Nanoplasmonics for solar cells

Doktorsavhandling, 2014

The main source of our energy system, the fossil fuels, will eventually be depleted and also pose environmental and climate hazards. There is thus a need for alternative, renewable energy sources. Solar (photovoltaic) cells will play an important role as one of them. The photovoltaics research is extensive today, but the relatively high cost of solar cells and/or insufficient efficiency make them still often loose in the energy market competition. This thesis explores novel concepts for solar cell research, enabled by advances in nanotechnology, specifically applying the phenomenon called plasmon resonance in metal nanoparticles, to study and improve thin-film solar cells. The plasmon resonance is a collective oscillation of conduction electrons in a metal nanostructure, which can be excited by light. It leads to interesting and potentially useful interactions between nanoparticles and light; in particular, the electric field in the vicinity of the nanoparticle is enhanced compared to that of the incident light. This work focuses on employing the enhanced field to (i) improve light absorption in thin amorphous silicon (a-Si:H) films and (ii) to sense adsorption and diffusion of dye molecules in TiO2 films, used for dye-sensitized solar cells. In the first part of the thesis, optical and photoconductivity measurements were performed on ultrathin a-Si:H films, with and without Ag plasmonic nanoparticles, in order to quantify the light absorption in a-Si:H films caused by the enhanced near-field around the nanoparticles. The effect was studied for (i) systems of Ag nanodiscs coated with a-Si:H films of various thicknesses, and (ii) Ag cone/a-Si:H nanocomposites placed on a reflector-spacer structure with varied geometric parameters. Finite-element method calculations were used to connect observed experimental features to specific plasmon resonance modes, and to explain mechanisms of absorption enhancement in the a-Si:H films. The second part of the thesis is focused on adsorption and diffusion kinetics of dye molecules on TiO2 films, studied by Indirect NanoPlasmonic Sensing (INPS) and Quartz crystal microbalance with dissipation monitoring (QCM-D) techniques. Measurements on flat film model systems revealed details of adsorption and desorption kinetics and allowed extracting the corresponding rate constants. Incorporating plasmonic sensing nanoparticles within mesoporous TiO2 films provides a unique opportunity to resolve adsorption kinetics locally in the film (in this case, at the bottom of the mesoporous TiO2 films). Diffusion times for dye molecules through the mesoporous films were measured and modelled with a diffusion-front model. This allowed deriving the effective diffusion coefficient of the dye molecules in this system.