PLASMONIC TiO2 THIN FILMS: INTERACTION BETWEEN Au NANOPARTICLES AND ORGANIC DYES FOR DSSC APPLICATIONS

The introduction of metal nanoparticles into the nanoporous TiO2 structure of the photoanode layer in DSSCs can result in enhanced photocurrent depending on the dye and the nanoparticle characteristics. The presence of gold nanoparticles (AuNPs) induces the plasmonic near-field absorption enhancement that allows the dye molecules located in their proximity to harvest more light increasing the quantity of electrons injected in the TiO2 layer and resulting in improved photocurrent. Among the various systems studied so far, AuNPs in combination with Ruthenium dyes have received increasing interest. Our aim is to clarify the process behind the interaction between the dye and the AuNPs by means of steady-state and time-resolved spectroscopy of plasmonic TiO2 films sensitized with purely organic dyes. AuNPs are able either to be excited and inject electrons to the TiO2 conduction band (CB) or to be acceptors for electrons in the CB of TiO2, the latter is a loss process for the DSSC, increasing the recombination rate. Gold is also sensible to corrosion if directly in contact with an iodine/triiodide-based electrolyte and might also promote recombination of the oxidized cationic dye with the injected electrons. These chemical and electronic interactions have been avoided creating a core-shell-shell Au-silica-titania system featuring an insulating silica shell between the Au and the TiO2. Specific methodologies were used to build up a shell thinner than 10 nm so that only the near-field electromagnetic interaction between the plasmon and the dye was allowed. Semitransparent TiO2 layers were needed to carry out the planned spectroscopic investigations. Highly homogeneous slurries of coated AuNPs/TiO2 and different methods of deposition and annealing were tested to optimize thin semitransparent films towards the desired properties. Spectroscopic characterization was carried out in solution and in thin film samples. The extent of absorption enhancement was investigated in various mixtures of organic dyes and AuNPs coated with different layers.