Nanostructured thin films for thermoelectric and sensing applications

Thermal energy constitutes a natural source of energy and most energy consumed is finally converted to heat. However, the overall energy conversion process remains inefficient creating the need for solutions that may be used to improve the efficiency. Thermoelectric materials offer an opportunity to improve the energy efficiency in thermal processes by converting the waste heat directly into electricity in a solid state material via the Seebeck effect. Since the achieved conversion efficiency remains still relatively low, a great challenge for materials research is to improve thermoelectric performance for example by microscopic tailoring of materials. Finally, nanotechnology has brought along many methods for tailoring the underlying conduction mechanisms that determine the performance of thermoelectric generators. Novel thermoelectric thin film materials and their applications are studied in this thesis. The presented research focuses on the development of metrology for the characterization of thermoelectric material properties, studies the thermoelectric properties of nanostructured materials and presents novel application of transparent thermoelectric thin films. In this thesis a method to measure the in-plane thermal conductivity of thin films using laser flash apparatus is presented. With this method one can expand the thickness range of samples that can be measured with laser flash analysis by more than two orders of magnitude. In addition, Al-doping of zinc oxide (ZnO) thin films is studied and the effect of the doping on thermoelectric performance of the films is determined. Nanostructuring of ZnO films is demonstrated by large-area conformal coatings by ALD. The work demonstrates the use of geometrical effects for the reduction the resistance of thermocouples fabricated in such a fashion. Accordingly, this work reports on the opportunity to double the generated power of any planar thermoelectric generator. Finally, the ZnO material is applied in a novel transparent thermal touch panel. The proposed concept takes advantage of the thermoelectric voltage generation allowing passive sensing and making the concept very interesting for large-area and low-power applications. The results presented in this thesis enable novel applications and solutions both for energy conversion and sensing. Consequently, the outcomes are forming the basis for a novel field of transparent and flexible thermoelectrics.