Linear Optics of Plasmonic Concepts to Enhance Solar Cell Performance

Abstract

Thin-film solar cells, with typical film thicknesses in the range of 1-2 μm, offer the benefit of reduced costs of materials and fabrication as compared to conventional silicon solar cells. However, they have the disadvantage of small absorbance at near-bandgap wavelengths (for example, in the red and near-infrared for silicon), resulting in small photocurrent densities. Common strategies to increase the absorbance are a reduction of reflection losses at the cell surface and a trapping of light in the absorbing layer, both through structuring of the cell surfaces. In recent years, interest has grown in replacing the randomly textured layers conventionally used for this purpose by nanostructures made of metals such as silver and aluminum. They offer the advantages of very compact dimensions, large scattering/diffraction efficiencies, and potentially useful optical near-field effects.The promising optical properties of nanostructures made of silver, aluminum, and a few other metals, for example, gold and copper, result from the fact that they support surface plasmons, which are collective excitations of conduction electrons at the metal surface and able to interact very strongly with the light field [1]. In metal nanoparticles, they are also called particle plasmons (PPs) or localized surface plasmons; on structured metal films, they are also known as surface-plasmon polaritons ( SPPs). Their strong interactions with the light field show themselves, for example, as huge optical cross-sections for light absorption and scattering. The large values of these cross-sections, which are caused by the great number of conduction electrons oscillating in concert even in relatively small optically excited metal nanoparticles [2], make them interesting for the incoupling of light into photovoltaic layers. In addition, the nanoparticles show a rich spectral behavior. Their spectra can be controlled by various factors: the choice of the metal, their shape and size, their dielectric environment, and the electromagnetic coupling between them. This control provides the potential for tuning the plasmon excitations into spectral regions where they are the most useful for applications in solar cells. An additional focus of interest are the optical near fields at and near the metal surface;