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Abstract

With the solution and thermal processability and coupled with the ability to tuning electronic properties through molecular doping, semiconducting conjugated polymers (CPs) have been increasingly explored in enabling organic thermoelectric (TE) for thermal energy harvesting and management. While the effort has been focused on optimizing TE material performance, functionally graded materials (FGMs) where the material properties are spatially controlled have been proven to further improve TE device performance, especially as thermoelectric (Peltier) coolers for thermal energy management and cooling applications. However, the concept of FGMs has not been explored in the context of organic materials.In this thesis, we aim to enable organic FGMs by leveraging molecularly doped semiconducting CPs through two approaches. The first approach is to spatially control the dopant composition across the film where the first series of organic FG polymer films, i.e., double-segmented and continuously graded thin films are achieved. Specifically, the focus of the Chapter 2 centers around a fundamental understanding of spatial structure-transport properties of molecularly doped poly(2,5-bis(3-alkyl-2-thienyl)thieno[3,2-b]thiophene) (PBTTT) through characterizations of double-segmented thin films. Moreover, Chapter 3 reports on continuously graded thin films relevant to improved thermoelectric (Peltier) cooling. Spatial compositional control of the molecular dopant, F4TCNQ (2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane), in PBTTT yield 1D profiles in α and σ. First principle calculations based on linear Onsager theory and conservation of charge and energy are used to model the cooling performance using the experimentally derived α and σ spatial profiles. The grading α and σ profiles allow for efficient redistribution of the Joule heating and Peltier cooling effects to improve cooling compared to equivalent uniform films. For the second route, we utilize the thermal processing to vary the microstructure of polymer films in order to enable the functional gradient. The variation in microstructure is first investigated in uniform poly(dodecyl-quaterthiophene) (PQT) thin films where polymorphs with interdigitated side chains coexist. The interdigitation of side-chain significantly affects the doping efficiency, which leads to a large variation in thermoelectric properties upon vapor doping with F4TCNQ. Chapter 5 follows the work from the previous chapter and realize spatial gradient in α and σ by coupling morphological change in PQT with molecular doping. The variation in morphology is achieved by apply a temperature gradient across the polymer film, which is captured directly through IR imaging. The cooling performance of the graded PQT film is predicted to be greater than the uniform equivalent. Lastly, we look into the α – σ relationship of doped thiophene-based polymers by applying Kang-Snyder charge transport model in Chapter 6. This fundamental investigation in modelling α – σ relationship of conjugated polymers enables more possibilities of future functional grading designs by precisely tailoring the values and profiles of conductivity and Seebeck coefficient to achieve enhanced device performance in TE applications.

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