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Title: Exploring the effect of polyfluorene copolymerisation for improved material design
Author: Lambeva, Nikol Todorova
ISNI:       0000 0005 0291 5340
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2020
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The discovery of electroluminescence from conjugated polymers was followed by tremendous academic and commercial interest in the field of polymer optoelectronics. Conjugated polymer materials have been shown to possess beneficial qualities for device fabrication such as low-cost, large area manufacturing and the potential for realizing flexible applications. Polyfluorenes, in particular, are a class of conjugated polymers which have been realized in a number of applications such as organic light emitting diodes, solar cells and field-effect transistors. The widespread commercialization of such products critically depends on improving device efficiencies. This can be accomplished by both advancing device engineering and improving material design. For the realization of the latter, fundamental understanding of the interplay between material chemistry and physics is required. This motivated the work in this thesis which is concerned with the structure-property relationship in polyfluorene containing copolymers. The well studied polyfluorene poly(9,9-dioctylfluorene) (PFO) was selected as the model polymer due to its complex phase morphology which allowed to additionally investigate the effect of film microstructure on the photophysics of the copolymers. The optoelectronic properties of PFO can be varied by the copolymerisation of suitable comonomers giving valuable insight into the relationship between chemical structure and photophysical properties that can guide future material design for device applications. In this work three different comonomers have been studied. Firstly, the hole mobility of four fluorene-arylamine copolymers has been investigated. It is shown that transport in these copolymers is controlled over a large range by the copolymerisation of different amounts of a lower ionisation potential transporting moiety. Next, the effect of increasing backbone rigidity on hole carrier transport is studied via the copolymerisation of fluorene and phenoxazine. The resulting copolymer has a high glass transition temperature suggesting a rigid polymer chain. Yet, its hole mobility is lower than for the chemically similar hole transporting material poly(9,9-dioctylfluorene-alt-N-(4-butylphenyl)-diphenylamine) (TFB) which possesses a greater number of torsions. Finally, the copolymerisation of PFO with a cyclometalated platinum complex has been investigated. The heavy platinum atom increases spin-orbit coupling leading to the radiative decay of the triplet state. It is shown that at low temperatures kinetic frustration of the triplet state is lifted in the planarised ß phase of the copolymer demonstrating that triplets can diffuse more effectively along highly conjugated polymer chains than along materials with short conjugation segments.
Supervisor: Riede, Moritz ; Bradley, Donal Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: semiconducting polymers ; polyfluorene ; organic semiconductors ; copolymers ; condensed matter physics