier mobility and thereby results into a high power factor, while multiscale hierarchical architecture accounts for the scattering of a wider spectrum of phonons leading to an ultra low thermal conductivity. In view of this promising thermoelectric performance together with the presence of copiously available constituent namely Se, the hot-pressed Bi2Se3 presents a technologically suitable and commercially viable alternative to the conventional Bi2Te3 which is based on expensive and scarcely available Te.