Two-dimensional electron systems have attracted attention as thermoelectric materials, which can directly convert waste heat into electricity. It has been theoretically predicted that thermoelectric power factor can be largely enhanced when the two-dimensional electron layer is far narrower than the de Broglie wavelength. Although many studies have been made, the effectiveness has not been experimentally clarified thus far. Here we experimentally clarify that an enhanced two-dimensionality is efficient to enhance thermoelectric power factor. We fabricated superlattices of [N unit cell SrTi_1−xNb _x O₃|11 unit cell SrTiO₃]₁₀—there are two different de Broglie wavelength in the SrTi_1−xNb _x O₃ system. The maximum power factor of the superlattice composed of the longer de Broglie wavelength SrTi_1−xNb _x O₃ exceeded ∼5 mW m⁻¹ K⁻², which doubles the value of optimized bulk SrTi_1−xNb _x O₃. The present approach—use of longer de Broglie wavelength—is epoch-making and is fruitful to design good thermoelectric materials showing high power factor.

Thermoelectric technology has emerged as a prominent area of research in the past few decades for harnessing waste heat and improving the efficiency of next-generation renewable energy technologies. There has been rapid progress in the development of high-performance thermoelectric materials, as measured by the dimensionless figure of merit (ZT = S² · σ · κ⁻¹). Several heavy-metal-based thermoelectric materials with commercial-level performance (ZT = 1) have so far been proposed. However, the extensive application of these materials still faces challenges due to their low thermal/chemical stability, high toxicity, and limited abundance in the Earth’s crust. In contrast, oxide-based thermoelectric materials, such as ZnO, SrTiO₃, layered cobalt oxides, etc., have attracted growing interest as they can overcome the limitations of their heavy-metal-based counterparts. In this review, we summarize the recent research progress and introduce improvement strategies in oxide-based thermoelectric materials. This will provide an overview of their development history and design schemes, ultimately aiding in enhancing the overall performance of oxide-based thermoelectric materials.

Among many thermoelectric materials, oxide-based materials draw significant interest due to their environmental compatibility. In particular, layered cobaltite, Na_0.75CoO₂, shows a large thermoelectric power factor parallel to the layers. However, the thermal conductivity (κ) is rather high (5–7 W m⁻¹ K⁻¹), and therefore, its thermoelectric figure of merit ZT is small (∼0.03) at room temperature. Here we show that substituting the Na⁺ ion with the Ba²⁺ ion in Na_xCoO₂ drastically reduces the κ while keeping the large power factor, resulting in a large enhancement in ZT. We fabricated epitaxial films of Na_0.75CoO₂ by the reactive solid-phase epitaxy method and performed ion-exchange treatment from Na⁺ to Ba²⁺ to synthesize Ba_0.27CoO₂ films. The room temperature electrical conductivity (σ), thermopower (S), and power factor of the c-axis oriented Ba_0.27CoO₂ films along the in-plane direction were 2310 S cm⁻¹, +72 μV K⁻¹, and 1.2 mW m⁻¹ K⁻², respectively, while the κ along the in-plane, which was clarified by measuring the κ of the c-axis inclined (55°) film, was 3.3 W m⁻¹ K⁻¹. This yields a ZT (= S²σT/κ) value along the in-plane as high as 0.11 at room temperature, which is the highest among those reported for oxide thermoelectric materials except oxychalcogenide (BiO)(CuSe).

Here, we present a thermoelectric phase diagram for the SrTi_1−xNb_xO₃ (0.05 ≤ x ≤ 1) solid solution system, which we derived from the characterization of epitaxial films. We observed two thermoelectric phase boundaries in the system, which originate from the step-like decrease in carrier effective mass at x ∼ 0.3 and from a local minimum in carrier relaxation time at x ∼ 0.5. The origins of these phase boundaries are considered to be related to isovalent/heterovalent B-site substitution: parabolic Ti 3d orbitals dominate electron conduction for compositions with x < 0.3, whereas the Nb 4 d orbital dominates when x > 0.3. At x ∼ 0.5, a tetragonal distortion of the lattice, in which the B-site is composed of Ti⁴⁺ and Nb⁴⁺ ions, leads to the formation of tail-like impurity bands, which maximizes the electron scattering. These results provide a foundation for further research into improving the thermoelectric performance of SrTi_1−xNb_xO₃.

Thermoelectric energy conversion technology has attracted attention as an energy harvesting technology that converts waste heat into electricity by means of the Seebeck effect. Oxide-based thermoelectric materials that show a high figure of merit are promising because of their good chemical and thermal stability as well as their harmless nature compared to chalcogenide-based state-of-the-art thermoelectric materials. Although several high-ZT thermoelectric oxides (ZT > 1) have been reported thus far, the reliability is low due to a lack of careful observation of their stability at elevated temperatures. Here, we show a reliable high-ZT thermoelectric oxide, Ba_1/3CoO₂. We fabricated Ba_1/3CoO₂ epitaxial films by the reactive solid-phase epitaxy method (Na_3/4CoO₂) followed by ion exchange (Na⁺ → Ba²⁺) treatment and performed thermal annealing of the film at high temperatures and structural and electrical measurements. The crystal structure and electrical resistivity of the Ba_1/3CoO₂ epitaxial films were found to be maintained up to 600 °C. The power factor gradually increased to ∼1.2 mW m^-1 K^-2 and the thermal conductivity gradually decreased to ∼1.9 W m^-1 K^-1 with increasing temperature up to 600 °C. Consequently, the ZT reached ∼0.55 at 600 °C in air.