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Title: Developing structural probes for designed molecular architectures
Author: James, David Thomas
ISNI:       0000 0004 2732 2229
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2013
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This thesis is concerned with the development of techniques to probe and control molecular morphology in organic semiconductors and investigate structure-property relationships. An angle-dependent polarised Raman technique is developed to probe molecular order and orientation in 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) thin lms, and applied to understand the role of blending with polystyrene on morphology and charge transport properties in inkjet-printed organic field-effect transistors (OFETs). Compared to pristine devices, blending improves film uniformity in terms of device coverage and molecular orientation and increases saturation mobility from [Mathematical formula appears here. To view, please open pdf attachment]. A zone-casting apparatus is developed to systematically control the grain size and molecular orientation of TIPS-pentacene thin films. Processing parameters such as solvent, solution temperature, substrate temperature and casting speed are systematically controlled to study film coverage, grain size, alignment and orientation, and applied to investigate charge transport properties in TIPS-pentacene and 6,13-bis(triethylsilylethynyl) pentacene (TES-pentacene) OFETs. Casting speed and grain orientation strongly influence device performance. TIPS- pentacene shows higher mobilities and grain-boundary-limited transport while TES-pentacene charge transport is limited by crystal orientation. The anisotropic structural and optical properties of TIPS-pentacene and TES-pentacene films are characterised using polarised UV-visible absorption and Raman spectroscopy. Evidence for J- and H-aggregation is found from a packing-induced red-shifted and blue-shifted absorption transition, respectively. Angle-dependent polarised Raman spectroscopy is used to probe the molecular orientation and order in the films. The pentacene long-axis orientation is found to be ±(50-60)° relative to the zone-casting direction for both materials, while TIPS-pentacene shows a higher Raman anisotropy than TES-pentacene. Quantum-chemical studies are performed on a range of conjugated polymer systems. Using simulated structure-property relationships, the effect of side chain placement & positioning, blending with small molecules, thermal annealing and heavy-atom substitution on the opto- electronic and charge transport properties of the polymers are discussed.
Supervisor: Kim, Ji-Seon Sponsor: Val O'Donoghue Scholarship in Natural Sciences ; Cambridge Display Technologies Ltd
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral