Plasmonics and nanophotonics for photovoltaics
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The scope for nanoscale light trapping Over the past 30 years, the cost of solar modules has reduced by over a factor of 10. About half of this reduction has been due to economies of scale; the other half by progress in increasing cell efficiencies and improved fabrication methods. The major cost component for silicon modules is now the materials cost of the silicon wafers, which is 40% of the cost of the entire module. This has led to a trend toward cheaper wafers, such as multicrystalline silicon, as well as thin-film cells, such as those based on polycrystalline silicon and cadmium telluride. As solar cells become thinner, absorption losses become more important. For example, Figure 1 shows the absorption in a 2 micron thick silicon film, with and without light trapping, compared to the irradiance available in the solar spectrum. Until recently, the main approach to reducing these losses was to texture the surface using features a few microns in size. The behavior of such structures can be described using geometrical optics (i.e., ray tracing). Some geometrical textures are known to be highly effective, for example, upright or inverted pyramids1 and random textures.2 However, these textures are only suitable for certain types of solar cells. Pyramids are only
suitable for monocrystalline silicon, and geometrical scale surface textures, in general, are not suitable for thin-film solar cells, which have thicknesses from a few hundred nanometers to a few microns. Many years ago, Yablonovitch and Cody showed that the limit of light trapping for a random, geometric texture was an increase in absorption for weakly absorbed light of a factor of 4n2 (where n is the index of refraction of the material) for isotropically incident light.3 This is a very large enhancement, around 50 for a high refractive index material such as silicon. The reason for the enhancement is the high density of optical modes in the silicon, compared to modes in air. The derivation assumes that the light is equally distributed among all the optical modes. This means that scattering processes tend to scatter light back into the high index material, leading to the large enhancement in absorption. According to the brightness theorem (see for example Reference 4), it is not possible to go beyond the 4n2 limit using geometrical optics except by narrowing the range of the angle of incidence of the light. Stuart and Hall extended the method to thin films, which only support a few waveguide modes.5 They showed that because there are fewer modes available, the
Kylie R. Catchpole, Australian National University; [email protected] Sudha Mokkapati, Australian National University; [email protected] Fiona Beck, Institut de Ciències Fotòniques, Barcelona, Spain; [email protected] Er-Chien Wang, Australian National University; [email protected] Arnold McKinley, Australian National University; [email protected] Angelika Basch, Institute of Physics, University of Graz, Austria; [email protected] Jaret Lee, Australian National Universit
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