Metal nanoparticles: light-induced charge separation and energy transfer

This thesis aims to explore the possibilities and limitations of employing metal nanoparticles for photovoltaic solar energy conversion. Plasmon-induced charge separation and energy transfer phenomena are explored in three experimental systems. In the first, 13 nm gold and 25 nm silver nanoparticles located at a TiO2/hole conductor interface are found to generate sustainable photocurrents upon excitation of the particles’ localized surface plasmon resonance. The photocurrent response closely follows the plasmonic absorption characteristics of the nanoparticles. A simple and fast method for the fabrication of these plasmonic solar cells is developed, based on electrostatic metal nanoparticle self-assembly. The method allows for the controlled creation of optically homogenous and dense nanoparticle monolayers with well-defined interparticle separations. The absorbed photon-to-electron conversion efficiency is found to significantly increase with deceasing particle size to a maximum of 15% for 5 nm Au nanoparticles, while it is found to be largely independent of the nanoparticle density. Three mechanisms for surface plasmon resonance based charge carrier separation are proposed and discussed. Self-assembly strategies based on molecular linkers are explored to enable a closer control over the nanoscale engineering of the TiO2/NP and NP/hole transport material interfaces. These strategies are also used to fabricate plasmonic solar cells based on anisotropic metal nanoparticles and self-assembled plasmonic nanostructures. In the second experimental system, the energy transfer between organic molecules and gold nanoparticles is studied in a colloidal model system. This energy transfer potentially enables the use of metal nanoparticles as ‘light harvesting antennas’ in photovoltaic devices. The distance and wavelength dependence of the electronic coupling between fluorophores and gold nanoparticles is determined theoretically and experimentally via steady state and time- resolved fluorescence spectroscopy. The fluorescence quenching of 4 different dye molecules, which absorb light at different wavelengths across the visible spectrum and into the near- infrared, is studied using a rigid silica shell as a spacer. A comprehensive experimental determination of the distance dependence from complete quenching to no coupling is carried out by a systematic variation of the silica shell thickness. Electrodynamic theory predicts the observed quenching quantitatively in terms of energy transfer from the molecular emitter to the gold nanoparticle. The plasmonic field enhancement in the vicinity of the 13 nm gold nanoparticle is calculated as a function of distance and excitation wavelength and is included in all calculations. Relative radiative and energy transfer rates are determined experimentally and are in good agreement with calculated rates. In control experiments, dye-dye interactions are found to have a severe effect on the fluorescence properties of dyes attached to the surface of a silica nanoparticle. This allows us to determine the experimental conditions, under which dye-dye interactions do not affect the experimental results. The energy transfer between gold nanoparticles and sensitizers in a dye-sensitized solar cell is explored in the third system and preliminary results are presented. The incorporation of gold nanoparticles in different dye-sensitized solar cell architectures is found to result in a decrease of the generated photocurrent in all cases. Different strategies for the incorporation of gold nanoparticles into dye-sensitized solar cells are evaluated and the importance of reference devices to evaluate the influence of the nanoparticle incorporation is highlighted.