Improved thermoelectric properties of n-type Bi₂Te₃ alloy deriving from two-phased heterostructure by the reduction of CuI with Sn

In this report, CuI and Sn co-doped n-type Bi₂Te₃ samples have been prepared by a high-temperature solid-state reaction, and the effect of co-doping on the thermoelectric properties was investigated from room temperature to 525 K. Sn single-doped and undoped Bi₂Te₃ were prepared for comparison. Detailed charge transport data including electrical conductivity, Seebeck coefficient, Hall coefficient, and thermal conductivity are presented. Microscopic observation of CuI/Sn co-doped samples revealed that numerous distinctive microstructures such as nanoprecipitates of the Cu and SnI-rich phase were generated in the matrix. The lattice thermal conductivity of CuI/Sn co-doped Bi₂Te₃ was substantially reduced compared to those of undoped and single doped Bi₂Te₃. Benefiting from the improved electrical transport properties by doping and the reduced lattice thermal conductivity by numerous microstructures, the ZT value of the Bi₂Te₃ doped with 1 at.% CuI/Sn is distinctly enhanced to 1.24 at 425 K. The average ZT value (ZT_ave ~ 1.02) at 300–525 K was clearly higher than those of Sn-doped Bi₂Te₃ (ZT_ave ~ 0.54) and CuI-doped Bi₂Te₃ (ZT_ave ~ 0.98). This work indicates that the average ZT can be improved over a broad temperature range using a co-doping approach.