"Current solar cells are not good at converting visible light to electrical power. The best efficiency is only around 20%," said Kyoto University researcher Takashi Asano, who uses optical technologies to improve energy production.
Higher temperatures emit light at shorter wavelengths, making short wavelengths an important target in the design of solar cells. "The problem is that heat dissipates light of all wavelengths, but a solar cell will only work in a narrow range," said Asano. "To solve this, we built a new nano-sized semiconductor that narrows the wavelength bandwidth to concentrate the energy."
Previously, Asano and colleagues at the Susumu Noda lab had taken a different approach. "Our first device worked at high wavelengths, but to narrow output for visible light required a new strategy, which is why we shifted to intrinsic silicon in this current collaboration with Osaka Gas," says Asano.
To emit visible wavelengths, a temperature of 1000C was needed, but conveniently silicon has a melting temperature of over 1400C. The scientists etched silicon plates to have a large number of identical and equidistantly-spaced rods, the height, radii, and spacing of which was optimized for the target bandwidth.
Using this material, the team has shown that their nanoscale semiconductor raises the energy conversion rate of solar cells to at least 40%.
"Our technology has two important benefits," adds lab head Noda. "First is energy efficiency: we can convert heat into electricity much more efficiently than before. Second is the design. We can now create much smaller and more robust transducers, which will be beneficial in a wide range of applications."
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