Stacking Solar Cells Is A Neat Trick To Maximise Efficiency

Solar power is already cheap and effective, and it’s taking on a larger role in supplying energy needs all over the world. The thing about humanity, though, is that we always want more! Too much, you say? It’s never enough!

The problem is that the sun only outputs so much energy per unit of area on Earth, and solar cells can only be so efficient thanks to some fundamental physical limits. However, there’s a way to get around that—with the magic of tandem solar cells!

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Overall design of retina-inspired NB perovskite PD for panchromatic imaging. (Credit: Yuchen Hou et al., 2023)

Perovskite Sensor Array Emulates Human Retina For Panchromatic Imaging

The mammalian retina is a complex system consisting out of cones (for color) and rods (for peripheral monochrome) that provide the raw image data which is then processed into successive layers of neurons before this preprocessed data is sent via the optical nerve to the brain’s visual cortex. In order to emulate this system as closely as possible, researchers at Penn State University have created a system that uses perovskite (methylammonium lead bromide, MAPbX3) RGB photodetectors and a neuromorphic processing algorithm that performs similar processing as the biological retina.

Panchromatic imaging is defined as being ‘sensitive to light of all colors in the visible spectrum’, which in imaging means enhancing the monochromatic (e.g. RGB) channels using panchromatic (intensity, not frequency) data. For the retina this means that the incoming light is not merely used to determine the separate colors, but also the intensity, which is what underlies the wide dynamic range of the Mark I eyeball. In this experiment, layers of these MAPbX3 (X being Cl, Br, I or combination thereof) perovskites formed stacked RGB sensors.

The output of these sensor layers was then processed in a pretrained convolutional neural network, to generate the final, panchromatic image which could then be used for a wide range of purposes. Some applications noted by the researchers include new types of digital cameras, as well as artificial retinas, limited mostly by how well the perovskite layers scale in resolution, and their longevity, which is a long-standing issue with perovskites. Another possibility raised is that of powering at least part of the system using the energy collected by the perovskite layers, akin to proposed perovskite-based solar panels.

(Heading: Overall design of retina-inspired NB perovskite PD for panchromatic imaging. (Credit: Yuchen Hou et al., 2023) )

Perovskite Solar: Coming Soon?

Making solar cells out of silicon is difficult. There’s plenty of manufacturing steps, many of them at very high temperatures, and you need a high vacuum and a clean room. However, perovskite solar cells–cells made with hybrid organic-inorganic materials in a perovskite crystal structure–are relatively easy to make using wet chemistry involving solvents or vapor deposition.

In theory, silicon solar cells could be 30% efficient, but in reality, 25% seems to be a practical limit with commercial cells typically topping out at 20%. Perovskite cells are nearly that high now, and could be higher by stacking thin layers, each sensitive to different wavelengths of light.

A recent development at the Lawrence Berkeley National Laboratory may lead to even more efficient perovskite cells. Researchers found that certain crystal structures had a much higher efficiency than other structures. The problem now is figuring out how to produce the crystals to increase the prevalence of that structure.

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