Managing the solar spectrum at sub-wavelength scales: nanowire enabled new PV concepts (Conference Presentation)

III-V semiconductor nanowires (NWs) are promising building blocks for next generation solar energy conversion at low cost. NW ensembles constitute a new class of metamaterial, where the optical properties of the array are tuned by the individual NW type, geometry and collective arrangement. Arising from the refractive index mismatch between NW and air and the sub-wavelength features, light propagation and distribution inside the nanostructure is strongly dependent on wavelength-geometry relation. While the photonic properties of small dielectric structures have been widely studied within the framework of Mie scattering, those of vertically standing nanowires cannot be explained with the same mathematical framework. In particular, coupling to poorly confined waveguided modes drives the absorption spectrum in NWs. The difficulty in obtaining III-V NWs on transparent substrates or self-standing, hampers the obtaining of their absorption properties experimentally. This work investigates how NWs interact with light from both theoretical and experimental methods. We experimentally illustrate the 3D extinction cross-section of GaAs NWs at different wavelengths by using fluorescence confocal microscopy. We demonstrate that by probing ordered arrays with fluorescence confocal microscopy, the effective absorption coefficient and cross-section can be obtained without the need of a transparent substrate. From these properties, we discuss a new variety of solar cell designs, that are not possible in the bulk form. Finally, we will also show that scanning probe microscopy is an emerging tool for the characterization of nanoscale solar cells, as well as a new fabrication approach to create 3D nanostructures on demand.