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If you look out across the SoMa district in San Francisco, you see many rooftops covered in large solar panel arrays. In the future, they might not be so obvious to the eye, as teams of researchers are working on transparent solar cells that could be worked into buildings’ windows.

The leading transparent solar cells work by absorbing as much light as possible outside of the visible range. If a cell absorbs visible light it becomes reflective, rendering it opaque instead of transparent.

At UCLA, researchers are working on cells that absorb infrared light, which can’t be seen by the unaided eye. They announced today that they have created a strip of transparent solar cells that converts 7.3 percent of the solar energy it receives into electricity. That is far below the 15 percent to 20 percent efficiency of conventional solar cells, but it is nearly twice as efficient as the 4 percent UCLA achieved last year. MIT hit 2 percent with their transparent solar cells this year.

They achieved the boost in efficiency by sandwiching two layers of solar cells together. One layer would absorb 40 percent of the infrared light that hits it, but together the two layers absorb 80 percent.

“Using two solar cells with the new interfacial materials in between produces close to two times the energy we originally observed,” team lead Yang Yang said in a release. “We anticipate this device will offer new directions for solar cells, including the creation of solar windows on homes and office buildings.”

Solar films like the UCLA team’s could also give gadgets like phones and computers a longer battery life by turning their screens into a solar panel. Wysips, for example, is testing a screen that could boost phone battery life by 20 percent.