Not so long ago, artists routinely made their own paints using all sorts of odd ingredients: clay, linseed oil, ground-up insects—whatever worked. It was a crude and rather ad hoc process, but the results were used to create some of the greatest paintings in the world.
Today I and other scientists are developing our own special paints. We’re not trying to compete with Vermeer or Gauguin, though. We hope to create masterpieces of a more technical nature: optoelectronic components that will make for better photovoltaic cells, imaging sensors, and optical communications equipment. And we’re not mixing and matching ingredients quite so haphazardly. Instead, we’re using our blossoming understanding of the world of nanomaterials to design the constituents of our paints at the molecular level.
For well over a decade, researchers have been investigating ways to make optoelectronic devices by painting, spraying, or printing the active materials onto an appropriate backing. This work has generated various commercial products, including flexible photovoltaic cells and ultrathin, high-contrast displays, Sony’s XEL-1 being a prime example of the latter. The organic polymers from which these devices are built absorb or emit light at visible wavelengths. But making paint-on optoelectronic materials that are sensitive to the infrared—that is, to wavelengths longer than those of visible light—opens up even further possibilities.
Infrared wavelengths are particularly valuable in solar cells. Such a cell must absorb infrared as well as visible light, lest it squander half the sun’s energy. And imaging devices that are sensitive to infrared provide a remarkable way to pierce through fog and to view outdoor scenes at night, using the faint infrared glow of the upper atmosphere as illumination. And for optical communications, the equipment must operate in the infrared, because outside of certain wavelength bands in this range, glass fibers tend to absorb or distort the light sent through them. Infrared is also useful for conducting secure line-of-sight optical communications.
My colleagues and I at the University of Toronto have made great progress in recent years building devices using what are essentially paints that respond well to infrared wavelengths. This work is still in the research stage—products remain between one and five years away, depending on the application—but the pace of advance has been so swift that it’s not too soon to look forward to the many exciting possibilities.
Yearly sales of photovoltaic panels now amount to tens of billions of dollars, and the overall energy market is measured in the trillions. The ideal that solar-cell developers are seeking is a device that is both efficient and inexpensive. Solar cells constructed from costly semiconductor wafers have yielded the greatest efficiencies—upward of 40 percent—but because they are so difficult to manufacture, such high-efficiency cells are too pricey for all but the most demanding applications, such as for the solar panels attached to spacecraft.
Photovoltaic cells made out of organic polymers cost far less, but the best efficiencies they’ve shown have typically been around 5 or 6 percent. That’s stunningly good for something that can be manufactured so cheaply, but it’s still less than the 10 percent figure experts say will be needed for this technology to take off commercially.
One common strategy to boost the efficiency of solar cells of any kind might be called the layer-cake method. The top layer of the cell absorbs photons of relatively short wavelengths, and thus of high energy, turning them into electricity. These wavelengths include visible light and some of the ultraviolet as well. Photons of lower wavelengths pass through this layer into a second one below, which is designed to absorb them and transform their energy into electric power. Some of these layer-cake designs include a third stratum at the bottom to capture the even lower-energy photons that penetrate the top two layers.