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Open Access Dissertation
Doctor of Philosophy (PhD)
Year Degree Awarded
Month Degree Awarded
Materials Chemistry | Organic Chemistry | Physical Chemistry | Polymer Chemistry
The thermoelectric properties of organic semiconductors allow them to directly convert heat into electricity without the use of moving parts and to directly convert electricity into heat without the use of working fluids. These properties offer opportunities for the generation of electricity from non-conventional or renewable sources of heat and for refrigeration without the risk of leaking harmful working fluids at any length scale down to the nanoscale. Since organic materials are lightweight, flexible, and made from abundant resources, these opportunities could one day become affordable for widespread use and could be expanded to include specialized and otherwise difficult to reach applications, such as wearable refrigeration and electricity generation from anthropogenic heat. Since these properties are a result of how charge carriers in organic materials transfer energy upon conduction, measuring these properties also allows us to better understand the factors that influence how charge carriers carry energy during charge transport.
One of the remaining challenges in developing organic thermoelectric materials for practical use is the preparation of n-type thermoelectric materials, which transport electrons as their charge carrier, since most n-type organic semiconductors are unstable in air due to electron transfer to oxygen gas. We synthesized three organic conjugated polymers based on electron deficient rylene diimides and a vinylene spacer – PDNDIV, PFNDIV, and PDIV – to study how these could be doped into n-type semiconductors and how long these persist in air. Each polymer was capable of electron transport, and PFNDIV was capable of remaining n-doped with electronic charges for at least one week in air.
Blending conducting fillers into organic thermoelectric materials can alter their thermoelectric properties by altering the mechanism of charge transport. We blended the conductive filler SWNT into the organic conjugated polymer PBTDV2 and measured the thermoelectric properties. Although PBTDV2 was originally developed for use as an electron transporting polymer, all blends of SWNT with PBTDV2 had p-type, hole transporting, thermoelectric properties similar to those of oxygen-doped SWNT.
We measured the thermoelectric properties of two p-type organic polymers – P3HT and PDPP4T – that were doped to achieve a high concentration of charge carriers as they spontaneously de-doped and decreased their charge carrier concentration. Modeling these thermoelectric properties revealed that the spatial distribution of dopants in the polymers impacted the how much energy was carried per charge carrier due to Coulombic interactions of the dopants with the charge carriers, and that more heterogeneous spatial distributions of dopants can limit the thermoelectric performance by limiting how much energy is carried per charge carrier. These findings aid in the future development of organic thermoelectric materials by highlighting the importance of doping organic thermoelectric materials in ways that achieve homogeneous spatial distributions of dopants.
Boyle, Connor J., "Impact of Chemical Doping on the Thermoelectric Charge Transport of Organic Semiconductors" (2018). Doctoral Dissertations. 1420.