Which Way Out? Solar Cells That Trap Light
Michael A. Greenwood
Although the supply of sunlight is endless, the ability to effectively harness this vast power source remains limited. Most of the photons that hit low-cost thin-film solar cells simply bounce off, and their energy is never captured.
In recent years, researchers have experimented with approaches to improving solar cell efficiency, essentially by capturing the photon and forcing it to cooperate. This is done by changing the basic architecture of solar cells so that photons become trapped — or at least temporarily delayed — when they come into contact with the cell, thus providing a greater chance that they will be absorbed and converted into usable electricity.
The geometry of the V-shaped solar cell is designed to trap light by serving as an optical funnel. As a result, photons have multiple interactions with the solar cell. The active layer of the cell is very thin compared with the substrate. Courtesy of Seung-Bum Rim.
These light-trapping designs include the use of metal gratings, buried nanoelectrodes and scattering elements to improve light absorption. However, these methods also tend to be expensive, and their performance degrades over time.
Researchers at Stanford University in California, led by Peter Peumans, are experimenting with a different approach to light-trapping solar cells in an effort to improve their quantum efficiency: thin-film cells outfitted with a zigzag pattern, like teeth on a saw. They describe their repeating V-shape design as “simple,” noting that it requires no modification to the device structure and that it results in improved quantum efficiency for all angles of incidence. A similar approach has been used with thick-film solar cells, but not with thin films, they said.
They deposited the cell’s active layer and reflective metal electrode onto a V-shaped transparent substrate coated with a transparent electrode. The V-fold acted like an optical funnel, drawing photons into the tiny cavities and causing them to bounce against the walls of the solar cell multiple times. The density of the reflections increased as the opening angle decreased. Each photon thus had multiple interactions with the cell (and multiple chances to be absorbed), instead of the single interaction that a photon typically has with a traditional planar solar cell.
To examine the performance of a V-shaped thin-film solar cell, the investigators compared it with similar cells that lacked the zigzag architecture. They found that the V-design had greater external quantum efficiency across a spectrum ranging from 400 to 800 nm and that it was effective for all angles of incidence; for instance, 170-nm-thick cells exhibited a 52 percent enhancement in quantum efficiency, from 2.2 percent in the planar configuration to 3.5 percent with a V-fold design with a 35° angle.
The technique does require more active material than the traditional planar approach. The researchers believe that, in many cases, this larger material consumption could be compensated by improved efficiency.
Applied Physics Letters, Dec. 10, 2007, Vol. 91, 243501.
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