Like picky eaters, silicon solar cells absorb only some of the Sun’s wavelengths and let the rest go to waste. Researchers at the University of California, Riverside, have now figured out how to capture some of these wasted wavelengths and convert them into electricity. They published their work in Nano Letters in July.
The idea should give existing solar technologies “a little extra oomph”, lead author Christopher Bardeen explains.
When a light photon hits a solar cell, it knocks free an electron that drives an electrical “photovoltaic” circuit. But only energetic, short-wavelength photons carry enough punch to do this. Almost 20% of the sunlight striking a silicon solar cell passes straight through because the wavelengths are too long and their energy is too low to budge electrons.
But Bardeen and colleagues have figured out how to turn low energy infrared rays into a higher energy form. Theoretically, this approach could broaden the proportion of sunlight that can be captured, so that only 5% is missed, he says.
The idea should give existing technologies "a little extra oomph".
Bardeen’s light converting technique combines lead selenide nanocrystals with a carbon-based molecule called rubrene. They work via a two-step process. The nanocrystals absorb the energy carried by long wavelength near-infrared photons. That energy is transferred to a rubrene molecule. On its own, a single rubrene molecule does nothing. But when two energised rubrene molecules collide, they join forces and kick out a higher energy photon that can be guzzled by the solar cell.
The way to use these light-converters is to place them below silicon cells like a tray to capture the light that passes through. The converted higher energy light would then bounce back to punch electrons out of the silicon.
So far the nanocrystals aren’t doing all that much to improve the efficiency of silicon cells. That’s because silicon can also capture infra-red photons, so overall, the combination is not capturing much more of the light spectrum. But the good news is that the nanoparticles can be tweaked to capture different parts of the spectrum, making them more a profitable partner to silicon.
On the other hand, the light converting nanoparticles make a good partner for a recently commercialised solar material called cadmium telluride that excels at capturing blue light but not infrared.
So far, the nanoparticles have only been tested in a solution. But Bardeen is confident they could be printed as thin films and incorporated as a layer in a solar cell.
Timothy Schmidt, a chemist at the University of New South Wales who also works on light-converting molecules, says bumping near-infrared light up to a more energetic wavelength has always been difficult. While he doesn’t believe Bardeen’s nanoparticles will be a game changer for solar cells, he says the idea of converting light wavelengths from long to short is exciting and should inspire other researchers, “It makes sense to use silicon as a scaffold, rather than starting from scratch.”
“The smart money is still on silicon,” Bardeen agrees. “But if we can ‘supercharge’ other materials like cadmium telluride with light conversion, they may be competitive with silicon.”
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Viviane Richter is a freelance science writer based in Melbourne.