Nanoparticle plasmon influence on the charge carrier generation in solar cells

Doktorsavhandling, 2008

Global warming is a potential threat to life on earth and to human society. It is by much evidence linked to the extensive use of fossil fuels in our present energy system. Replacing the fossil fuels by alternative sources of energy poses a tremendous challenge for mankind, but is not at all undoable. Of the many alternatives that can be employed, electricity producing photovoltaic solar cells are one of the most attractive long term solutions due to their scalability, flexibility, simplicity, environmental friendliness and a huge, reliable, physical potential for providing energy. However, they are not yet cost competitive for on-grid applications. An important route to reduce the cost of photovoltaic solar cells, is to accomplish the same or even improved efficiency as in today’s solar cells, with less (thinner) photoactive material. An interesting possibility in this respect is to employ localized surface plasmon resonances (LSPRs) for the initial light capture. A plasmon resonance is a collective oscillation of the conduction electrons which can be excited by electromagnetic radiation in small metal particles. Especially noble metal particles of nanoscale dimensions create LSPRs with very large optical cross sections in the visible and near infrared spectrum, which is the range of interest for photovoltaic applications. There are three possible mechanisms for plasmon enhanced conversion of incident light to free charge carriers in an adjacent photovoltaic junction: i) via a favorably modified electromagnetic (EM) far-field distribution, ii) via EM near-field enhancement close to the particles and iii) via charge carrier photoemission from the particles to the semiconductor substrate. In this thesis, the influence of noble metal nanoparticles on the charge carrier generation in two different types of photovoltaic solar cells, representing two extreme cases, were investigated experimentally and theoretically. The first is a planar version of the dye sensitized, Grätzel solar cell (DSSC), which relies on surface absorption of light in dye molecules. The second is a more conventional silicon (Si) pn-junction solar cell, where absorption takes place in the bulk of the material. Experiments and theory (including finite element calculations) primarily address the EM mechanisms i) and ii) outlined above, and indicate that near-field enhanced performance at the LSP resonance is possible for the surface absorption (DSSC) case, but that negative effects dominate for the bulk absorption (Si) case. On the other hand, enhanced coupling into the bulk substrate modes results in higher photocurrents for longer wavelengths. A possibility related to this is to couple energy into waveguided modes of thin photovoltaic layers via the plasmonic particles. In a parallell study dealing with laser induced restructuring of thin gold films, we found a very concrete example of such coupling; gold particles were moved and reshaped into a grating pattern, due to the formation of a standing wave involving guided modes of the thin Si3N4 substrate.