Using metallic nanostructures to trap light and enhance absorption in organic solar cells

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.