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Sunlight is sustainable, clean and readily available energy source, which is one of the potential alternatives to the traditional energy sources. Recently, the organic photovoltaics (OPVs), in particular polymer solar cells (PSCs), have attracted increasing attention owing to their outstanding properties such as low cost, lightweight, flexible, allowing vacuum-free fabrication process and thin-film architecture. These advantageous material and manufacturing features of PSCs provide the opportunities for many novel applications. However, the lower power conversion efficiencies (PCEs) of PSCs compared to inorganic solar cells hinder their competition in the marketplace. This thesis covers the basic principles of the PSC, strategies for enhancing PCEs as well as the recent development of PSCs. The importance of the source materials has been also demonstrated and discussed. Due to a large number of possibilities, limited resources and time, it is not feasible to do all the work experimentally. Therefore, for continuing advance development of PSCs, the device performance should be modeled as a function of material parameters, which requires the knowledge of material properties, in particular the complex index of refraction N= n - ik. Accurate determination of the optical functions of the active layers and light trapping layers commonly used in PSCs by using the spectroscopic ellipsometry (SE) has been demonstrated. In order to acquire reliable solutions, the methodology including multiple sample analysis, combinations of different measurement techniques, selection of models, the rigorous fitting procedures and the independent verification have been proposed. The obtained information can be used in the simulation to optimize device architectures, model device performance as well as characterize novel materials. / published_or_final_version / Physics / Doctoral / Doctor of Philosophy

The work presented in this thesis provides a new route to a colloidal form of polyaniline, which uses colloidal silica as a dispersant. We obtained stable colloidal dispersions of polyaniline-silica composite particles with a 'raspberry' morphology. Compressed pellets of these particles exhibit solid-state conductivities of 1O-!_10-2 S crrr l, which is approximately 1-2 orders of magnitude lower than that of polyaniline bulk powder. This novel colloidal form of polyaniline has significantly improved processability compared to conventionally synthesised polyaniline. The synthesis and chemical characterisations are presented for various polyaniline-silica colloidal nanocomposites. The quantity of polyaniline incorporated into the nanocomposite particles can be controlled by varying the diameter of the silica dispersant, approximately 20% and 60 % polyaniline content being obtained using 120 nm and 10 nm diameter silica respectively. The average particle size ranges of nanocomposites was found to be 150 to 700 nm and 330 to 560 nm, as determined by transmission electron microscopy (TEM) and disc centrifuge photosedimentometry (DCP) respectively. The nanomorphology and surface composition of the polyanilinesilica particles were determined by small angle X-ray scattering (SAXS) and X-ray photoelectron spectroscopy (XPS) respectively. The average inter-particle separation distance of the silica particles within the polyaniline-silica raspberries was determined by SAXS to be 4 nm, a dimension equivalent to molecular polyaniline. The XPS data suggests that the surface of the particles is silica rich, this is consistant with their long term colloidal stability in 1.2 mole dm-3 HCl. The kinetics of polymerisation was studied using 1H NMR spectroscopy to monitor the disappearance of aniline monomer. Polymerisation rates during the synthesis of polyaniline-silica nanocomposites were appreciably faster than the corresponding precipitation polymerisations carried out in the absence of silica dispersants, due primarily to an increase in the second auto-catalytic step of the reaction. Rate constants were determined for both these types of synthesis; the values obtained for the precipitation polymerisations were in reasonably good agreement with literature values.

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This thesis presents the results of a comprehensive study on the transport and optical properties of polyaniline (PANI) films. The films are derived by protonation (doping) of the emeraldine base form of polyaniline, as synthesized in Durham, with either 2-acrylamido-2-methyl-l-propanesulfonic acid (AMPSA) or 10- camphorsulfonic acid. Thus, two distinct PANI systems are obtained: PANI-CSA and PANI-AMPSA. The variation of the doping level can affect the metallic properties of the final system, so that samples close to the boundary as well as samples at either side of a disorder induced metal-insulator can be obtained. The relation between the doping level and the degree of disorder, along with the existence of an inherently metallic behaviour in PANI, are investigated through a series of experiments. Temperature dependent dc conductivity measurements ranging from 10-295 K are performed using a closed loop helium cryostat under dynamic vacuum (~10(^-5) mbar). From the conductivity data curves, typical fingerprints of the metallic behaviour are detected for certain samples and an initial estimate of the degree of disorder is implicitly attained. More specific information regarding the microscopic contributions to the transport mechanisms is obtained via low temperature (down to 1.5 K) magnetocon- ductance measurements on selected samples. The magnetic field dependence of conductivity for fields up to 14 T is measured and the suitability of the localization- interaction model for the understanding of the transport mechanism in PANI is examined. Infrared reflectivity (20-9000 cm(^-1)) measurements on samples of both PANI systems are performed. The experimental configuration permits the determination of the sample’s absolute reflectivity. The optical constants are deduced from Kramers- Kronig analysis of the reflectivity data. Typical features of metallic behaviour are examined and analysed in the context of the localization modified Drude model. The results are shown to be consistent with the transport measurements, indicating that PANI is a disordered metal close to the boundary of a disorder induced metal- insulator transition.