THE NEXT SOLAR

The incredible shrinking solar cell; With lilliputian collectors, almost anything could be sun-powered
Janet Raloff, July 31, 2010 (Science News)

"The next generation of solar cells will be small. About the size of lint. But the anticipated impact [when they get to the marketplace in 3-to-5 years]: That’s huge…[They] could be embedded in windows without obscuring the view…[or] molded onto the tops of cars or maybe the roofs of buildings…[They] could be rubber-stamped by the millions onto a yard of fabric…[I]t becomes hard to imagine what they couldn’t electrify…

"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…[Instead of] rigid wafers of costly silicon…Sandia’s little cells are less than 10 percent as chunky…[with] minute refractive lenses into glass or plastic plates…Silicon is needed only at the focal point of each lens…The Sandia program, which began in early 2008, is already turning out prototype cells with an energy conversion efficiency of about 15 percent…"

From Science News (click to enlarge)

"At the University of Illinois at Urbana-Champaign, John Rogers works with even thinner silicon…like a strand of hair…By backing the cells with a reflective material, however, photons that initially evaded the silicon will bounce back for a second chance at collection…The Illinois microcells also rely on concentrators to focus sunlight…

"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…[T]he only thing holding the cells…are tiny anchors of material left at either end of each cell…The scientists then place a soft piece of slightly tacky rubber onto the batch of cells and press down…When they lift…the freed cells come with it…"

Scehmatic of the Cal Tech concept (From Science News - click to enlarge)

"Caltech scientists have upended the silicon elements in their microcells and jettisoned the concentrator…[for] a sparse carpet of tiny fibers that stretch up toward the light. In the latest designs the fibers are 100 micrometers long and 1 or 2 percent as wide…Some photons entering the carpet will immediately hit a semiconductor fiber. Many more will miss…But by making the wires effectively long and the carpet’s bottom reflective, photons not initially collected will ricochet repeatedly within the carpet until the silicon collects most of them…

"To protect and hold the fibers, the Caltech scientists pour a liquid akin to clear bathroom caulk (a polymer that solidifies into a pliable plastic) to fill space separating the carpet’s sparse pile…[It is a] composite array of wires and polymer…[like] a piece of [malleable] Scotch tape…By maximizing photon ricochets within the carpet…[it is] the same light absorption as…100 percent silicon…using only 1 percent as much [silicon]…"