Public defence in Advanced Materials and Photonics, M.Sc.(Tech.) Tomi Koskinen

The title of the thesis: Nanostructured thermoelectric materials for transparent and flexible applications

Doctoral student: Tomi Koskinen
Opponent: Prof. Nini Pryds, DTU, Denmark
Custos: Prof. Ilkka Tittonen, Aalto University School of Electrical Engineering, Department of Electronics and Nanoengineering

Thesis available for public display 10 days prior to the defence at: https://aaltodoc.aalto.fi/doc_public/eonly/riiputus/

Nanostructures enable transparent and flexible materials for converting heat into electricity 

Thermoelectric materials enable the conversion of temperature gradients into electricity. These materials can be exploited in electronics both as power sources and as sensing elements integrated to the application. This thesis investigated nanostructured thermoelectric materials for transparent and flexible applications. 

First, the thermal properties of GaAs-AlAs nanowire arrays were investigated, finding that the introduction of an AlAs shell reduces the thermal conductivity of the heterostructure. The control of the thickness of the shell could be applied to reduce the power consumption of optoelectronic devices utilizing similar structures. Atomic layer deposition was exploited in fabricating zinc oxide thin films for transparent thermoelectric applications. The method was used to dope zinc oxide first with zircon and then with aluminum. The latter material was also conformally deposited on grass-like alumina surfaces, which was found to improve both the transparency and the electrical conductivity of zinc oxide compared to films grown on planar glass substrates. 

Materials suitable for flexible thermoelectric applications were also investigated. InAs nanowire networks grown by metal-organic vapor phase epitaxy were found to maintain their thermoelectric properties despite of repetitive bending. A flexible thermoelectric generator based on nanowire networks was demonstrated utilizing patterning taking place already in the growth step. Finally, spray-coated graphene ink was exploited in preparing a flexible thermal distribution sensor. The sensor was shown to be capable of detecting temperature distributions on large areas and detecting shapes based on differences in temperature. 

This thesis presents new methods to prepare materials for functional thermoelectric applications as well as introduces ways of exploiting nanostructures in improving the transparency and flexibility of thermoelectric materials.

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Doctoral theses in the School of Electrical Engineering: https://aaltodoc.aalto.fi/handle/123456789/53