Browsing by Author "Yamauchi, Hisao, Prof., Aalto University, Department of Chemistry, Finland"

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Atomic Layer Deposition of Thermoelectric ZnO Thin Films
(2013) Tynell, Tommi
School of Chemical Technology | Doctoral dissertation (article-based)
Thermoelectric energy harvesters hold great potential for reducing our dependence on conventional energy sources by making use of untapped heat sources and converting them into electricity. For the technology to be able to make a significant impact though, more efficient materials need to be developed. This is a huge challenge because of the interdependence of the physical properties that affect the performance of a thermoelectric material, and nanostructuring might be the only way to overcome this obstacle. This thesis presents research on the effect of inorganic dopants and inorganic-organic hybrid superlattice structures on the thermoelectric properties of ZnO thin films. Atomic layer deposition (ALD) was used to fabricate the thin films in this study due to the suitability of the technique for the deposition of precisely controlled nanostructures. Zinc oxide is one of the most promising thermoelectric oxide materials, especially when doped with for instance Al or Ga, but it is somewhat held back by the lack of a stable p-type ZnO material. The effects of aluminum and phosphorus doping on the thermoelectric properties of ALD-grown ZnO thin films were investigated in this work, and both were found to increase the carrier concentration of ZnO, with Al turning out to be the better dopant in terms of improving thermoelectric performance. An attempt was also made to induce p-type conductivity in ZnO through thermal treatment of the P-doped films, but a deterioration of the electrical properties of the films was observed instead. The fabrication of super lattice structures of organic layers within ZnO with a combination of the atomic and molecular layer deposition (MLD) techniques was successfully demonstrated for three different organic precursors: hydroquinone, 4-aminophenol and 4,4'-oxydianiline. All of the organic molecules were found to have an effect on the electrical and thermoelectric properties of ZnO, the magnitude of which varied noticeably between the different organic constituents despite their relatively similar structures. The observed effects from the introduction of the hybrid superlattice structures amounted to only small changes in the thermoelectric power factor of ZnO, and these changes were not cumulative with the effects of Al doping when hybrid superlattices were combined with Al doping. The net effect from the organic layers on the thermoelectric performance of ZnO is predicted to be greater than implied by the slight changes in power factor due to the expected decrease in thermal conductivity resulting from phonon inhibition by the organic layers.