Nonreciprocal thermoelectric cooling by spatially grading Seebeck coefficient for cooling enhancement

Public summary

* An extended figure of merit was proposed considering the total electronic entropy change.

* A compositionally graded n-type Bi2Te2.7Se0.3 was grown to achieve spatial grading of the Seebeck coefficient.

* Nonreciprocal thermoelectric cooling and a 10% enhancement were realized in the as-grown single-leg device.

* The strategy can be applied to any dopable materials, providing a new pathway for thermoelectric cooling.

Abstract

Within a unified thermoelectric description, the Peltier and Thomson effects essentially involve changes in electronic entropy along the electron transport pathway, manifested as a spatial gradient in the Seebeck coefficient (∇xS). The former originates from the chemical potential gradient and the latter from the temperature gradient. However, the Thomson effect is usually negligible for a conventional thermoelectric device because ∇xS across the operating temperature range is typically small. This work proposes an extended figure of merit (zextended) considering the electronic entropy change throughout the bulk thermoelectric material, which originates from the total ∇S with respect to all relevant independent variables, including but not limited to temperature (T), chemical potential (μ), magnetic field (B), and others. Experimentally, the electronic entropy change was realized by spatially grading chemical potential in a compositionally graded n-type Bi2Te2.7Se0.3 single-leg device, resulting in nonreciprocity in thermoelectric cooling with respect to the electric current direction. By aligning the direction of ∇xμ with that of the electric current, nonreciprocal thermoelectric cooling yields a 10% improvement in the maximum temperature span (ΔTmax) compared to the cooler without nonreciprocity. Moreover, this work reveals a trade-off between maximizing additional heat absorption and preserving the optimal zT, providing a refined approach to cooling enhancement using graded materials. The strategy of nonreciprocal thermoelectric cooling can, in principle, be applied to any dopable material, introducing new degrees of freedom to improve thermoelectric cooling.