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RESEARCH HIGHLIGHT: Capture sunlight in your window


Milano, August 24 2015 – Luminescent solar concentrators (LSCs) are an emerging sunlight harvesting technology that has the potential to disrupt the way we think about energy by turning any window into a daytime power source. In these devices, a fraction of light transmitted through the window is absorbed by nanosized particles (semiconductor quantum dots) dispersed in a glass window, re-emitted at the infrared wavelength invisible to the human eye, and wave-guided to a solar cell at the edge of the window. Using this design, a still nearly transparent window becomes an electrical generator, which can power your room’s air conditioner on a hot day or a heater on a cold one. This is what becomes possible with new devices – quantum dot LSCs – reported in the study “Highly efficient large-area colourless luminescent solar concentrators using heavy-metal-free colloidal quantum dots” (DOI: 10.1038/NNANO.2015.178) realized by the research team coordinated by Prof. Sergio Brovelli and Prof. Francesco Meinardi of the Department of Materials Science of the University of Milan-Bicocca (UNIMIB) in Italy and researchers at the Center for Advanced Solar Photophysics (CASP) of Los Alamos National Laboratory led by Victor Klimov.


In April 2014, by using special composite quantum dots, the Italian-American collaboration demonstrated the first example of large-area luminescent solar concentrators free from optical losses due to reabsorption of the guided light by the nanoparticles (see News Archive). This represented a fundamental advancement with respect to state-of-the-art technology based on organic emitters, that allowed for the realization of concentrators only a few centimeters in size. However, the quantum dots used for those proof-of-principle devices were still unsuitable for real world applications, as they comprised toxic heavy metals (cadmium) and were capable of absorbing only a portion of the solar light. This resulted in intrinsically limited efficiency and strong yellow/red coloring of the concentrators, which complicated and even inhibited their application in residential environments.

“In order for this technology to leave the research laboratories and reach its full potential in sustainable architecture”, explains Francesco Meinardi, professor of Physics at UNIMIB ”it is necessary to realize non-toxic concentrators capable of harvesting the whole solar spectrum, while still preserving the key ability to transmit the guided luminescence without reabsorption losses, so as to conjugate high photovoltaic efficiency with dimensions compatible with real windows. The aesthetic factor is also of critical importance for the desirability of an emerging technology.”

Victor Klimov, CASP’s director, explains, “Our new devices utilize quantum dots of a complex composition which includes copper (Cu), indium (In), selenium (Se) and sulfur (S). This composition is often abbreviated as CISeS. Importantly, these particles do not contain any toxic metals that are typically present in previously demonstrated LSCs. Furthermore, the CISeS quantum dots provide a uniform coverage of the solar spectrum thus adding only a neutral tint to a window without introducing any distortion to perceived colors. In addition, their near-infrared emission is invisible to a human eye, but at the same time is ideally suited for most common solar cells based on silicon.”

Hunter McDaniel, formerly a CASP postdoc and presently a quantum dot entrepreneur (UbiQD Founder and President), adds, “with a new class of low-cost, low-hazard quantum dots composed of CuInSeS, we have overcome some of the biggest roadblocks to commercial deployment of this technology. One of the remaining problems to tackle is reducing cost, but already this material is significantly less expensive to manufacture than alternative quantum dots used in previous LSC demonstrations.”

A key element of this work is a new UNIMIB protocol for encapsulating quantum dots into a high-optical quality transparent polymer matrix using a procedure comparable to the cell casting industrial method used for fabricating high optical quality polymer windows. The polymer used in this study is a cross-linked polylaurylmethacrylate, which belongs to the family of acrylate polymers. Its long side-chains prevent agglomeration of the quantum dots and provide them with the “friendly” local environment, which is similar to that of the original colloidal suspension. This allows one to preserve light emission properties of the quantum dots upon encapsulation into the polymer.

Sergio Brovelli, the lead researcher on the Italian team, concludes: «Quantum dot solar window technology, of which we had demonstrated the feasibility just one year ago, now becomes a reality that can be transferred to the industry in the short to medium term, allowing us to convert not only rooftops, as we do now, but the whole body of urban buildings, including windows, into solar energy generators. This is especially important in densely populated urban area where the rooftop surfaces are too small for collecting all the energy required for the building operations. Our estimations indicate that by replacing the passive glazing of a skyscraper such as the One World Trade Center in NYC (also known as Freedom Tower, 72.000 square meters divided into 12.000 windows) with our technology, it would be possible to generate the equivalent of the energy need of over 350 apartments. Add to these remarkable figures, the energy that would be saved by the reduced need for air conditioning thanks to the filtering effect by the LSC, which lowers the heating of indoor spaces by sunlight, and you have a potentially game-changing technology towards zero energy cities”.

The study was funded by Fondazione Cariplo, The European Research Council and by the US Department of Energy.

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