Solar cells cross new threshold

Imagine powering your cell phone by leaving it on the window sill. Sounds like science fiction? Actually, this might soon turn into reality. Scientists have been exploring the potential of solar energy for decades. One of the cheapest ways to turn solar energy into electricity is by creating solar cells from organic polymers, which are easily manipulated by scientists. However, such cells are not efficient at converting sunlight to electricity. But now, researchers at UCLA’s Department of Materials Science and Engineering have come up with a new type of solar cell that sets the record in changing sunlight into power. Many organic solar cells are single-layer—they contain a film-thin layer of the organic polymer between two transport layers. When photons strike the cell, they energize electrons, turning them into excitons. The energy in excitons can be converted to electricity. The reason why such cells are not efficient at the sun-to-electricity conversion is because the polymer only absorbs a part of the solar spectrum—a certain bandgap—not all of it. This means that much of the solar energy, which could be potentially turned into electricity, is lost. Then researchers created the “tandem” cell. This device has two layers of organic polymers, connected by an electrically-conductive interconnecting layer. The two organic layers capture light from different parts of the solar spectrum, resulting in more light captured and less wasted—a single-layer cell has conversion efficiency of 6% while for a tandem cell, this number is 8.62%. The UCLA team that created the cell, including Jing-Bi You, Letian Dou, and Gang Li working under the supervision of Professor Yang Yang, has published its research in a recent issue of Nature Photonics. Afterwards, solar energy received another boost when Sumitomo Chemical of Japan created a polymer capable of absorbing wavelengths in the infrared spectrum. When researchers incorporated this polymer into their tandem cell, their conversion efficiency jumped to 10.6%, a new record. “Devices with more than 10% efficiency,” says co-author Jing-Bi You, “could be incorporated into the products” that are commercially viable. For example, such cells can be used to provide electricity for cars, buildings, even planes. “We also can make semi-transparent organic solar cell and put it on windows,” adds the paper’s co-author Letian Dou. “Then we will have colorful windows that can generate electricity during the daytime.” However, such feats might be a while away. For now, scientists are only able to produce cells that have the maximum area of 1 cm2—definitely way too small to power a plane. But they are working hard to overcome this and other challenges so to that one day, we’ll just be able to charge our phones as we are strolling through a park on a nice day.