Synthesis, Characterization, and Applications of Nucleobase-Functionalized Conjugated Polymers

Understanding the effect of the functional groups at the terminus of the side chains is important for developing conjugated polymers through side chain engineering. Nucleobases, which are known for their multi-functionality, have not been deeply studied as functionality in conjugated polymers due to synthetic challenges. The overarching goal of my dissertation is to design, synthesize, characterize conjugated polymers bearing nucleobase functionality in their side chains and demonstrate their utility in various applications. Stille cross-coupling and direct arylation polymerization are used to synthesize adenine- and thymine-containing conjugated polymers. Monomer design requirements for successful polymerization are studied and conditions that optimize polymerization are identified. Structural, thermal, and photophysical properties of nucleobase-functionalized polymers are studied and compared to non-functionalized homologs to assess the impact of nucleobase functionality. In addition to establishing design-structure-property relationships for these polymers, the ability of nucleobases to chelate metal ions and hydrogen bond were examined. My work highlights that nucleobases in the side chains can help to control thin film properties such as packing, which impacts transport properties, and responsive behaviors and surface assembly. This work shows that the properties of conjugated polymers can be improved and their applicability can be expanded through molecular design that utilizes the functionality and specificity of nucleobases. These outcomes are achieved through careful monomer design and optimization of polymerization conditions

Enhancing Thermal Stability of Perovskite Solar Cells through Thermal Transition and Thin Film Crystallization Engineering of Polymeric Hole Transport Layers

Organic hole transport layers (HTLs) have been known to be susceptible to thermal stress, leading to poor long- term stability in perovskite solar cells (PSCs). We synthesized three 2,5-dialkoxy-substituted, 1,4-bis(2-thienyl)phenylene (TPT)-based conjugated polymers (CPs) linked with thiophene-based (thiophene (T) and thienothiophene (TT)) comonomers and evaluated them as HTLs in n-i-p PSCs. TPT-T (MB/C6), which has branched 2- methylbutyl and linear hexyl (MB/C6) side chains, emerged as a promising HTL candidate, enabling power conversion efficiencies (PCEs) greater than 15%. In addition, PSCs with this HTL showed an improvement in long-term stability at elevated temperatures of 65 °C when compared to those with the state-of-art HTL, 2,2′,7,7′- tetrakis(N,N-p-dimethoxyphenylamino)-9,9′-spirobifluorene (spiro- OMeTAD). This improvement is ascribed to the lack of thermal transitions within the operational temperature range of PSCs for TPT-T (MB/C6), which is attributed to the relatively short branched side chains of this polymer. We propose that the elimination of thermal transitions below 200 °C leads to HTLs without cracking as-deposited and after conducting a stress test at 65 °C, which can serve as a new design guideline for HTL development

Lyotropic Liquid Crystal Mediated Assembly of Donor Polymers Enhances Efficiency and Stability of Blade‐Coated Organic Solar Cells

Conjugated polymers can undergo complex, concentration-dependent self-assembly during solution processing, yet little is known about its impact on film morphology and device performance of organic solar cells. Herein, lyotropic liquid crystal (LLC) mediated assembly across multiple conjugated polymers is reported, which generally gives rise to improved device performance of blade-coated non-fullerene bulk heterojunction solar cells. Using D18 as a model system, the formation mechanism of LLC is unveiled employing solution X-ray scattering and microscopic imaging tools: D18 first aggregates into semicrystalline nanofibers, then assemble into achiral nematic LLC which goes through symmetry breaking to yield a chiral twist-bent LLC. The assembly pathway is driven by increasing solution concentration - a common driving force during evaporative assembly relevant to scalable manufacturing. This assembly pathway can be largely modulated by coating regimes to give 1) lyotropic liquid crystalline assembly in the evaporation regime and 2) random fiber aggregation pathway in the Landau-Levich regime. The chiral liquid crystalline assembly pathway resulted in films with crystallinity 2.63 times that of films from the random fiber aggregation pathway, significantly enhancing the T80 lifetime by 50-fold. The generality of LLC-mediated assembly and enhanced device performance is further validated using polythiophene and quinoxaline-based donor polymers

Synthesis of a soluble adenine-functionalized polythiophene through direct arylation polymerization and its fluorescence responsive behavior

An adenine-functionalized polythiophene is synthesized via direct arylation polymerization using Boc-protection to overcome catalyst deactivation. The resulting copolymer is highly soluble and shows reversible fluorescence quenching.</p