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    Photovoltaic windows, the next frontier in renewable energy

    Windows that can efficiently collect solar energy are a step closer to reality with researchers at the University of Minnesota and University of Milano-Bicocca embedding high tech silicon nanoparticles into luminescent solar concentrators (LSCs).

    These LSCs are the key element of windows that can efficiently collect solar energy; when light shines through the surface, the useful frequencies of light are trapped inside and concentrated to the edges where small solar cells can be put in place to capture the energy.

    Considered the next frontier in renewable energy technologies, photovoltaic windows can potentially increase the surface of buildings that can generate energy without any impact to the aesthetics. Unlike conventional systems, LSC-based photovoltaic windows do not require any bulky structure to be applied onto their surface; since the photovoltaic cells are concealed in the window frame, they blend invisibly into the built environment.

    Though the concept of solar concentrators and solar cells integrated into building design is not new, the latest research stands apart for its use of silicon nanoparticles. An abundant material in the environment and also non-toxic, silicon works more efficiently by absorbing light at different wavelengths than it emits.

    University of Minnesota mechanical engineering professor Uwe Kortshagen explains that their process involves shrinking the dimension of silicon crystals to a few nanometers, about one ten-thousandths of the diameter of human hair. At this size, silicon's properties change and it becomes an efficient light emitter, without reabsorbing its own luminescence. This is the key feature that makes silicon nanoparticles ideally suited for LSC applications. Kortshagen, one of the senior authors of the study, is the inventor of the process for creating silicon nanoparticles.

    Sergio Brovelli, physics professor at the University of Milano-Bicocca and co-author of the study says LSC technology has, in recent years, experienced rapid advancement, thanks to pioneering studies conducted in Italy, but finding suitable materials for harvesting and concentrating solar light was still an open challenge. Now, it’s possible to replace these elements with silicon nanoparticles. Brovelli is the co-founder of the spin-off company Glass to Power that is industrialising LSCs for photovoltaic windows.

    Researchers say the optical features of silicon nanoparticles and their nearly perfect compatibility with the industrial process for producing the polymer LSCs can create efficient photovoltaic windows with the ability to capture more than 5 percent of the sun's energy at unprecedented low costs.

    Francesco Meinardi, physics professor at the University of Milano-Bicocca and one of the first authors of the paper, believes this will make LSC-based photovoltaic windows a real technology for the building-integrated photovoltaics market without the potential limitations of other classes of nanoparticles based on relatively rare materials.

    The silicon nanoparticles are produced in a high-tech process using a plasma reactor and formed into a powder with each particle made up of less than two thousand silicon atoms, says Samantha Ehrenberg, a University of Minnesota mechanical Ph.D. student and another first author of the study. The powder is turned into an ink-like solution and then embedded into a polymer, either forming a sheet of flexible plastic material or coating a surface with a thin film.

    The University of Minnesota invented the process for creating silicon nanoparticles about a dozen years ago and holds a number of patents on this technology. In 2015, Kortshagen met Brovelli, an expert in LSC fabrication who had already demonstrated various successful approaches to efficient LSCs based on other nanoparticle systems. The potential of silicon nanoparticles for this technology led to the partnership with the University of Minnesota producing the particles and researchers in Italy successfully fabricating the LSCs by embedding them in polymers through an industrial method.

    This research is published in Nature Photonics.

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