Development of Hierarchically-Structured TiO2 for Multi-Purpose Building Blocks in Dye-Sensitized Solar Cells

Abstract

Solar power has been received global interest nowadays with the growing demand for the sustainable and renewable energies. However, high cost of conventional semiconductor solar cells limits their wider applications for the electricity production. Since the release of the dye-sensitized solar cells (DSCs) in the 1991 by O’Regan and Grätzel, the DSCs have surfaced as competitive alternative to silicon-based photovoltaic devices by virtue of their low cost of production and competitive conversion efficiency. Up to now, extensive studies have been made to improve the DSC performance in the field of materials chemistry and engineering. One of major research trends driving innovations in the DSCs is modification of the oxide nanostructures. Since the beginning of the nanotechnology era in 2000s, various oxide nanostructures were proposed as photoelectrode materials in order to achieve an efficient power conversion in the DSCs. Nonetheless, traditional mesoporous TiO2 nanoparticles seem to be still the most reliable material for the photoanode of the DSCs. However, TiO2 nanoparticles are not soley used as working electrodes because of their high transparency in the visible range. This may result in a huge loss of the incident light, eventually limiting the photocurrent generation in the DSCs. Thus, efficient light scattering has become one of the major issues in designing the photoelectrodes. In recent years, researchers have discovered hierarchically-structured oxide nanomaterials that include ZnO aggregates, nano-embossed hollow spherical TiO2, mesoporous spherical beads, and so on. These porous spherical colloids are comprised of nanocrystallites that are clustered together to form larger secondary structures. This structural nature inherently enables them to have similar or even higher internal surface area, as compared with a conventional nanocrystalline film. In this thesis, a unique synthetic route was developed to prepare hierarchically-structured TiO2 for multi-purpose building blocks in the DSCs. By combination of ‘dilute-mixing’-assisted sol-gel and solvothermal process, tunable secondary particle diameters could be produced (size-tunable mesoporous spherical TiO2). In the optimized particle size (587 nm), the best dual effects of light scattering and dye-loading was demonstrated to exhibit the maximum efficiency of 9.37 % (versus 6.80 % for the reference nanocrystalline TiO2). As a next step, the degree of the hierarchical order was carefully tailored by modification of the primary pore structures. This methodology provided graded-series of optical properties essential for constructing a multi-layer architecture. Consequently, the hierarchically-structured multi-layer (HSM) was fabricated by layering these three nanomaterials on the FTO substrates. The present HSM–DSC exhibited the synergetic effect of the enhanced optical absorption and charge collection with world class level of efficiency: 11.43 % under 1 Sun, 12.16% under 1/8 Sun illumination. As-prepared hierarchical nanostructure was also utilized as an additive to an amphiphilic polymer gel electrolyte (LS-APGE). This strategy would potentially grant compactness and flexibility to the DSCs by replacing conventional scattering layers, and assure long term stability during practical operation: maximum 8.2 % with LS-APGE versus 7.7 % for the reference cell made with the conventional liquid electrolyte.