Ultralow Lattice Thermal Conductivity and Improved Thermoelectric Performance in Cl-Doped Bi 2 Te 3- x Se x Alloys.

Bi 2 Te 3 -based alloys are the main materials for the construction of low- and medium-temperature thermoelectric modules. In this work, the microstructure and thermoelectric properties of Cl-doped Bi 2 Te 3- x Se x alloys were systematically investigated considering the high anisotropy inherent in these materials. The prepared samples have a highly oriented microstructure morphology, which results in very different thermal transport properties in two pressing directions. To accurately separate the lattice, electronic, and bipolar components of the thermal conductivity over the entire temperature range, we employed a two-band Kane model to the Cl-doped Bi 2 Te 3- x Se x alloys. It was established that Cl atoms act as electron donors, which tune the carrier concentration and effectively suppress the minority carrier transport in Bi 2 Te 3- x Se x alloys. The estimated value of the lattice thermal conductivity was found to be as low as 0.15 Wm -1 K -1 for Bi 2 Te 3- x - y Se x Cl y with x = 0.6 and y = 0.015 at 673 K in parallel to the pressing direction, which is among the lowest values reported for crystalline materials. The large reduction of the lattice thermal conductivity in both pressing directions for the investigated Bi 2 Te 3- x Se x alloys is connected with the different polarities of the Bi-(Te/Se)1 and Bi-(Te/Se)2 bonds, while the lone-pair (Te/Se) interactions are mainly responsible for the extremely low lattice thermal conductivity in the parallel direction. As a result of the enhanced power factor, suppressed bipolar conduction, and ultralow lattice thermal conductivity, a maximum ZT of 1.0 at 473 K has been received in the Bi 2 Te 2.385 Se 0.6 Cl 0.015 sample.