Summary of this article on solar energy?
The incredible shrinking solar cell
With lilliputian collectors, almost anything could be sun-powered
By Janet Raloff
THE INCREDIBLE SHRINKING SOLAR CELL
The next generation of solar cells will be small. About the size of lint. But the anticipated impact: That’s huge.
Some of these emerging electricity-generating cells could be embedded in windows without obscuring the view. Engineers envision incorporating slightly larger ones into resins that would be molded onto the tops of cars or maybe the roofs of buildings. One team of materials scientists is developing microcells that could be rubber-stamped by the millions onto a yard of fabric. When such cells shrink in size — but not efficiency — it becomes hard to imagine what they couldn’t electrify.
“The idea is to develop ubiquitous solar power,” says Greg Nielson of Sandia National Laboratories in Albuquerque. Foldable and moldable modules crammed full of photovoltaic cells could directly power devices or recharge batteries. “You can imagine putting them onto every surface,” he says. “Your cell phone, laptop, backpack, tent — whatever.”
The U.S. Department of Energy is funding more than a dozen labs to investigate photovoltaic physics “at the nanoscale,” notes Linda Horton, who works in the agency’s Office of Science in Washington, D.C. “Our goal,” she says, “is to understand and improve at a very fundamental level the process by which energy from sunlight is translated into electrical energy.”
Concentrate on this
The real trick to creating useful and affordable lilliputian solar cells is not just shrinking their overall size, but cutting the amount of silicon (or another costly semiconductor) that is needed for them to deliver a watt of power.
Most photovoltaic devices today are crafted from rigid wafers of costly silicon. At 20 micrometers thick, Sandia’s little cells are less than 10 percent as chunky as the ones used in conventional photovoltaic devices. “And because ours are not just thin, but small laterally, we can do interesting tricks with them optically,” Nielson says. For instance, his group has begun studding minute refractive lenses into glass or plastic plates. Each lens concentrates sunlight onto a solar cell, nearly as small as a pinpoint, that sits directly below.
Silicon is needed only at the focal point of each lens, further diminishing the required quantity to about 1 percent of what’s needed per unit of light-collecting area with commercial photovoltaics. “So silicon is no longer the dominant cost, but a negligible one,” Nielson says.
His group grows thin, pure crystalline silicon, then etch-cuts each wafer into a mass of separate hexagons anywhere from 250 micrometers to 10 millimeters in diameter. “We call them glitter,” says Sandia’s Murat Okandan, and they do sparkle in hues ranging from gold and green to dark purple. Each batch yields uniform and remarkably rugged cells. “We can easily pick them up with a tweezers, and they don’t break,” the electrical engineer says.
The Sandia program, which began in early 2008, is already turning out prototype cells with an energy conversion efficiency of about 15 percent. “And we anticipate getting over 20 percent,” Nielson says. That wouldn’t be far from the best commercial solar cells today, which sport efficiencies somewhat more than 25 percent, Okandan adds.
The small print
At the University of Illinois at Urbana-Champaign, John Rogers works with even thinner silicon — 10 to 15 micrometers thick — because when it’s slim enough it flexes like a strand of hair. Although he’s testing silicon even thinner than that, the material presents special challenges, he notes, “because even at 10 to 15 micrometers the silicon won’t absorb all of the incident light.” Much passes through.
By backing the cells with a reflective material, however, photons that initially evaded the silicon will bounce back for a second chance at collection. “We found that 15 micrometers is just about the right thickness for that kind of double-pass configuration,” Rogers says. “It will collect about 90 percent of the light.” And the efficiency of these cells is already good, he says, on the order of 12 percent.
The Illinois microcells also rely on concentrators to focus sunlight. Another key to keeping cell costs low, Rogers contends, will be avoiding a need to “pick and place” each cell individually within a module of perhaps legions of others, which is what the integrated circuit industry does today. In the February Energy & Environmental Science, Rogers’ team describes a way to simultaneously lift and transfer thousands of microcells.
After building a block of pure crystalline silicon, the researchers etch out thousands of tiny cells from its surface by cutting around the sides of each one and even underneath. Theirs more but this is enough. Can someone please explain this article to me?
- John WLv 78 years agoFavorite Answer
It means they can make the solar cells really small but still focus the light to it and therefore make the solar panels really cheap and flexible so it can be everywhere.
- cieroLv 44 years ago
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