Functionally Graded Semiconducting Polymers as Organic Thermoelectrics

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

Pre-Synthetic Redox Gated Metal-to-Insulator Transition and Photothermoelec-tric Properties in Nickel Tetrathiafulvalene-Tetrathiolate Coordination Polymers

Photothermoelectric (PTE) materials are promising candidates for solar energy harvesting and photodetection applications, especially for near-infrared (NIR) wavelengths. Although the processability and tunability of organic materials is highly advantageous, examples of organic PTE materials are comparatively rare and their PTE performance is typically limited by poor photothermal (PT) conversion. Here we report the use of redox-active Sn complexes of tetrathiafulvalene-tetrathiolate (TTFtt) as transmetalating agents for the synthesis of pre-synthetically redox tuned NiTTFtt materials. Unlike the neutral material NiTTFtt, which exhibits n-type glassy-metallic conductivity, the reduced materials Li1.2Ni0.4[NiTTFtt] and [Li(THF)1.5]1.2Ni0.4[NiTTFtt] (THF = tetrahydrofuran) display physical characteristics more consistent with p-type semi-conductors. The broad spectral absorption and electrically conducting nature of these TTFtt-based materials enable highly efficient NIR-thermal conversion and good PTE performance. Furthermore, in contrast to conventional PTE composites, these NiTTFtt coordination polymers are nota-ble as single-component PTE materials. The pre-synthetically tuned metal-to-insulator transition in these NiTTFtt systems directly modulates to their PT and PTE properties

Linker Redox Mediated Control of Morphology and Properties in Semiconducting Iron-Semiquinoid Coordination Polymers

The emergence of conductive 2D, and less commonly 3D, coordination polymers (CPs) and metal–organic frameworks (MOFs) promises novel applications in chemical sensing, energy storage, optoelectronics, thermoelectrics, and spintronics. While classic CPs and MOFs now have relatively sophisticated synthetic parameters to control morphology, crystallinity, and phase purity, similar parameters are not thoroughly understood for electronically more complex materials. In particular, many linkers used in conducting CPs have multiple accessible redox states and the relationship between starting linker oxidation state and final material structure and properties is not well understood. Here we report a new 3D semiconducting coordination polymer, Fe5(C6O6)3, which is composed of hexagonal Fe2(C6O6)3 layers which are bridged by additional Fe ions. This material, which is a fusion of 2D Fe-semiquinoid materials and recently reported 3D cubic Fex(C6O6)y materials, is obtained by using a different initial redox-state of the C6O6 linker. The material displays high electrical conductivity (0.02 S cm–1), broad electronic transitions in the visible to middle-infrared region, promising thermoelectric behavior (S2σ = 4.2×10–9 W m–1 K–2), and strong antiferromagnetic interactions even at room temperature. The unique structure and properties of this material illustrates that controlling the oxidation states of redox-active components in conducting CPs can be a “presynthetic” strategy to carefully tune material topologies, properties, and functionalities in contrast to more commonly encountered post-synthetic modifications

N-doped activated carbon from used dyeing wastewater adsorbent as a metal-free catalyst for acetylene hydrochlorination

Dyeing wastewater led to the water pollution and mercury contamination originated from polyvinyl chloride (PVC) production are both environmental problems derived with industrial development. In this work, the coconut activated carbon (CAC) was used to adsorb neutral red (NR), a representative dye in dyeing wastewater. Then, the spent CAC was calcined to be N-doped metal-free catalyst to realize the resource recovery. The metal-free catalyst showed a superior catalytic performance in acetylene hydrochlorination which is the important reaction for PVC production industry. The optimal 3NR/4CAC catalyst exhibited preferable catalytic activity with C 2 H 2 conversion of 97.9% and competitive stability in the 200 h lifetime test. A series of experimental characterizations combined with ReaxFF molecular dynamics simulations as well as density functional theory (DFT) calculations have been carried out to reveal the structural and electronic properties of the N-doped CAC catalysts, the nitrogen doping process as well as the catalytic mechanism of different N species for the acetylene hydrochlorination. This work provides a novel way for the reutilization of the waste adsorbent produced from dyeing wastewater treatment to support the sustainable development of PVC industry