TECH FOCUS PHOTOVOLTAICS


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would be possible”


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occurs roughly between 8 and 13µm, with a secondary window falling at slightly longer wavelengths. The earth is therefore indeed an ideal heat source for these cells, as its surface, which ranges in temperature from about 220 to 320K, emits strongly within this infrared window. It is this that enables the planet to regulate its temperature and remain habitable. In order to absorb the


radiation, the bottom surface of the proposed thermoradiative cell must therefore be thermally conductive, the researchers report in their paper. The cell should also contain a back- reflector and an infrared window on the front to direct its radiation toward the sky. In addition, the cell would also have to be encapsulated to protect itself from the environment and restrict conductive and convective heat loss.


28 Electro Optics June 2020


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How low can the bandgap go? The proposed cells should also be made using a material with an ultra low bandgap in order to maximise their power output, according to the researchers. While most modern commercial solar cells are made from silicon, which has a 1.1eV bandgap; this is too high for use in the nighttime thermoradiative cells. If an ultra low bandgap material were instead used to make these devices, then in ideal conditions (an extraterrestrial cell held at 300K coupled with deep space at 3K) an output of up to 54W/ m2 would be possible, the researchers report, while for a terrestrial cell in typical sky conditions, more than 10W/m2 may be possible. ‘For the active material,


there are several possible low bandgap semiconductors that could serve as starting points for investigation,’ the researchers state in their paper.


‘InSb [indium antimonide] can reach a bandgap below 0.1eV, which can be useful in proof of principle devices. However, for optimal power, even lower bandgaps are needed.’ The researchers therefore suggest that Hg1−


heavily characterised material used in the infrared sensing industry, could be suitable, or alternatively, newer materials such as graphene−hBN heterostructures. ‘Although difficult to fabricate, it has been shown that alternately stacking sheets of graphene with hBN can open a bandgap of 0.04eV in the graphene layer, which would be an excellent bandgap for a nighttime PV cell,’ they confirmed. ‘However, any chosen material may likely need additional engineering in order to suppress nonradiative generation/recombination.’


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Around-the-clock power In the paper, the researchers suggest that one possibility with the proposed thermoradiative technology would be to use it in a tandem system with a standard solar cell operating during the day –


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producing higher total around- the-clock power. The nighttime device could, for example, be rolled out on top of the standard solar modules after sunset. ‘For example, in an


average US climate, such as Boulder, Colorado, the National Renewable Energy Laboratory database records an average solar irradiance of about 5kWh/m2 per day, of which a commercial solar cell could harvest 1kWh/m2,’ the researchers say. ‘A nighttime PV cell in this climate could produce an average of 120Wh/m2 (if operated during only 12 hours), adding approximately 12 per cent more power to the 24-hour cycle.’


In conclusion, as the global push toward carbon neutrality continues, the sun is seemingly not the only sky-facing option for power generation. Through the clever use of photonics, optics, and materials science, thermoradiative photovoltaic devices exhibiting strong absorption and emission at thermal wavelengths offer the possibility of nighttime power generation by optically coupling with the cold of deep space. EO


“In typical sky conditions, more than 10W/m2


References 1.


www.irena.org/publications/2020/Mar/Renewable-Capacity-Statistics-2020 2. ACS Photonics 2020, 7, 1-9: Nighttime Photovoltaic Cells: Electrical Power Generation by Optically Coupling with Deep Space - Tristan Deppe and Jeremy Munday.


@electrooptics | www.electrooptics.com


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