Antireflective coatings (ARC) are a necessary element of solar cells and infrared (IR) optical applications. Current state-of-the-art coatings consist of materials such as TiO2 and Si3N4 deposited on silicon via chemical vapor deposition (CVD). This method of coating is undesirable due to the costly, tedious, and time intensive nature of the process. Herein, we have developed a novel antireflective coating (ARC) that is facile in nature. Through the implementation of high sulfur content polymers, with ultrahigh refractive index, and near to mid infrared (IR) transparency, we have been able to demonstrate high-quality films via spin/dip coating. This polymer was produced via inverse vulcanization of elemental sulfur, a byproduct of petroleum refining, and the organic monomer 1,3-diisopropenylbenzene (DIB) to yield poly(sulfur-r-(1,3-diisopropenylbenzene)) (poly(s-r-DIB)). The reaction product (poly(s-r-DIB)) was taken up into solution and deposited directly onto a silicon substrate. To verify the performance of a quarter wave antireflection coating, the spectra of the coated silicon wafer was taken. In this we found we could create a high performance, single layer, antireflection coating. Due to the unique nature of the polymer (poly(s-r-DIB)) used in this system we were able to fine tune the thickness of the coating, and therefore the target wavelength in which desired performance could be observed. Further investigation of the system is underway, as the polymer’s index can be tuned to fit a variety of substrates, making this system ideal for a multitude of antireflective applications.
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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