Inhibiting the bipolar effect via band gap engineering to improve the thermoelectric performance in n-type Bi2-Sb Te3 for solid-state refrigeration

To date,the benchmark Bi2Te3-based alloys are still the only commercial material system used for ther-moelectric solid-state refrigeration.Nonetheless,the conspicuous performance imbalance between the p-type Bi2-xSbxTe3 and n-type Bi2Te3-xSex legs has become a major obstacle for the improvement of cooling devices to achieve higher efficiency.In our previous study,novel n-type Bi2-xSbxTe3 alloy has been pro-posed via manipulating donor-like effect as an alternative to mainstream n-type Bi2Te3-xSex.However,the narrow bandgap of Bi2-xSbxTe3 provoked severe bipolar effect that constrained the further improvement of zT near room temperature.Herein,we have implemented band gap engineering in n-type Bi1.5Sb0.5Te3 by employing isovalent Se substitution to inhibit the undesired intrinsic excitation and achieve the dis-tinguished room-temperature zT.First,the preferential occupancy of Se at Te2 site appropriately enlarges the band gap,thereby concurrently improving the Seebeck coefficient and depressing the bipolar thermal conductivity.In addition,the Se alloying mildly suppresses the compensation mechanism and essentially preserves the already optimized carrier concentration,which maintains the peak zT near room tempera-ture.Moreover,the large strain field and mass fluctuation generated by Se alloying leads to the remark-able reduction of lattice thermal conductivity.Accordingly,the zT value of Bi1.5Sb0.5Te2.8Se0.2 reaches 1.0 at 300 K and peaks 1.1 at 360 K,which surpasses that of most well-known room-temperature n-type thermoelectric materials.These results pave the way for n-type Bi2-xSbxTe3 alloys to become a new and promising top candidate for large-scale solid-state cooling applications.