Plasmonic nanostructures for absorption enhancements close to the GaAs band edge

In this paper we present the optical characterisation of periodic arrays of Au nanoparticles fabricated on both GaAs substrates and a triple junction solar cell structure. The nanoparticle arrays were designed to employ the localised surface plasmon resonances of the nanoparticles and diffraction effects arising from the periodicity of the arrays, at wavelengths close to the GaAs band edge. In principle effects such as these may be used to improve the absorption of light in solar cell structures and hence increase their efficiency. In particular, we aim to build upon the success of high efficiency GaAs based photovoltaic cells by targeting the spectral region close to the band edge, where the absorption strength in conventional GaAs solar cells is poor. We demonstrate from numerical simulations and experimental observation that by careful adjustment of the nanoparticle dimensions we can tune the localised surface plasmon resonance to the desired wavelength. Furthermore, we show evidence of the diffraction of incident light into lateral modes within the absorbing material, increasing the optical path length. This effect is associated with the periodic nature of the arrays, and can therefore also be spectrally tuned by controlling the spacing between the nanoparticles. Such periodic nanoparticle arrays therefore provide two methods of absorption enhancement that may be employed in the same structure.