Control of phonon transport by the formation of the Al₂O₃ interlayer in Al₂O₃–ZnO superlattice thin films and their in-plane thermoelectric energy generator performance

Abstract

Recently, significant progress has been made in increasing the figure-of-merit (ZT) of various nanostructured materials, including thin-film and quantum dot superlattice structures. Studies have focused on the size reduction and control of the surface or interface of nanostructured materials since these approaches enhance the thermopower and phonon scattering in quantum and superlattice structures. Currently, bismuth-tellurium-based semiconductor materials are widely employed for thermoelectric (TE) devices such as TE energy generators and coolers, in addition to other sensors, for use at temperatures under 400 K. However, new and promising TE materials with enhanced TE performance, including doped zinc oxide (ZnO) multilayer or superlattice thin films, are also required for designing solid-state TE power generating devices with the maximum output power density and for investigating the physics of in-plane TE generators. Herein, we report the growth of AlO/ZnO (AO/ZnO) superlattice thin films, which were prepared by atomic layer deposition (ALD), and the evaluation of their electrical and TE properties. All the in-plane TE properties, including the Seebeck coefficient (S), electrical conductivity (σ), and thermal conductivity (κ), of the AO/ZnO superlattice (with a 0.82 nm-thick AO layer) and AO/ZnO films (with a 0.13 nm-thick AO layer) were evaluated in the temperature range 40-300 K, and the measured S, σ, and κ were -62.4 and -17.5 μV K, 113 and 847 (Ω cm), and 0.96 and 1.04 W m K, respectively, at 300 K. Consequently, the in-plane TE ZT factor of AO/ZnO superlattice films was found to be ∼0.014, which is approximately two times more than that of AO/ZnO films (ZT of ∼0.007) at 300 K. Furthermore, the electrical power generation efficiency of the TE energy generator consisting of four couples of n-AO/ZnO superlattice films and p-BiSbTe (p-BST) thin-film legs on the substrate was demonstrated. Surprisingly, the output power of the 100 nm-thick n-AO/ZnO superlattice film/p-BST TE energy generator was determined to be ∼1.0 nW at a temperature difference of 80 K, corresponding to a significant improvement of ∼130% and ∼220% compared to the 100 nm-thick AO/ZnO film/p-BST and n-BT/p-BST film generators, respectively, owing to the enhancement of the TE properties, including the power factor of the superlattice film.

Improving the Efficiency of Thin Film Thermoelectric Generators under Constant Heat Flux by Using Substrates of Low Thermal Conductivity

“…For more information about real contacts on thin film thermoelectric generator with heat flux parallel to the films we refer to refs. . At this point we would like to emphasize that the goodness of this theoretical results will be the maximum limit at which real devices could aspire under the described conditions.…”

“…This approach has been indirectly suggested as a conclusion on the work of Yamamoto et al, where the effect of the thermoelectric film thickness on the output power is discussed but without a deep analysis of the role of the substrate and the relationship between thicknesses and thermal conductivities of both thin film and substrate. In addition, other authors as Park et al remark the importance of the power factor on this kind of in‐plane thermoelectric generators, in this case by comparing the behavior of Al 2 O 3 /ZnO thin films and superlattices on the same substrate. In this work, we present a theoretical proposal of the relationship between film and substrates thicknesses and thermal conductivities in order to maximize this decoupling when operating under constant flux conditions.…”

“…A less common configuration with the heat flow running parallel to the thin film surface (Figureb) overcomes this drawback, avoiding the use of extra energy and, subsequently, increasing the efficiency. We propose the use of this last configuration to get a further improvement of the thin film thermoelectric generator efficiency through an effective “decoupling” of the thermal and electrical parameters.…”

Flexible thermoelectric modules based on ALD-grown ZnO on different substrates

“…24 So far, ALD-grown thin films of both conventional and hybrid materials have been investigated for their basic thermoelectric properties, but there have been few reports on the performance of thermoelectric modules made using ALD (Ref. 25) or ALD/MLD. 26 This is most likely because of the difficulty in achieving high power outputs due to the small temperature gradients obtained with thin-film materials.…”

A sandwich structure assisted by defect engineering for higher thermoelectric performance in ZnO‐based films

“…Consequently, with excellent electrical conductivity originated from higher carrier concentration and Seebeck coefficient due to large DOS effective mass, GZO-3 nm ZnO-GZO-V thin film presents better power factor values over the entire temperature range with the highest value of 434 μW m −1 K −2 at 623 K, as shown in Figure 7A. Compared with other ZnO-based thin films taken from the references 10,[29][30][31][32] (Figure 7B), the PF value of our GZO-ZnO-GZO-V thin film is still the highest and competitive not only at higher temperatures but also at room temperature. As a result, sandwich structure design combined with defect engineering is an effective approach to realize simultaneous increase of the Seebeck coefficient and electrical conductivity and thus further improve the thermoelectric performance for ZnO-based thin films.…”