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

Abstract

• Se replacement can enlarge the E g and thus effectively mitigate the bipolar effect. • The manipulation of donor-like effect retains the optimized carrier concentration. • The large strain field and mass fluctuation reduce lattice thermal conductivity. • A state-of-the-art zT = 1.0 at 300 K is attained for n -type Bi 1.5 Sb 0.5 Te 2.8 Se 0.2 . • n -type Bi 2- x Sb x Te 3 becomes a promising candidate for solid-state cooling field. To date, the benchmark Bi 2 Te 3 -based alloys are still the only commercial material system used for thermoelectric solid-state refrigeration. Nonetheless, the conspicuous performance imbalance between the p -type Bi 2- x Sb x Te 3 and n -type Bi 2 Te 3- x Se x legs has become a major obstacle for the improvement of cooling devices to achieve higher efficiency. In our previous study, novel n -type Bi 2- x Sb x Te 3 alloy has been proposed via manipulating donor-like effect as an alternative to mainstream n -type Bi 2 Te 3- x Se x . However, the narrow bandgap of Bi 2- x Sb x Te 3 provoked severe bipolar effect that constrained the further improvement of zT near room temperature. Herein, we have implemented band gap engineering in n -type Bi 1.5 Sb 0.5 Te 3 by employing isovalent Se substitution to inhibit the undesired intrinsic excitation and achieve the distinguished room-temperature zT . First, the preferential occupancy of Se at Te 2 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 temperature. Moreover, the large strain field and mass fluctuation generated by Se alloying leads to the remarkable reduction of lattice thermal conductivity. Accordingly, the zT value of Bi 1.5 Sb 0.5 Te 2.8 Se 0.2 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 Bi 2- x Sb x Te 3 alloys to become a new and promising top candidate for large-scale solid-state cooling applications.