Solar cells convert light into electricity. While the sun is one source of light, the burning of natural resources like oil and natural gas can also be harnessed.
However, solar cells do not convert all light to power equally, which has inspired a joint industry-academia effort to develop a potentially game-changing solution.
“Current solar cells are not good at converting visible light to electrical power. The best efficiency is only around 20%,” explains Kyoto University’s Takashi Asano, who uses optical technologies to improve energy production.
Higher temperatures emit light at shorter wavelengths, which is why the flame of a gas burner will shift from red to blue as the heat increases. The higher heat offers more energy, making short wavelengths an important target in the design of solar cells.
“The problem,” continues Asano, “is that heat dissipates light of all wavelengths, but a solar cell will only work in a narrow range.”To solve this, we built a new nano-sized semiconductor that narrows the wavelength bandwidth to concentrate the energy.”
Previously, Asano and colleagues of 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.
According to Asano, “the cylinders determined the emissivity,” describing the wavelengths emitted by the heated device.
Using this material, the team has shown in Science Advances 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. Secondly is design. We can now create much smaller and more robust transducers, which will be beneficial in a wide range of applications.”
The paper “Near-infrared-to-visible highly selective thermal emitters based on an intrinsic semiconductor” appeared 23 December 2016 in Science Advances, with doi: 10.1126/sciadv.1600499
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Control of the thermal emission spectra of emitters will result in improved energy utilization efficiency in a broad range of fields, including lighting, energy harvesting, and sensing. In particular, it is challenging to realize a highly selective thermal emitter in the near-infrared–to–visible range, in which unwanted thermal emission spectral components at longer wavelengths are significantly suppressed, whereas strong emission in the near-infrared–to–visible range is retained.
To achieve this, we propose an emitter based on interband transitions in a nanostructured intrinsic semiconductor. The electron thermal fluctuations are first limited to the higher-frequency side of the spectrum, above the semiconductor bandgap, and are then enhanced by the photonic resonance of the structure. Theoretical calculations indicate that optimized intrinsic Si rod-array emitters with a rod radius of 105 nm can convert 59% of the input power into emission of wavelengths shorter than 1100 nm at 1400 K. It is also theoretically indicated that emitters with a rod radius of 190 nm can convert 84% of the input power into emission of <1800-nm wavelength at 1400 K. Experimentally, we fabricated a Si rod-array emitter that exhibited a high peak emissivity of 0.77 at a wavelength of 790 nm and a very low background emissivity of <0.02 to 0.05 at 1100 to 7000 nm, under operation at 1273 K.
Use of a nanostructured intrinsic semiconductor that can withstand high temperatures is promising for the development of highly efficient thermal emitters operating in the near-infrared–to–visible range.
(A) Spectral emissivity and (B) spectral radiance of Si rod-array emitter (solid lines) in the direction normal to the surface at 1400 K with a = 500 nm, h = 450 nm, r = 110 nm, and tSi = 50 nm. The inset in (A) illustrates the emitter structure and parameters. (C) Radiation angle dependence of emission intensity at 1400 K for the emitter with a 1-m2 area. (D) Radiation spectra of Si rod-array emitters at 1400 K integrated over the upper hemisphere with structural parameters a = 500 nm, h = 450 nm, r = 110 nm, and tSi = 50 nm (solid black line); a = 600 nm, h = 600 nm, r = 105 nm, and tSi = 0 nm (solid red line); and a = 700 nm, h = 800 nm, r = 190 nm, and tSi = 0 nm (solid green line). The blackbody spectrum at 1400 K integrated over the upper hemisphere (dashed line) and the irradiance of sunlight at AM1.5G (gray line) are also plotted.