Photonic crystals cool solar cells while maintaining sunlight absorption

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Researchers from Stanford University have developed a material made from photonics crystals that can cool solar cells by 13°C. The material, which cools solar cells while absorbing the same amount of sunlight, could lead to improved efficiencies and lifetimes.

The scientists will present their results at the Conference on Lasers and Electro-Optics (CLEO), which is held 5-10 June in San Jose, California.

Solar cells turn sunlight into electricity, but they’re only about 20 per cent efficient. Much of the leftover energy turns into heat, which can damage the solar cell.

One way to keep objects cool in the sun is to reflect the light back into the atmosphere. This approach works for white cars and mirrored rooftops, but it not for solar cells, as they need to absorb as much light as possible to generate electricity.

An alternative is to make it easier for heat to escape, using an approach called radiative cooling. The new material developed at Stanford keeps the solar cell cooler even as the solar cell absorbs the same amount of sunlight.

‘What’s unique about our work is that we demonstrate radiative cooling while preserving the amount of solar absorption,’ said Linxiao Zhu, a graduate student in the research group of electrical engineering professor, Shanhui Fan.

The researchers managed to cool the cells while maintaining sunlight absorption by using a wafer made of silica. Tapered holes, about six micrometres across and 10 micrometres deep, were etched in the wafer in order to smooth the path the thermal radiation takes to escape.

The team tested the silica layer by placing it on top of a solar cell mimic – a polished silicon wafer with an antireflection surface and aluminium back that has similar absorption characteristics to standard solar cells, but wasn’t actually wired to produce electricity.

The testing verified that because the silica layer is transparent, approximately the same amount of sunlight still reaches the solar cell mimic. In fact, there was a slight increase in absorption because of anti-reflection and light trapping effects of the etched silica.

The researchers also found that the etched silica layer lowered the temperature 13°C compared to the bare solar cell mimic.

Cold solar cells function better than hot ones, so the cooler the better. The researchers estimate that the 13°C cooling would result in an absolute efficiency improvement of more than one per cent. Aaswath Raman, a co-author of the study, also noted that heat can speed up the degradation of solar cell parts, so cooling could lengthen their lifespan and likely save costs.

Solar cells aren’t the only applications that could benefit from this cooling approach, especially since the new research shows it can work without significantly altering the sunlight absorption characteristics of an underlying material, Zhu said. Cooling cars, clothing, and outdoor equipment are all possible applications, he added.

The next step for Zhu and his colleagues is to test the etched silica layer with a real solar cell to demonstrate the predicted efficiency improvements. The team is also talking to industry partners who could be interested in commercialising the approach.

Further information 

CLEO conference