School of Chemistry - Theses
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ItemCore-shell structures in dye-sensitised solar cells( 2014)The growing energy demand in the world combined with global warming and pollution is increasing the need for new energy sources. Energy production through capturing sun light is emerging as a good and feasible alternative to fossil fuels. An alternative to the conventional silicon-based devices is the dye-sensitised solar cells (DSCs) with a photoelectrode generally made from nanoparticles of titania. This type of solar cell offers many advantages such as low materials and manufacturing cost, light weight, flexibility and transparency. However, so far only 12% efficiency has been achieved for laboratory cells with a liquid hole conductor. The cell efficiency can be increased by enhancing the internal light-scattering ability of the porous semiconductor photoelectrode. The overall aim of this thesis was to increase the light scattering in the photoelectrode of the DSC by introducing core-shell structures into the photoelectrode. The objective was to improve the light-scattering ability of current mesoporous structures by synthesising core-shell structures where a core of different refractive index was introduced into mesoporous spheroids to increase their light-scattering power. A semi-batch coating method for coating particles with amorphous titania was developed with which particles of varying sizes (80-300 nm) and materials could be successfully coated. Under optimal conditions the thickness of the coating (50-220 nm) could be controlled to a great extent by varying the feed time of the water and precursor feed solutions as well as by varying the concentration of the core particles. Coating thicknesses of up to 550 nm could be achieved when four coating layers were grown on the same particles. A batch method for the synthesis of monodisperse amorphous titania was developed to produce titania cores with low porosity. A solvothermal treatment method was developed where the morphology of the crystallised particles together with their pore size distribution was controlled by the oven temperature and the ammonia concentration. Successful synthesis of core-shell structures of crystalline titania core-particles and a thick amorphous titania coating was obtained by first priming the crystalline particle surface with a thin layer of amorphous titania. A solvothermal treatment method was developed for the crystallisation of the thick titania coating. The specific surface area of the solvothermally treated coatings was significantly higher (higher than 60 m2 g-1) than for the calcined coatings. The specific surface area, pore size and morphology of the solvothermally treated coatings were dependent on ammonia concentration, treatment time and temperature. The silica core-particles dissolved during the solvothermal treatment. The amount of Si incorporated into the titania coating depended on the treatment temperature. Six different core-shell structured materials were synthesised (three dense-core and three hollow-core materials) for testing of their performance in DSCs. All the core-shell structured materials had a better light-scattering ability than the control photoelectrode consisting of only nanoparticles. The dense-core materials had the lowest solar cell performance with no difference in diffuse reflectance observed for the different core-shell structures with dense cores for a 6.5 µm photoelectrode film. The similarity in the diffuse reflectance was attributed to the low structural contrast between the core and the mesoporous coating. For a 6.5 µm film, the hollow-core materials displayed a higher diffuse reflectance compared to the dense-core materials with the silica-templated material reaching more than 75% for light with a wavelength of 400 nm. The better light-scattering ability was attributed to the higher structural contrast between the core and the shell compared to the dense-core materials. The polystyrene-templated material with a thick mesoporous coating (240 nm) had the best photovoltaic performance of all the tested materials. The cell efficiency of 8.0 ± 0.3% was attributed to the high incident-photon-to-current conversion efficiency given by a high dye concentration, good light-scattering properties and low mass transport limitations for the electrolyte.