Nanoparticle plasmonics for solar cell applications

Licentiate thesis, 2011

The energy demand of society increases rapidly, while the main source of today’s energy, the fossil fuels, eventually will be depleted and also poses environmental and climate hazards (though the global warming). Therefore there is a need for alternative, renewable energy sources, and solar photovoltaics (solar cells) will play an important role as one of them. The photovoltaics research is very extensive today, but the relatively high cost of solar cells and their intermittent production of electricity make solar cells still loose in the energy market competition. Efficiency is also an issue. This thesis explores novel concepts, enabled by advances in nanotechnology, in the solar cell research. In particular, it focuses on 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. This phenomenon leads to interesting ways through which the nanoparticle interacts with light and with its own nanorange environment; in particular, the electric field in the close vicinity of the particle is largely enhanced compared to the incident light. This thesis focuses on employing this enhanced field to (i) enhance light absorption in thin amorphous Silicon (a-Si) films and (ii) sense adsorption and diffusion of dye molecules in titania (TiO2) films, used for dye sensitized solar cells (DSC). In the first part of the thesis, photoconductivity measurements were performed on a-Si films, with and without plasmonic Ag nanoparticles, in order to quantify the ‘extra’ light absorption in a-Si films caused by the enhanced near-field around the nanoparticles. The plasmon induced light absorption was studied as a function of a-Si film thickness, and was found to be maximal (15% absolute increase of absorptance, from 22% to 37%) in a 9 nm thick a-Si film. The finite-element method calculations reproduced the experimental results reasonably well. The observed plasmon-induced increase in the light absorption is substantial and it has a large potential toward realizing an ultrathin (about 20 nm) a-Si solar cell with efficiency similar to that of the standard (about 300 nm thick) a-Si cell. In the second part of the thesis, Indirect Nanoplasmonic Sensing was used to study adsorption kinetics of dye molecules on TiO2 films. The concept was first demonstrated on thin (12-70 nm) compact TiO2 films that serve as a model system, and was extended toward thick (10 µm) mesoporous TiO2 film of the kind used in dye solar cells. The short-range sensitivity of the plasmonic nanoparticles allows monitoring processes locally in their vicinity, i.e. at the interface between the TiO2 film and the support, mimicking the electron collecting electrode. This technique has a large potential for studying combined diffusion and adsorption kinetics for DSCs, as well as for similar phenomena in a broad range of other (meso-)porous materials.