Less is more for highly absorbing, flexible, cheaper solar cells
Using arrays of long, thin silicon wires embedded in a polymer substrate, a team of researchers at the California Institute of Technology (Caltech) have created a new type of flexible solar cell. Promising enhanced sunlight absorption and efficient conversion of photons into electrons, the new solar cell uses only a fraction of the expensive semiconductor materials required by conventional solar cells, and because they are flexible, they will be cheaper to manufacture.
Independently each silicon wire is a high efficiency, high quality solar cell. By bringing them together in an array the researchers were able to make them even more effective, because they interact to increase the cell’s ability to absorb light. So much so that the new solar cells have surpassed the conventional light-trapping limit for absorbing materials, which refers to how much sunlight it is able to absorb. The silicon-wire arrays absorb up to 96 percent of incident sunlight at a single wavelength and 85 percent of total collectible sunlight.
However, as Harry Atwater, Howard Hughes Professor, professor of applied physics and materials science, and director of Caltech's Resnick Institute, points out, “Many materials can absorb light quite well but not generate electricity - like, for instance, black paint. What's most important in a solar cell is whether that absorption leads to the creation of charge carriers."
The silicon wire arrays created by Atwater and his colleagues are able to convert between 90 and 100 percent of the photons they absorb into electrons—in technical terms, the wires have a near-perfect internal quantum efficiency. "High absorption plus good conversion makes for a high-quality solar cell," says Atwater.
These conversion rates are surprising given that the wires cover only between two and 10 percent of the cell’s surface area. When light comes into each wire, a portion is absorbed and another portion scatters. According to Atwater it is the collective scattering interactions between the wires that make the array very absorbing despite the sparseness of the wires.
"When we first considered silicon wire-array solar cells, we assumed that sunlight would be wasted on the space between wires," explains graduate student Michael Kelzenberg. "So our initial plan was to grow the wires as close together as possible. But when we started quantifying their absorption, we realized that more light could be absorbed than predicted by the wire-packing fraction alone. By developing light-trapping techniques for relatively sparse wire arrays, not only did we achieve suitable absorption, we also demonstrated effective optical concentration - an exciting prospect for further enhancing the efficiency of silicon-wire-array solar cells."
Each wire measures between 30 and 100 microns in length and only 1 micron in diameter. “The entire thickness of the array is the length of the wire,” notes Atwater. “But in terms of area or volume, just 2 percent of it is silicon, and 98 percent is polymer.”
In other words, while these arrays have the thickness of a conventional crystalline solar cell, their volume is equivalent to that of a two-micron-thick film.
Since the silicon material is an expensive component of a conventional solar cell, a cell that requires just one-fiftieth of the amount of this semiconductor will be much cheaper to produce. The composite nature of the solar cells means that they are also flexible, meaning they could be manufactured in a roll-to-roll process. As this is an inherently lower-cost process than one that involves brittle wafers, like those used to make conventional solar cells, costs can be further reduced.
The next steps, Atwater says, are to increase the operating voltage and the overall size of the solar cell. "The structures we've made are square centimeters in size," he explains. "We're now scaling up to make cells that will be hundreds of square centimeters—the size of a normal cell."
Atwater says that the team is already "on its way" to showing that large-area cells work just as well as these smaller versions.