Discotic compounds and columnar materials within organic photovoltaic material blends: synthesis and characterisation
AffiliationSchool of Chemistry
Document TypePhD thesis
Access StatusOpen Access
© 2019 Dr. Quentin Hong
Discotic organic polyaromatic semiconducting materials have been utilized in different organic electronic applications, specifically as light harvesting material for organic photovoltaics (OPV), the development of which, alongside other alternative energy sources, plays an integral role in rising to the challenge of climate change. Within OPVs, the photovoltaic process heavily depends on the nano-structure of the light harvesting material blend; ideally the nanostructure consists of interconnected tendrils with clear charge transport pathways. The formation of supramolecular stacks, a property inherent to many discotic polyaromatics, have been shown to be useful towards the formation of this ideal nanostructure. This thesis focusses on the synthesis and characterisation of novel discotic polyaromatic compounds as well as its applications in OPVs. The first project attempts to combine the unique stack forming properties of discotic polyaromatic compounds with crosslinking techniques. Previous studies indicated that FHBC was a moderately successful electron donor material (PCE = 1.46%), but the devices suffered from poor efficiency retention over time due to changes in the nano-structure of the organic material blend. Crosslinking of FHBC, which has been shown to form supramolecular columns (which can also be described as supramolecular polymers), within the organic photovoltaic active layer was hypothesized to help stabilize the nano-structure. Four derivatives of FHBC containing aldehyde functional groups were synthesized and characterized. The aldehyde functional groups were reacted with diamine crosslinkers, to form crosslinked material films which demonstrated resilience to solvent. Photovoltaic devices were subsequently constructed but no improvement to the efficiency retention was observed. The second project aims to at the development of novel materials with supramolecular stacking capabilities and modern design principles. While HBC shows evidence towards the ideal nanostructure, the efficiency of photovoltaic devices fabricated from these materials is ultimately overshadowed by more recent publications. Instead HBC was imagined as a columnar scaffold to which chromophores can be affixed to, taking advantage of both quality chromophores and the self-assembly of HBC. Three FHBC derivative electron acceptors, demonstrating capability to form columns via NMR studies, were synthesized and characterized. Spectroscopic analysis showed independent photophysical properties of the HBC core and the affixed chromophore. Photocurrent was detected in initial photovoltaic devices (with P3HT as the electron donor material) with efficiencies between 0.3% and 0.5%. The final project details the improvement of photophysical properties to the bulk material by taking advantage of the strong π-π interactions which result in the self-assembly of discotic polyaromatic compounds. Optimizing the absorption and emission profile of organic semiconducting materials is one the core tenants of research and development of organic electronics as a whole. Columnar stacks of materials offer unique opportunity to change interactions between molecules to induce different photophysical phenomena. Truxene derivatives were synthesized with different frontier orbital energy levels which were then shown to have charge transfer characteristics in bulk films when mixed. Overall, this thesis discusses different approaches to improving discotic polyaromatic compounds for OPVs focusing on the synthesis and characterisation of novel materials.
Keywordsfunctional organic materials; chemical synthesis; organic photovoltaics; supramolecular interactions
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