Windows that can effectively and cheaply double as solar panels are getting closer and closer every day, as recent work from the Los Alamos National Laboratory shows.
The new work has demonstrated that quantum dots can be utilized to great effect in the harvesting of solar energy via large-area luminescent solar concentrators.
“The key accomplishment is the demonstration of large-area luminescent solar concentrators that use a new generation of specially engineered quantum dots,” stated lead researcher Victor Klimov of the Center for Advanced Solar Photophysics (CASP) at Los Alamos.
The press release from the DOE/Los Alamos National Laboratory provides some background:
Quantum dots are ultra-small bits of semiconductor matter that can be synthesized with nearly atomic precision via modern methods of colloidal chemistry. Their emission color can be tuned by simply varying their dimensions. Color tunability is combined with high emission efficiencies approaching 100%.
A luminescent solar concentrator (LSC) is a photon management device, representing a slab of transparent material that contains highly efficient emitters such as 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.
Klimov noted: “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.”
“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,” explained Sergio Brovelli, who worked at Los Alamos until 2012 and is now a faculty member at UNIMIB.
Quantum dots — thanks to their highly efficient, color-tunable emission and solution processability — are very attractive as a potential material for inexpensive, large-area LSCs. There are some roadblocks to their use, though — for instance, there’s an overlap between emission and absorption bands in the dots, which leads to significant light losses as the dots reabsorb some of the light that they produce.
That’s where the new work comes in, the researchers from Los Alamos and UNIMIB have addressed the issue through the development of “LSCs based on quantum dots with artificially induced large separation between emission and absorption bands (called a large Stokes shift).”
These “Stokes-shift” engineered quantum dots represent 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.
To implement this concept, Los Alamos researchers created a series of thick-shell (so-called “giant”) CdSe/CdS quantum dots, which were incorporated by their Italian partners into large slabs (sized in tens of centimeters) of polymethylmethacrylate (PMMA). While being large by quantum dot standards, the active particles are still tiny — only about hundred angstroms across. For comparison, a human hair is about 500,000 angstroms wide.
“A key to the success of this project was the use of a modified industrial method of cell-casting, we developed at UNIMIB Materials Science Department” explained Francesco Meinardi, professor of Physics at UNIMIB.