Thesis presented December 18, 2017

Abstract:
With the rising demand of flexible, low cost and environmentally friendly materials for future technologies, organic materials are becoming an interesting alternative to already existing inorganic ones. Organic photovoltaics, organic light emitting diodes, organic field effect transistors, organic thermoelectricity, organic transparent electrodes are all evidences of how organic materials are sought for tomorrow. Materials which can fulfill the requirements specifications of future technologies are conducting polymers, which owe their popularity to their outstanding electrical, optoelectronic, thermochromic, lighting and mechanical properties. Moreover, they exhibit good processability even on flexible substrates and low environmental impact. Poly(3,4-ethylenedioxythiophene) (PEDOT) is certainly the most known and most used conducting polymer because it is commercially available and shows great potential for organic electronics. Studies dedicated to PEDOT films have led to high conductivity enhancements. However, an exhaustive understanding of the mechanisms governing such enhancement is still lacking, hindered by the semi-crystalline nature of the material itself. In such a context, this thesis has four objectives which are (1) the synthesis of PEDOT materials with an optimized and controlled structure to enhance the electrical properties, (2) the thorough characterization of the as-synthesized PEDOT in order to understand the charge transport mechanisms, (3) the study of their thermoelectric properties and (4) the study of their stability under different environments and stresses. Thus, after a literature review on PEDOT materials, we present the enhancement of the electrical conductivity of PEDOT:OTf and PEDOT:Sulf up to 5400 S cm^-1via a structure and dopant engineering, and then thoroughly study their electrical and electronic transport properties. Subsequently, two thermoelectric properties of PEDOT are investigated, namely its resistive Joule heating ability and its Seebeck effect, for both heating and energy harvesting applications. A novel application of PEDOT as flexible transparent heater is demonstrated in the first case. PEDOT:Sulf for example exhibited a sheet resistance of 57 Ω sq^-1 at 87.8 % transmittance and reached a steady state temperature of 138 °C under 12 V bias. Finally, this thesis is concluded with the ageing and stability of our PEDOT based materials under different environmental stresses. While PEDOT is stable under mild conditions, heavy degradations can occur under harsh conditions. The degradation mechanisms are then investigated in this last part.

Keywords:
Thermoelectric materials, PEDOT, Organic semiconductors, Thick films, Materials characterization.

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