New Designs Could Boost Solar Cells Beyond Their Limits
Date Posted: July 16, 2019
Posted In: Ideas,
From Wired: The sun blankest the Earth with enough photons every hour to meet the entire world’s energy needs for a year. The question is how to efficiently convert them into electricity. Even under small-scale laboratory conditions, the world’s best single-junction solar cells—the kind found in most solar panels—still max out at capturing 29 percent of the sun’s energy. That puts them just shy of the hard limit of about one third that solar researchers calculated half a century ago. But scientists studying photovoltaics—the process by which sunlight is converted into electricity—have also long suspected that this limit is not as hard as it once seemed.
The ceiling on solar cell efficiency, known as the Shockley-Queisser limit, is between 29 and 33 percent, depending on how you measure it. It assumes a single-junction cell, meaning it’s made using only one type of semiconductor and is energized by direct sunlight. To nose past the limit, researchers have tried stacking multiple types of semiconductors or using lenses to concentrate light so that the cell receives a blast hundreds of times more powerful than the sun. Earlier this year, the National Renewable Energy Lab set a world record when it used a six-junction solar cell and a beam 143 times more concentrated than sunlight to achieve a whopping 47.1 percent energy efficiency.
But this technology will never be deployed at scale. The reason, says Marc Baldo, a professor of electrical engineering and computer science at MIT, is that these ultra-high-efficiency, multilayer solar cells are far too complex and expensive to produce as solar panels. To actually get more solar energy on the electric grid requires figuring out how to hit the Shockley-Queisser limit with single-junction, silicon-based solar cells, which are comparatively easy and cheap to produce. Better yet would be finding a way to bump the limit higher. And after a decade of work, Baldo and his colleagues may have finally figured out how.
As detailed in a paper published last week in Nature, Baldo’s team coated solar cells in a thin layer of tetracene, an organic molecule that effectively splits incoming photons in two. This process is known as exciton fission and means that the solar cell is able to use high energy photons from the blue-green part of the visible spectrum.