One way to squeeze more power out of sunlight is to ensure that it always hits a solar panel at the ideal angle. This means either tracking the sun and maneuvering a panel to face it, or using complex optics to redirect the sun's rays to hit the panel's surface from above.
Researchers at the University of Illinois have now come up with self-assembling spherical solar cells capable of capturing more sunlight than flat ones. The shape is a simpler way to make more use of the sun's rays, but has been difficult to realize in a solar cell. These new microscale solar cells are made using conventional lithography combined with self-assembly. If they prove practical, the devices could be wired up into large arrays that have the same power output as conventional cells, but that save on materials costs by using less silicon.
Fold-up silicon: In these images, three thin films of silicon fold up into 3-D shapes under the force of surface tension as water droplets placed in their centers evaporate. The top row depicts the first step, when the water droplets are large, and the images below it show a time progression as the water droplets shrinks.
"Instead of a big slab of semiconductor fitted with concentrating lenses and motors to move it around, we want to make compact cells that still have a significant power output," says Ralph Nuzzo, professor of chemistry at the University of Illinois at Urbana-Champaign.
Curved surfaces capture more light than flat ones because they have a greater surface area. But making solar cells that are curved or spherical is challenging, says Nuzzo, because the techniques used to process semiconducting materials such as silicon work best on flat surfaces. Nuzzo's group has overcome this problem by making microscale 3-D structures that self-assemble from flat sheets.
The Illinois researchers start by treating the surface of a thin, high-quality silicon wafer and using conventional lithography to etch out a thin, two-dimensional shape. To make a sphere, the researchers cut the silicon into a flower shape. They then use an adhesive to secure a small piece of glass inside. The glass helps the structure maintain its shape once it is assembled. Finally, as a drop of water placed in the center of the flower shape evaporates, surface tension pulls its petals up, eventually bringing them together to form a sphere."The challenge in this is, how do you get things to follow the necessary sequence of steps to fold into the desired shape?" says Nuzzo. The Illinois group came up with mathematical models to help predict the mechanical properties of silicon sheets of different shapes and thicknesses, as well as how they interact with water, which can be tuned by chemically treating their surfaces.
Nuzzo's group used the techniques to make functioning microscopic spherical solar cells, as a proof of the functionality of what he calls "materials origami." Before cutting the silicon into the petal shape, the team treated it to form the conductive regions that make a solar cell work. After the flower had folded up into a sphere, electrical contacts were added. The group used a similar technique to make cylindrical micro-solar cells as well.
These devices convert only about 1 percent of the light that hits them into electricity--a poor return for a solar cell--but this is better than a planar solar cell made using the same relatively crude techniques using the same amount of silicon. The researchers say the technique can be applied to other materials besides silicon, and could be used to make new forms of solar cells. The work is described online this week in the Proceedings of the National Academy of Sciences.
"Folding is very appealing because you can make fantastic, complicated three-dimensional shapes," says George Barbastathis, professor of mechanical engineering at MIT.
There are other ways of improving solar cells' ability to capture light, such as antireflective coatings and surface texturing. The main advantage of the new approach is that it requires less material, says Nuzzo. Planar solar cells just a few micrometers thick can't be textured--there's simply not enough material. And antireflective coatings add more manufacturing costs and complexity. Self-assembly, Nuzzo hopes, could offer an alternative.
The Illinois group will now work to improve the process, and make designs that further improve the cells' light management. "We want to bring forward form factors that rely on high performance materials like silicon but provide a substantial economy" by using as little of these expensive materials as possible, says Nuzzo.
By Katherine Bourzac