Enhanced thermoelectric performance of Ga-doped ZnO film by controlling crystal quality for transparent thermoelectric films

Highlights

• We studied transparent thermoelectric Ga-doped ZnO films by PLD and sol-gel method.

• Both GZO films had highly (0001)-oriented nanometric crystal domains.

• They have higher power factors and lower thermal conductivities than GZO bulks.

Abstract

ZnO, a wide bandgap (3.3 eV) semiconductor has been expected to be a transparent thermoelectric material for the purpose of energy harvesting application because it is a low-cost and ubiquitous element material with a high optical transmittance and a high power factor. Bulk Ga-doped ZnO (GZO) is expected to have higher electrical conductivity and lower thermal conductivity than bulk Al-doped ZnO. However, because reports on their thermoelectric properties of GZO films have been scarce up to now, it has been unclear what film characters affect the thermoelectric properties effectively. In this work, GZO thin films with different characters (c-axis orientation, crystal domains and the domain interfaces, and carrier activation rate) were fabricated by two different methods, sol-gel method and pulsed laser deposition. All samples have optical transmittance over 80% in visible range. The highly-oriented GZO films exhibit the highest power factors up to 2.8 μWcm⁻¹ K⁻² in the reported GZO materials and low thermal conductivities of 8.4 Wm⁻¹ K⁻¹ (1/4 as high as that of bulk GZO). This enhanced thermoelectric performance is attributed to the high carrier activation rate, and the interfaces of highly-oriented crystal domains with the small carrier scattering effect.

Introduction

Thermoelectric materials have attracted much attention as an application for energy harvesting from global waste heat sources, such as stand-alone power sources of sensors for IoT society or supplemental clean energy sources for solving global warming issue. Among many applications of thermoelectric materials, we focused on transparent thermoelectric film devices for opening up an application to the stand-alone energy sources placed on the hot glass windows of the building and cars, and on displays of PC or smart phone. Materials for the above applications have to satisfy the following conditions. First, they must be thermoelectric films with high thermoelectric dimensionless figure of merit ZT (=S²σT/κ), where S is a Seebeck coefficient, σ is an electrical conductivity, T is an absolute temperature, and κ is a thermal conductivity. Second, these films must have a high optical transmittance in the visible range for the transparency. Third, the film materials should be non-toxic and cost-effective for global applications.

In this sense, conventional heavy metal thermoelectric materials, such as Bi₂Te₃ and PbTe [1,2], are not suitable for the global energy harvesting applications because they are expensive and toxic. Recently, it was theoretically predicted that the use of nanostructures or film structures could elevate ZT value of materials, where S is enhanced by quantum confinement [3] or energy filtering effect [4] and κ is reduced by phonon scattering effect [[5], [6], [7]]. Therefore, the nanostructure [[8], [9], [10], [11]] or film structure composed of ubiquitous materials with transparent, cost-effective, and non-toxic nature became a potential thermoelectric material for the global energy harvesting applications.

A transparent wide bandgap (~3.3 eV) semiconductor zinc oxide (ZnO), which bulk [12,13] and nanostructures [[14], [15], [16], [17]] have been intensively studied, is a promising material in various application fields: transparent electrode [18], gas sensor [19,20], piezoelectric device [21], and so on. Bulk ZnO materials exhibit a high power factor S²σ (~ 10–20 μWcm⁻¹ K⁻²) [12,22,23] but also high κ [12,22,24]. The high κ bottlenecked their thermoelectric applications. As for the films, J. Loureiro, et al., reported thermoelectric properties of Al-doped ZnO (AZO) films fabricated by the magnetron sputtering where low κ value was reported at 300 K [25]. However, κ reduction sacrifices their S²σ values to some extent [25]. Thus, S²σ enhancement as well as κ reduction of ZnO thin films are required for their transparent thermoelectric applications.

We focus on the Ga-doped ZnO (GZO) films instead of AZO films for the following reasons. In terms of electron transport, GZO is expected to realize a higher electron mobility than AZO because a substitutional Ga at Zn site in GZO induces smaller local lattice deformation than that in the AZO case due to the bond length of GaO (0.192 nm) closer to ZnO (0.197 nm) than that of AlO (0.181 nm). [26] As for the phonon transport, GZO is expected to have lower than AZO due to the effective phonon scattering at heavy Ga ions. [12] Moreover, GZO has an application advantage: higher resistivity to oxidation degrading the electrical conductivity than AZO [27], and lower stress-strain constraints on doping, and so on. [28] Unlike AZO films, however, the reports on the thermoelectric properties of GZO films are scarce up to now [29,30]. A report about non-oriented sol-gel GZO films grown by isopropyl alcohol (IPA) solution process [29] exhibited low thermoelectric power factor, S²σ (1.9 μWcm⁻¹ K⁻²) due to poor electron mobility in low crystal quality films. This implies a possibility that enhancement of film crystal quality increases the electron mobility resulting in large S²σ. Thus, the systematic investigation of the relation between film crystal quality and thermoelectric performance is important, but there have been no reports about impacts of film characters on thermoelectric properties in GZO films: crystal orientations, crystal domain sizes and the interfaces. Unlike non-oriented sol-gel films [29,30], oriented GZO films by pulsed laser deposition (PLD) and sol gel processes have not been studied for its thermoelectric application although films have better controllability of crystal domain orientation than bulk GZO [31].

In this work, we thoroughly investigate the thermoelectric properties depending on the structural characters of GZO films. To control the orientation, size and interfaces of crystal domains, oriented sol-gel films synthesized by 2-methoxyethanol solution process [31] and highly-oriented PLD films were fabricated. The crystal orientation of GZO films would rather be a key for the thermoelectric performance than the interface density and the domain size. As a result, highly-oriented GZO films exhibited the highest S²σ of ~2.8 μWcm⁻¹ K⁻² in the reported GZO materials although the carrier concentration is not optimized yet. The thermal conductivity at the out-of-plane direction exhibited ~1/4 reduction (~8.4 Wm⁻¹ K⁻¹) compared with bulk GZO (κ~29 Wm⁻¹ K⁻¹) [32], demonstrating that the highly-oriented films is a promising for thermoelectric film devices.