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In solar applications, traditional crystalline silicon photovoltaic (PV) cells are the most commonly used technology to harvest solar energy. The efficiency of Si PV is fundamentally limited to around 33% and in practice, these cells have an outdoor efficiency of less than 22%. Concentrated PV technology uses multi-junction PV cells that collect a broader spectrum of the sun with high efficiency (>40% has been reported). However, due to the different semiconductors used, multi-junction cell costs are higher than traditional PV cells. Increasing the solar concentration not only reduces the cost of electricity produced by multi-junction cells, by reducing the required area, but can also maximize the IV efficiency of the cells. There exist different methods to concentrate solar energy such as large parabolic mirrors, which have tracking challenges due their size and weight; or spherical lens arrays, which have limited optical geometrical concentration ratios. In this respect, freeform optical devices can be used to enhance the optical throughput for multi-junction cells and reduce the space required to achieve large concentration ratios. In this work, we discuss a novel optical design combining aspherical lens arrays and arrays of optical waveguides, which constitute broadband, freeform non-imaging optical devices. We compare different waveguide designs which have been optimized using non-sequential ray tracing software. The relationship between the optical surface quality and the optical efficiency is also investigated. Finally, we present the results of the experimental characterization of these waveguides under laboratory conditions using different techniques to measure optical throughput and stray light losses.
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