Functional Materials for Energy

Thermopower Enhancement in Designer Oxide Superlattices

A precise tuning of the 2D carrier density by using fractional δ-doping of 3d electrons at the interface of SrTiO3/LaTiO3 superlattices is found to improve greatly the thermoelectric properties of oxide heterostructures.

A layer-by-layer design of 2D oxide superlattices with precisely controlled interface compositions has improved the thermopower of oxide thermoelectrics by 300% compared to that of bulk counterparts. Controlling the 2D carrier density through a new materials design strategy is critical for developing highly efficient thermoelectrics.

For high-performance thermoelectrics, achieving a large value of the thermoelectric figure of merit (ZT) is greatly desirable. However, the controllability of ZT is always limited due to the difficulty in decoupling both the thermal conductivity and thermopower from the electrical conductivity. A deliberate crystal design capability enabled by advanced pulsed-laser epitaxy enhanced the thermopower of thin-film thermoelectrics by the 2D confinement effect, without a significant reduction in the electron mobility. The enhancement of thermopower resulted in a tripling of the power factor for a moderate value of carrier density. This result demonstrates that the low-dimensional effect together with the precise controllability of carrier characteristic is an important factor to be considered in oxide thermoelectric materials. This promising enhancement, which can be attributed to the anisotropic band structure in the 2D system, highlights the importance of materials discovery by design. 

W. S. Choi, H. Ohta, and H. N. Lee, “Thermopower Enhancement by Fractional Layer Control in 2D Oxide Superlattices,” Advanced Materials (2014).     DOI: 10.1002/adma.201401676

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