PORTLAND, Ore. — Quantum dots are already revolutionizing displays, such as those used in widely praised Kindle Fire e-readers whose backlight uses a quantum-dot enhanced film (QDEF) manufactured by Nanosys. Now researchers are poised to revolutionizesolar energycollectors with quantum dots.

By harvesting light coming from the sun with embedded quantum dots, the researchers hope to turn windows into efficient solar-panel concentrators. Their strategy is to place photovoltaic (PV) solar cells around the edges of quantum-dot-impregnated windows, thus turning them into luminescent solar concentrators (LSCs). Los Alamos National Laboratory, in cooperation with Italy's University of Milano-Bicocca (UNIMIB), recently demonstrated optical efficiencies for such LSC windows of greater than 10 percent and an effective concentration factor of more than four.

Normally transparent quantum-dot-impregnated window pane material luminescences under ultraviolet illumination.

(Source: Los Alamos National Laboratory)

"Our device is a light-harvester -- a concentrator that captures light from a large area and directs it to a much smaller PV cell," Victor Klimov, lead researcher on the project at the Center for Advanced Solar Photophysics (CASP) at Los Alamos National Laboratory, told EETimes.

For the proof-of-concept demonstration, Klimov's team embedded the quantum dots into a transparent plastic material with PV solar cells around its edges, with the help of colleagues at UNIMIB, including Sergio Brovelli, who worked at Los Alamos National Labs until 2012 but who is now a faculty member at UNIMIB.

"The quantum dots re-emit absorbed solar light at a longer wavelength, which then propagates in the regime of total internal reflection towards the PV cell installed at the edge of the LSC device," says Klimov.

Quantum dots are highly efficient emitters, demonstrating emission efficiencies approaching 100 percent, but previous attempts to use them in LSCs of practical dimensions were not successful. The problem was that the quantum dots reabsorbed many of the re-emitted photons that were intended to be harvested by the edge-mounted PV cells. To solve that problem, Klimov and colleagues engineered quantum dots that shifted the wavelength of the re-emitted photons using an approach of Stokes-shift-engineering, named after the 19th century Irish physicist George Stokes.

A giant Stokes-shift was engineered into the quantum dots by combining two different materials, cadmium selenide (CdSe) and cadmium sulfide (CdS), in a core-shell geometry. A small CdSe core served as an emitter while a thick CdS shell played the role of a light-harvesting antenna. Since CdS has a wider bandgap than CdSe, the light re-emitted by the CdSe core exhibited a large low-energy shift with respect to the onset of strong optical absorption defined by the CdS shell. This strategy resulted in a giant Stokes shift, which helped eliminate losses to re-absorption.

The researchers demonstrated that the resultant LSC devices had light harvesting efficiencies of about 10 percent, with virtually no losses to reabsorption for slabs measuring tens of centimeters. And simulations showed that practical dimensions of these devices can be extended to over a meter.

Cadmium-selenium (CdSe) quantum dots absorb photons, then re-emit the energy at a different wavelength from their cadmium-sulfur (CdS) inner cores when embedded in a polymethylmethacrylate (PMMA) window pane that directs the concentrated solar energy to phovoltaic cells at the window's edge.

The Los Alamos Nation Lab researchers created the thick shell CdSe/CdS quantum dots, while their Italian partners embedded them into the large slabs of PMMA. Details can be found in the Nature Photonics paper titled "Large-area luminescent solar concentrators based on Stokes-shift-engineered nanocrystals in a mass-polymerized PMMA matrix."

Funding was provided by the US Department of Energy's Office of Science through the Center for Advanced Solar Photophysics (CASP), an Energy Frontier Research Center at Los Alamos National Laboratory. The work at UNIMIB was funded by Fondazione Cariplo and the European Community’s Seventh Framework Programme.

— R. Colin Johnson, Advanced Technology Editor,EE Times

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