Designing plasmonic nanoparticles for light trapping applications in solar cells

Abstract

Optically driven localised surface plasmons can be excited on sub-wavelength metal particles, which can strongly scatter light. These particles have interesting optical properties: the nanoparticle shape and size, and the local dielectric environment determine the wavelength dependent scattering behaviour. If these particles are fabricated on a high-index substrate, a large fraction of light is scattered into the optically dense medium. This can be exploited to couple incident sunlight into trapped modes in a solar cell, increasing the absorption in the active region. The challenge is then to design plasmonic structures that are strongly scattering, and that couple efficiently to underlying substrates. In this thesis, the design of plasmonic nanoparticles is investigated for light trapping applications in solar cells. By changing the type of dielectric spacer layer separating the particles from the semiconductor surface, tunable light trapping can be provided for Si solar cells by random arrays of self-assembled, Ag nanoparticles, using materials compatible with Si solar cell fabrication. Particle arrays can also be located on the rear of the cell, avoiding reductions in the transmission' of light into the solar cell at wavelengths below resonance that occur for front located particles. This configuration also has other advantages; namely that it allows the independent optimisation of light trapping and anti-reflection effects in a solar cell. In comparing the light trapping provided by front and rear-located arrays, asymmetry is observed in the scattering behaviour of particles with light incident from air, or through a Si substrate. By using 3D numerical modelling, in conjunction with a simple dipole model of scattering above a substrate, it is established that this is due to differences in the strength of the electric field driving the plasmonic oscillations. However, the dipole model breaks down when the nanoparticles are on very thin spacer layers. Anomolously high scattering cross-sections are calculated for particles directly on the substrate, and additionally asymmetry, over and above the differences in the driving field, is observed in the scattering from front and rear-located particles. By investigating the interaction of light with nanoparticles on high index substrates, and comparing this to the excitation of surface plasmon polaritons (SPPs) at metal/dielectric interfaces, different types of scattering resonances are identified. The ~ain plasmonic resonance is then attributed to geometrical resonances of SPP modes, confined to the volume of the particle. This new physical interpretation of the scattering behaviour of plasmonic particles on highindex substrates offers insights into the sensitivity of these modes to changes in the particle shape, and the details of the local dielectric environment. It also provides new design criteria for plasmonic structures for efficient light trapping, beyond the dipole model. These results indicate that it would be possible to design arrays of nanoparticles which would scatter strongly over the wavelengths at which light trapping is needed for a particular photovoltaic device. These types of arrays could be usefully employed to increase the absorption, and hence the efficiency of thin solar cells.