Silicon and sun: Sandia researcher Gregory Nielson holds up an array of microscale, multi-crystalline silicon solar cells.
Microscale solar cells offer new possibilities for light concentrating and tracking, which could further boost the cells’ efficiency. Conventional tracking systems are large and heavy and have to be moved by motors. An array of micro solar cells could be topped with a microlens array that needs to move only a fraction of a millimeter to track the sun.
The microscopic cells could also be combined with more efficient lenses. Instead of Fresnel lenses, which are bulky and capture only about 80 percent of the light that hits them, the micro cells could use refractive lenses, which capture 90 percent of incoming light. It isn’t practical to use refractive lenses with conventional solar cells because such lenses would become too expensive and bulky at the size required (the larger the lens, the farther it must be mounted from the surface of the cell). But for the Sandia cells, a refractive microlens array could match each silicon device with one lens just a few micrometers in diameter. Such arrays are already commercially available.
Nielson says developers could eventually suspend the cells in a liquid to make an ink that could be printed onto flexible substrates coated with electrical contacts to create flexible solar modules.
For solar cells, flexibility usually hurts efficiency. For example, the company Konarka makes flexible solar cells from organic materials, but these only operate at about 4 percent efficiency. “We think we can use high-efficiency materials to provide the same flexibility using five times less area,” says Nielson.
Nielson expects the project, which is funded through the U.S. Department of Energy Solar Technologies Program, to yield modules for military use (for example, in energy-harvesting tents and backpacks) in about three years. The rest of the solar market has stringent lifetime requirements, so it may take another few years to develop modules that are durable enough. The national lab will likely license the technology to a company after it’s more mature.