Windows that can efficiently collect solar energy are one step closer to becoming a reality thanks to high-tech silicon nanoparticles. Researchers at the University of Minnesota and University of Milano-Bicocca have developed technology to embed the silicon nanoparticles into what they call efficient 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. Photovoltaic windows — windows that can collect solar energy — have the potential to largely increase the surface of buildings suitable for energy generation without impacting their aesthetics. LSC-based photovoltaic windows do not require any bulky structure to be applied onto their surface and since the photovoltaic cells are hidden in the window frame, they blend invisibly into the built environment. While most of the light concentrated to the edge of the silicon-based luminescent solar concentrator is actually invisible, the concentration effect can be seen better when the slab is illuminated by a “black light” which is composed of mostly ultraviolet wavelengths. Courtesy of Uwe Kortshagen, College of Science and Engineering. The silicon nanoparticles set this study apart from others. Until recently, the best results had been achieved using relatively complex nanostructures based either on potentially toxic elements, such as cadmium or lead, or on rare substances like indium, which is already massively utilized for other technologies. Silicon is abundant in the environment and nontoxic; it also works more efficiently by absorbing light at different wavelengths than it emits. Uwe Kortshagen, University of Minnesota mechanical engineering professor and inventor of the process for creating silicon nanoparticles, said because silicon in its conventional bulk form does not emit light or luminesce, they had to make some changes. "In our lab, we 'trick' nature by shrinking the dimension of silicon crystals to a few nanometers, that is about one ten-thousandths of the diameter of human hair," said Kortshagen. "At this size, silicon's properties change and it becomes an efficient light emitter, with the important property not to re-absorb its own luminescence. This is the key feature that makes silicon nanoparticles ideally suited for LSC applications." Using the silicon nanoparticles opened up many new possibilities for the research team. "Over the last few years, the LSC technology has experienced rapid acceleration, thanks also to pioneering studies conducted in Italy, but finding suitable materials for harvesting and concentrating solar light was still an open challenge," said Sergio Brovelli, physics professor at the University of Milano-Bicocca. "Now, it is possible to replace these elements with silicon nanoparticles." University of Minnesota researcher Samantha Ehrenberg uses a plasma reactor to create silicon nanoparticles that are the key ingredient in the solar concentrators. Courtesy of Patrick O'Leary, University of Minnesota. Researchers say the optical features of silicon nanoparticles and their nearly perfect compatibility with the industrial process for producing the polymer LSCs create a clear path to creating efficient photovoltaic windows that can capture more than 5 percent of the sun's energy at unprecedented low costs. "This will make LSC-based photovoltaic windows a real technology for the building-integrated photovoltaic market without the potential limitations of other classes of nanoparticles based on relatively rare materials," said Francesco Meinardi, physics professor at the University of Milano-Bicocca and one of the first authors of the paper. The silicon nanoparticles are produced in a high-tech process using a plasma reactor and formed into a powder. The powder is then turned into an ink-like solution and embedded into a polymer that can be formed into a sheet of plastic material or used to coat a surface with a thin film. The research has been published in the journal Nature Photonics (doi:10.1038/nphoton.2017.5).