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Title: The effect of highly-ordered molecular conformations on the optoelectronic properties of conjugated polymers
Author: Cheetham, Nathan James
ISNI:       0000 0004 9350 5218
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2018
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This thesis examines the effect of well-defined, highly-ordered molecular conformations on conjugated polymer optoelectronic properties. Understanding the links between chemical and physical structure and optoelectronic properties is critical for the continued development of conjugated polymer electronic devices and applications. This is achieved using a variety of spectroscopic techniques, including time-resolved and temperature-dependent absorption and photoluminescence spectroscopies. The first results chapter uses ultrafast transient absorption spectroscopy to resolve exciton dissociation dynamics in the polyfluorene poly(9,9-dioctylfluorene), PFO, as the amount of chain segments adopting the highly-ordered β-phase conformation is systematically varied. Increased polaron generation yield when β-phase chains are present is assigned to more efficient exciton dissociation at the interface between disordered glassy and ordered β-phase conformational phases, attributed to significant local energetic offsets, the higher electronic delocalisation of the β-phase and mismatches in charge carrier mobilities. The second results chapter examines a second, recently reported β-phase forming polyfluorene, PODPF. Formation of β-phase is characterised by absorption and photoluminescence spectroscopy following a series of thermal annealing steps. In addition to β-phase, formation of crystalline regions is observed at higher annealing temperatures, from which a second variant of β-phase can also form. Generation of higher-ordered molecular conformations is found to be sensitive to heating rate. The third results chapter compares photophysical and structural properties of PODPF and PFO. The means by which backbone planarisation is achieved, key to β-phase formation, appears to be very different. The β-phase in PODPF is reliant upon interchain interactions and so is inherently 3-dimensional, rather than a solely intrachain phenomenon as for PFO. This leads to lower efficiency energy transfer in PODPF than PFO due to larger PODPF β-phase domains. Finally, the optical properties of the homoconjugated polymer, PPM, and methyl-substituted derivatives, are characterised to probe the conformational origins of homoconjugation. The concentration of the two distinct homoconjugated chromophores in PPM is found to be very low. Chain length-dependent absorption suggests that formation of one homoconjugated species is confined to chain ends, while the second species requires a certain substitution pattern between polymer repeat units.
Supervisor: Stavrinou, Paul ; Bradley, Donal ; Nelson, Jenny Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral