Recent quantum dot research at Los Alamos National Laboratory could lead to windows that also act as solar panels. The work shows that the greater light emitting properties of quantum dots are applicable to solar energy, helping more efficiently harvest sunlight.
Quantum dots, extremely small bits of semiconductor material, can be synthesized with almost atomic precision through advanced methods in colloidal chemistry.
The emission color is tunable by varying their dimensions. Color tunability combines with high emission efficiencies approaching 100 percent. These properties have recently become the basis of a new technology, quantum dot displays. These displays are used, for instance, in the newest generation of the Kindle Fire.
“The key accomplishment is the demonstration of large-area luminescent solar concentrators that use a new generation of specially engineered quantum dots,” said Victor Klimov, lead researcher.
Luminescent Solar Concentrators
A luminescent solar concentrator, or LSC, is a photon management device. It is a slab of transparent material holding very efficient emitters like dye molecules or quantum dots.
Sunlight absorbed in the slab is re-radiated at longer wavelengths and guided towards the slab edge equipped with a solar cell.
“The LSC serves as a light-harvesting antenna which concentrates solar radiation collected from a large area onto a much smaller solar cell, and this increases its power output,” said Klimov.
“LSCs are especially attractive because in addition to gains in efficiency, they can enable new interesting concepts such as photovoltaic windows that can transform house facades into large-area energy generation units,” said team member Sergio Brovelli, of University of Milano-Bicocca .
Due to their highly efficient, color-tunable emission and solution processability, quantum dots are ideal materials for use in inexpensive, large-area LSCs.
Engineered Stokes Shift
An overlap between emission and absorption bands in the dots is a challenge, which leads to considerable light losses because the dots re-absorb some of the light they produce.
To overcome this problem the researchers have created their LSCs based on quantum dots with artificially induced large separation between emission and absorption bands, called a large Stokes shift.
The Stokes-shift engineered quantum dots are cadmium selenide/cadmium sulfide (CdSe/CdS) structures in which light absorption is dominated by an ultra-thick outer shell of CdS, while emission occurs from the inner core of a narrower-gap CdSe.
The separation of light-absorption and light-emission functions between the two different parts of the nanostructure results in a large spectral shift of emission with respect to absorption, which greatly reduces losses to re-absorption.
Giant Quantum Dots
To use this concept, researchers fabricated a series of thick-shell, or “giant” CdSe/CdS quantum dots. The dots were incorporated into large slabs, sized in tens of centimeters, of polymethylmethacrylate.
Although giant by quantum dot standards, the active particles are still miniscule, about one hundred angstroms across. (A human hair is about 500,000 angstroms wide.)
Spectroscopic measurements indicated virtually no losses to re-absorption on distances of tens of centimeters. In spite of their high transparency, the fabricated structures showed significant enhancement of solar flux with the concentration factor of more than four.
These results mean that Stokes-shift-engineered quantum dots are a promising materials platform. They could enable the creation of solution processable large-area LSCs with independently tunable emission and absorption spectra.
Francesco Meinardi, Annalisa Colombo, Kirill A. Velizhanin, Roberto Simonutti, Monica Lorenzon, Luca Beverina, Ranjani Viswanatha, Victor I. Klimov, Sergio Brovelli.
Large-area luminescent solar concentrators based on ‘Stokes-shift-engineered’ nanocrystals in a mass-polymerized PMMA matrix.
Nature Photonics, 2014; DOI: 10.1038/nphoton.2014.54
Image courtesy of DOE/Los Alamos National Laboratory