Tailoring Functional Π-Conjugated Polymers and Molecules for Organic Photovoltaic Applications
Lo, Chi Kin
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The field of organic photovoltaics (OPVs) has developed into an active research topic in applied polymer, materials, and chemical sciences over the past decade due to the continuous improvement in device performance. The solution-processability of the organic active layer materials allows for the development of low-cost, flexible, and lightweight solar devices which have the potential to be manufactured through a variety of high-throughput roll-to-roll printing methods. Understanding the structure-property relationship in polymer photovoltaic materials is crucial to continue the recent improvement in OPV device performance. However, many high performing donor polymers with power conversion efficiencies (PCEs) of 8-10% have different molecular structures, frontier energy levels, and physical properties. Even for polymers with structural similarities, they may not be easily compared as a result of their batch-to-batch variances, an intrinsic drawback of polymeric materials. This dissertation will first present a family of photoactive conjugated polymers with only a “one-atom” minimal change. This will showcase the use of thorough cross-coupling polymer synthesis and purification, structural and physical characterization techniques to elucidate structure-property understanding in designing OPV materials. The dissertation will continue with the theme of structure-property investigation, focusing on a series of donor-acceptor polymers containing various election-deficient conjugated moieties such as isoindigo, diketopyrrolopyrrole, and thienoisoindigo. It will be used to explain how the structures and frontier energy levels of the acceptor units affect the open-circuit voltages, the active layer morphologies, and device performances. Supramolecular assembly of π-conjugated materials is crucial in high performance organic electronic device fabrication. Materials that self-assemble into ordered domains between 5 and 100 nm length scale can bridge the gap between single molecule electronics, where molecular orientation and conformation dictate charge carrier direction and mobility, and polymer electronics, where dispersities and backbone defects can hinder device performance. Many of these materials are amphiphilic nature, leading to high degrees of intermolecular organization as a result of the nanoscale phase separation between the hydrophilic and hydrophobic moieties which assembles into highly ordered domains. Non-covalent interactions such as π-π interactions, hydrogen bond, dipole-dipole interactions, and steric effect determine the spontaneous self-organization processes. However, this self-assembly process, both in solution and during film formation, is thermodynamically driven and is controlled by the choice of deposition technique. To obtain full control in bottom-up assembly, layer-by-layer (LbL) deposition emerges as an excellent candidate for depositing organic materials. Langmuir-Blodgett (LB) is an LbL technique that can achieve a true molecular monolayer buildup. In the next section of this dissertation, thin-film characterizations will be the main focus, especially on morphological and structural studies on the Langmuir-Blodgett films of two amphiphilic conjugated molecules. Both OPV and OFET performances will be discussed, showing the use of Langmuir-Blodgett deposition to create well-ordered material layers in organic electronic applications. Despite its popularity as a universal acceptor in an OPV device, fullerene derivatives have high production cost, weak absorption, and limited chemical stability. To improve device performance, the OPV field has focused on designing non-fullerene acceptors. The next section of this dissertation will first introduce the design for a family of polymeric acceptors. The synthesis of the polymers based on acceptor moieties including isoindigo, thienoisoindigo, diketopyrrolopyrrole, and thienopyrrolodione will highlight the use of direct arylation reaction to achieve pure and high molecular weight conjugated polymers. The different acceptor moieties are selected to adjust the energy levels in order to enhance the absorption of low energy photons. Structural designs and morphological investigations in AFM will be presented to correlate the transport property, which is proven to be critical for the development of high performance non-fullerene acceptors. Finally, an outlook of the organic photovoltaics will be discussed. Niche applications will be suggested and serve as examples for future market opportunities of the OPV technology. This chapter will also highlight the potentials of each material class in this dissertation for real world organic photovoltaics market applications.
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