Bi8Se9: effective reduction of bipolar diffusion via increasing band gap

The binary Bi/Se system with the general formula [Bi-2](m)[Bi2Se3](n) exhibits intriguing properties, such as topological insulator and thermoelectric properties, and attracts intense attention. Herein, we found that the thermoelectric properties of BiSe at elevated temperatures have been deteriorated, because of the strong bipolar diffusion effect caused by its small band gap. Subsequently, with the aid of the crystal structure engineering strategy, we purposely involve less metallic [Bi-2] layers in the crystal structure, and successfully synthesize a new n-type Bi8Se9, and investigate, for the first time, its thermoelectric properties. Bi8Se9 exhibits a significantly widened band gap from 0.085 eV in BiSe to 0.146 eV that effectively reduces the bipolar thermal conductivity from the 0.37 W m(-1) K-1 in BiSe to 0.14 W m(-1) K-1. As a result, Bi8Se9 realizes a balance between the electrical resistance, the Seebeck coefficient, and a lower thermal conductivity, and achieves a ZT value of 0.40 at 673 K, which is twice that of BiSe at 673 K.