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.[1] The surface plasmon resonance phenomena due to the presence of gold nanoparticles (AuNPs) induces the plasmonic near-field absorption enhancement which allows the dye molecules located in their proximity to harvest more light; thus increasing the quantity of electrons injected in the semiconductor oxide layer resulting in improved photocurrent. Among the various systems studied so far, gold nanoparticles in combination with Ruthenium dyes have received increasing interest.[2] 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 [3] , the latter a loss process for the DSSC, increasing the recombination rate. Unfortunately 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 creating a double Schottky barrier between the Au and the TiO2.[4] Specific methodologies [5] 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. Thus 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 different organic dyes and AuNPs coated with different layers. Comparison with analogue, well-known, Ruthenium dyes was performed to investigate possible additional phenomena due to the presence of a dye metal center.