A structural and spectroscopic investigation of polyfluorene copolymers in solution and the solid state
Creators: name: Stanley, Robert Peter
Publisher: name: Imperial College London
Year: 2012

description: This thesis involves a structural and spectroscopic investigation of the electroluminescent polymer poly(9,9-dioctylfluorene), F8T2, and its copolymers poly(9,9-dioctyl)fluorene, PFO, and poly(9,9-dioctylfluorene-alt-benzothiadiazole), F8BT. Small angle neutron scattering (SANS) studies have shown that the three polymers take up very rigid, rod-like conformations in toluene and chloroform solutions. Aggregation effects of F8T2 in toluene were studied over 16 hours with a sheet-like aggregate structure suggested. Light-scattering (LS) measurements were used to investigate the molecular weight, backbone conformation on different length scales, coil size and the effect of chain aggregation in different solvents, and at different temperatures of the three polymers. Again, the polymers appear to be very rigid, with precise molecular weights and radii of gyrations found. F8T2 was extensively studied using optical absorption, photoluminescent (PL) emission, PL quantum efficiency (PLQE) and PL lifetime measurements of solutions and films deposited from them. Strong PL quenching effects were seen which have been attributed to interchain interactions at concentrations around 0.01 mg/ml, self-absorption effects dominate at higher concentrations above ~0.1 mg/ml. Large spectral changes were seen due to solvent choice, aggregation effects and thermal dissolution histories. Deposited films were also investigated with temperature dependent PL, which allowed some thermal transition temperatures to be found, atomic force microscopy and as bottom-gate/top-contact organic field-effect transistors. Synchrotron Grazing Incidence Wide-Angle X-ray scattering (GIWAXS) studies were conducted on films deposited on flat substrates and polyimide alignment layers, as well as annealed within an in-situ vapour cell. F8T2 was found to be a poor scatterer of X-rays but F8BT appears to stack with a vertical interchain distance of 4.4 Å, with an interchain seperation of 5.8 Å due to sidechains. Other copolymers of F8T2, poly[2,7-(9,9-dihexylfluorene)-alt-bithiophene] F6T2, and SC005 were found to be good candidates for GIWAXS study. Thermally treated PFO was very well indexed, with clear changes seen due to vapour exposure in situ. Chapter 1 discusses vital, general concepts used to describe electro-conductive polymers, as well as a thorough review of the background of organic semiconductor characterisation. Further relevant details are covered in each results chapter. Chapter 2 describes and defines the polymer samples and processing methods used throughout this thesis. The experimental techniques used are described in detail, as are the polymers involved and sample fabrication methods. In Chapter 3 small-angle neutron scattering undertaken at RAL is discussed, with results shown from F8T2 in various solvents, as well as studies of F8BT and PFO. Chapter 4 covers both the results of multi-angle light scattering on F8T2 and other polyfluorenes in a variety of solvents. The effect of the passage of time of dissolved polymer solutions is investigated, along with size and molecular weight characteristics in different solvents. SAXS results from F8T2 solutions are also briefly discussed. In Chapter 5 a broad variety of spectroscopic investigations of F8T2 are discussed, both in solution and as thin-films. The effects of thermal treatments on thin-films are studied, including DSC results and the consequences on the electrical properties of thermal and solvent annealing on field-effect transistors. In Chapter 6 the results from a GIWAXS study on aligned and non-aligned thin-films of PFO and three co-polymers are shown. Unit cell size parameters are determined, and the effect of exposing PFO thin-films to solvent vapour is studied.