Electronic and structural modification of titanium dioxide/zinc oxide photoanode for dye-sensitized solar cells

Dye-sensitized solar cells (DSC) are a new class of molecular photovoltaics that mimics the natural photosynthesis, for the direct conversion of sunlight into electricity. A typical DSC is a sandwich of a dye sensitized nanoparticle TiO2 film and a catalyst coated counter electrode, with a redox electrolyte filling the space in between. The high internal surface area of the mesoporous TiO2 film ensures higher dye uptake for an efficient light harvesting. Following the light illumination, an electron from the highest occupied molecular orbitals (HOMO) of the dye is excited to its lowest unoccupied molecular orbitals (LUMO). Due to an effective electronic orbital coupling between dye and TiO2, the excited electrons are injected into TiO2. The oxidized dye is regenerated by the electron donor present in the electrolyte. The circuit is completed by the transfer of photogenerated electrons in TiO2 via an external load, to counter electrode regenerating the oxidized electrolyte species. In a conventional DSC, the transport rate of the photogenerated electrons in TiO2 competes with the parasitic electron recombination, as these two processes occur at a similar ms time domain. This thesis work is aimed at the fundamental understanding of these electron transfer processes at sensitized photoanodes that were subject to different bulk and surface electronic structural modifications. The implications were shown to be of profound interest for the fabrication of high efficiency and low cost dye-sensitized solar cells. In the first part of the thesis, the bulk electronic structure of the anatase TiO2 nanoparticles is modified by the incorporation of different concentrations of aliovalent cations (Nb5+, Ga3+, Y3+) in the crystal lattice. The cationic doping/substitution in the nanoparticles is achieved by hydrothermal process. The modified nanoparticles retained the parent anatase crystal structure of TiO2. The transparent dye-sensitized solar cells made with these modified films, using our standard heteroleptic metal complex sensitizers and iodide/triiodide redox electrolyte, displayed enhanced photovoltaic performances, at certain added concentrations. Charge extraction measurements in DSC, reveals a formation of deeper trap states below the conduction band of titanium dioxide when Nb ion is incorporated into the TiO2 lattice. On the other hand, Ga3+ and Y3+ did not significantly affect the trap states distribution compared to the reference device. The modification in the trap states and the oxygen stoichiometry due to these cation inclusions significantly influenced the electron recombination and transport rates. These properties are investigated in detail using transient decay techniques. Following the study on the electronic structure of the bulk TiO2, the influence of the surface modification of the TiO2 nanoparticles on the electron transfer dynamics is investigated. The kinetics of electron recombination from TiO2 to the single electron redox shuttles is