Lateral Nanoconcentrator Nanowire Multijunction Photovoltaic Cells

In the past year we focused on developing the metal nanostructure simulation, nanowire growth, and device fabrication. We explored a new geometry with Hertzianlike disc shaped antennas to replace the bowtie antennas we had initially investigated to achieve better performance. Light absorption enhancement of the nanodimers in terms of size and inter space was simulated. A testbed structure based on Si nanowire was developed to quantify the absorption enhancement in single NWs due to the presence of optical feed-gap antenna structures. We also studied a top down etching process for Si NW fabrication, which can be also used for other materials. The Ge core-shell nanowire device was fabricated and tested. The fabrication process is compatible to the integration with metal nanostructures and can be applied to other nanowire devices. An MOCVD tool is under construction which will be used to explore the high efficient III-V nanowire solar cells. GaP nanowire growth has been carried out using other MOCVD facilities to explore the conditions. Single crystalline anatase TiO2 nanowires were fabricated successfully using block copolymer self-assembled templates and dye-sensitized solar cell (DSSC) based on TiO2 nanowires were fabricated and tested. Introduction The objective of this research project is to develop a novel type of multijunction photovoltaic cell that uses lateral arrays of semiconductor nanowires (NWs) of various bandgaps as the elements that convert optical energy into electrical energy. In contrast to conventional stacking multijunction cells, the NWs of varying bandgap will not be connected in series in our approach. Instead, a specially-designed nanostructured metal film is used to split the incident broadband solar spectrum and localize spectral energy in different lateral spatial locations (spectral splitting and concentration) coinciding with the location of the NWs of the optimized bandgap. The same nanostructured metal film also allows for current extraction from each nanowire separately such that photocurrent matching is not required. This allows us to use a wide range of bandgaps (depending on the performance of the lateral metal spectral splitter and concentrator) without requiring current matching. It also allows broader choices on materials which can approach the ideal performance limits due to the less spectral mismatch losses. This removes the most GCEP Progress Report 2009 Wong, Peumans, Brongersma and Nishi