Thermoelectric Properties of Organic Polymers
Thermoelectric devices present an enticing solution for harnessing waste thermal energy from industrial processes and converting it into electrical power. Due to their cost-effectiveness and potential in flexible electronic applications, organic thermoelectrics have attracted considerable attention. Nonetheless, further enhancing their thermoelectric performance remains a great challenge. Solutions have been proposed, encompassing molecular structure design, uniaxial alignment, and advanced doping techniques. This dissertation explores the relationship between the enhanced thermoelectric properties and the structure of organic polymers. It delves into both innovative new materials systems and established typical materials. First it introduces the background and basic experimental techniques in Chapters 1-2. Then it delves into the function of uniaxial alignment and ion exchange doping to optimize the thermoelectric properties of organic polymers in Chapter 3. Uniaxial alignment achieves anisotropic charge transport by orienting the polymer backbones, which facilitates charge movement along backbones. Ion exchange doping has demonstrated superiority over traditional molecular and electrochemical doping methods, increasing charge carrier densities. By integrating these two techniques, we've observed marked improvements in the thermoelectric attributes of some typical conjugated polymers such as poly[2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT) and diketopyrrolopyrrole (DPP)-based polymers. In Chapter 4, we report a new model system for better understanding the key factors governing their thermoelectric properties: aligned, ribbon-phase PBTTT doped by ion-exchange doping. Using a range of microstructural and spectroscopic methods we study the effect of controlled incorporation of tie-chains between the crystalline domains through blending of high and low molecular weight chains. The tie chains provide efficient transport pathways between crystalline domains and lead to significantly enhanced electrical conductivity, that is not accompanied by a reduction in Seebeck coefficient nor a large increase in thermal conductivity. We demonstrate respectable power factors of 172.6 μW m-1 K-2 in this model system. Our approach is generally applicable to a wide range of semicrystalline conjugated polymers and could provide an effective pathway for further enhancing their thermoelectric properties and overcome traditional trade-offs in optimization of thermoelectric performance. Furthermore, this thesis explores the relationship between molecular structure and thermoelectric properties of some specially designed novel polymers in Chapters 5-6. This includes Poly-3-hexyl-thiophene (P3HT)-based random co-polymers, P[(3HT)1-x-stat-(T)x] containing different proportions of unsubstituted thiophene units (x ranging from 0 to 0.36). It also includes naphthalene diimide (NDI)-based copolymers modified by substituting side chains, incorporating Selenium atoms, and incorporating Fluorine atoms. Finally, Chapter 7 of this thesis provides a summary and offers an outlook, highlighting how our methodology is broadly applicable across various semicrystalline polymers, which can potentially and substantially improve the thermoelectric figure-of-merit in these emerging materials. Furthermore, the techniques discussed in this thesis show great promise for expanding the applications of organic thermoelectric devices in potential future industrial scenarios.
charge transport, Organic electronics, polymers
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