Enhanced cryogenic thermoelectric cooling of Bi0.5Sb1.5Te3 by carrier optimization

As the best-performing materials for thermoelectric cooling, Bi₂Te₃-based alloys have long attracted attention to optimizing the room-temperature performance of Bi₂Te₃ for both power generation and refrigeration applications. This focus leads to less emphasis and fewer reports on the cooling capability below room temperature. Given that the optimal carrier concentration (n_opt) for maximizing the cooling power is highly temperature dependent, roughly following the relationship n_opt∝T^3/2, lowering the carrier concentration is essential to improve the cooling capability at cryogenic temperatures. Taking p-type Bi_0.5Sb_1.5Te₃ as an example, careful control of doping in this work enables a reduction in carrier concentration to 1.7 × 10¹⁹ cm⁻³ from its optimum at 300 K of 3.4 × 10¹⁹ cm⁻³. This work successfully shifts the temperature at which the thermoelectric figure of merit (zT) peaks down to 315 K, with an average zT as high as 0.8 from 180 to 300 K. Further pairing with commercial n-type Bi₂Te₃-alloys, the cooling device realizes a temperature drop as large as 68 K from 300 K and 24 K from 180 K, demonstrating the extended cooling capability of thermoelectric coolers at cryogenic temperatures.