Photovoltaic (PV) windows could help increase the surface areas of buildings that can be exploited to generate energy. One of the advantages of LSC-based windows is that they do not require any bulky structure to be applied to their surface since the photovoltaic cells are hidden in the window frame itself. This means that they blend in aesthetically with their environment.

A team of researchers led by Uwe Kortshagen of the University of Minnesota and Sergio Brovelli and Franco Meinardi at the University of Milano-Bicocca has now discovered that silicon nanocrystals or colloidal quantum dots (QDs) are an excellent material for LSCs. Not only is silicon extremely abundant on Earth, it is also non-toxic and its nanocrystalline form has an optical profile that is perfect for producing large-area concentrators that can be paired with low-cost silicon solar cells. Indeed, at the nanoscale, silicon’s properties change – it efficiently emits light at certain frequencies, but does not strongly absorb light at these same frequencies. This means that emitted and trapped light within the concentrator is not easily “lost” during reabsorption events by neighbouring QDs as the light makes its way to the device edge.

“This is an uncommon material property and has been a major hurdle for the technology in the past,” explains team member and co-lead author of the study Samantha Ehrenberg.

The researchers made their silicon quantum dots in a plasma reactor that can produce various nanomaterials from gaseous precursors in a bottom-up method to grow the crystals atom by atom. They collected the silicon as a dry powder from the plasma reactor and then chemically treated the particle surfaces in a solution to produce highly luminescent and transparent silicon ink that can be used to dope a polymer waveguide.

“This approach is based on a common cell-casting synthesis technique that we adapted so as to minimize harmful interactions of the QDs with radical polymerization initiators,” explains Meinardi. “Using this fairly gentle procedure, combined with the robustness of our Si-QDs, allows us to produce particles that keep their light-emitting properties when they are incorporated into the photopolymerized cross-linked PLMA matrix. And that is not all: by incorporating the QDs into PLMA and suitably selecting the cross-linking level, we obtain both rigid and flexible freestanding polymer slabs with an excellent optical quality that are well suited to making PV windows and other architectural elements.”

“Once in a slab, the majority of light that the particles emit becomes trapped within the slab, only escaping at the small area edges, where it is concentrated,” adds Ehrenberg.

“The fabrication technique allows for Si-QDs that are also stable under UV light and to high temperatures of up to 100°C,” she says, and we can also choose the emission wavelength by tuning the nanocrystal diameter.”

Disruptive technology

Our technology is disruptive in the field of LSCs, say Meinardi and Ehrenberg, since no other type of existing chromophore (be it top-notch organic dyes or engineered nanostructures) can produce such a power-efficient small device that could easily be reproduced on the industrial scale. We believe that silicon QDs will ultimately allow us to bring this technology to life in the short term, where it will find applications in converting urban windows into energy harvesting surfaces.

Spurred on by its results, the team is now busy trying to build real-world semi-transparent PV windows. “Our target is to install a fully working system prototype in an operational environment by the end of this year and then to certify and hopefully commercialize it,” says Meinardi.

“Our computer simulations on the system predict that we can more than double the performance of our LSCs by adjusting how much of the Si-QDs we add to the plastic slabs,” adds Ehrenberg. We hope to demonstrate such performance in the lab while we continue to study the long-term performance of the devices and devise ways of improving the silicon nanocrystals further, by improving their emission efficiencies.”

The Si-QDs LSCs are detailed in Nature Photonics doi:10.1038/nphoton.2017.5.