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Dunbar, Ricky (2012): Using metallic nanostructures to trap light and enhance absorption in organic solar cells. Dissertation, LMU München: Faculty of Physics
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Abstract

Solar cells generate clean electricity from sunlight. However, they remain significantly more expensive than other, less environmentally-friendly, energy generation technologies. Although the emergence of thin-film solar cells, low-cost alternatives to the prevailing crystalline silicon solar cells, has been a significant advance in photovoltaic technology, these devices typically suffer from low absorption. If this absorption could be enhanced, it would enable an increase in power conversion efficiency and hence a reduction in cost/kW of generating capacity. This is the motivation of the work presented in this doctoral thesis. Metallic nanostructures are used to trap light within the semiconductor film in organic solar cells. By increasing the optical path length, the probability that photons are absorbed before exiting the film is increased. A novel process is developed to fabricate nanostructured metallic electrode organic solar cells. These devices feature a nanovoid array interface between the metallic electrode and the semiconduc- tor film. Absorption enhancements over conventional, planar architectures as high as 45% are demonstrated. This light-trapping is found to be largely enabled by localized void plasmons. The experimental investigations are supported by finite element simulations of absorption in solar cells, which display very good agreement with experimental results. It is found that light trapped in organic solar cell architectures is very efficiently absorbed by the organic film - in- creases in the exciton generation rate per unit volume of semiconductor material of up to 17% are observed. The simulation routine is additionally used to compare and contrast common plasmonic architectures in organic solar cells. The role of the metallic nanostructure geometry on the dominant light-trapping mechanism is assessed for various size domains and optimum architectures are identified. When implemented according to the findings of this thesis, light- trapping will have the potential to vastly increase the efficiency and hence decrease the price of thin-film solar cells.