Dec 16 2008
Solar cells made from a new class of materials, called quantum dots, could capture much more sunlight than existing photovoltaic panels. If produced in bulk, an upcoming generation of solar technology promises to be cheap enough to compete with fossil fuels for electricity generation by using a fundamentally different approach.
These nanocrystalline particles have electrical properties similar to those of established solar cell materials, such as silicon and cadmium telluride. The difference is that quantum dots can be designed and arranged with more flexibility than ordinary silicon cells. Importantly, they can be made in different sizes, where the size of the dot determines its ability to absorb a different wavelength of light. Smaller dots absorb shorter wavelengths, while larger dots absorb longer ones. If the particles are arranged properly, they may be able to harvest much more sunlight than ordinary solar cells. Existing solar cells convert between 15 and 20 percent of the available light—these promise about 30 percent efficiency. Researchers built the first multi-spectrum solar converters, called “rainbow” solar cells, last March.
Now this month, researchers report in Nature Nanotechnology that they have uncovered another property of quantum dots. Not only does their size tune the particles’ properties, but so does the amount of strain they are placed under. It turns out that straining a network of quantum dots can also change the wavelength of light that the particles respond to. This is important because the strain-tuned quantum dots can be made from more benign materials than the size-sensitive ones. While the first generation of quantum dots typically used cadmium, a toxic metal, the new variety can be made from zinc and selenium, two human-friendly elements that may allow the nano-particles to be used safely in biomedical applications.
As tentative as these updates may sound, quantum dot research is not just an academic exercise. A few companies are already working on quantum dot solar cells, though the programs are still a decade or more away from commercialization. NASA, for one, has gotten behind nano-structured solar cells with a development grant awarded last week to Kopin Corp., a photonics company that makes displays and other devices for the telecommunications industry. A number of small solar start-ups also appear to be making headway using venture capital funding, though many of them are still in stealth mode—or at least very cagey about the details of their technology.