# Plant-Based Strategy for Harvesting Light

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A new photodetector design borrows its light-harvesting architecture from plants and offers a potential path to more efficient solar cells.

When it comes to harvesting energy from sunlight, plants and solar cells are wired differently. One of the main differences is that a plant has separate structures for absorbing sunlight and converting that energy into a usable electrical charge, while a solar cell has a structure that does both. However, calculations suggest that light-harvesting devices such as solar cells and photodetectors could be made more efficient by adopting the planting strategy. A new experiment by University of Michigan’s Stephen Forrest and colleagues has replicated plant-like separation in an organic photodetector [1]. The reported efficiency is better than that of simpler photodetectors, but more work is needed to make this plant mimic competitive with current solar cell technology.

The two most important photosynthetic structures in plants are the antenna complexes (ACs) and the reaction centers (RCs). When a photon from the sun encounters an alternating current, its energy is absorbed, creating an excited molecular state in the alternating current called an exciton. This exciton travels through the plant to an RC where the exciton is converted into a free electron that the plant uses to create chemical energy. One benefit of this division of labor is that plants can grow relatively small RCs, reducing a background noise called dark current. A low dark current is a key factor for efficient light harvesting.

Like a plant, a solar cell absorbs sunlight and creates excitons, but it doesn’t then send the excitons elsewhere to convert them into free electrons. “It’s all happening in the same place,” Forrest says. This means that the conversion area cannot be made smaller than the absorption area to improve the efficiency. Researchers have considered alternative solar cell designs that mimic how plants work, but the challenge has been moving the excitons from absorption sites to conversion sites. “The problem with excitons is that they can only travel a few nanometers,” explains Forrest. Plants overcome this roadblock by having a large network of RCs as well as a protein scaffold that helps with energy transfer. However, these solutions are not available to solar cell designers.

The idea that Forrest and his team came up with is to swap excitons for polaritons. A polariton is a hybrid particle in which an exciton couples to a photon. This light-matter mix allows a polariton to travel fast and far like a photon, while being able to connect to an electronic device with the same ease as an exciton.

After years of trial and error, Forrest and colleagues have now produced the first polariton-based photodetector. The device consists of an organic film placed on a mirror-like surface. To create polaritons, the researchers shine light at a focused spot on the film. “If you get the setup right, a polariton formed by optical excitation will whiz across the surface at very high speeds,” says Forrest. “It doesn’t live long, but it doesn’t have to live very long to travel a long distance.” To observe polariton propagation, the team placed a detector – one that converts excitons into charge – on one side of the organic film. They found that polaritons can travel up to 0.3 mm from the focused beam spot to the detector – more than 1000 times further than excitons. The team measured the photodetector’s efficiency in terms of current produced per unit of optical power received. They found the plant-inspired device outperformed simple silicon photodiodes but underperformed state-of-the-art photodetectors.

The team is now trying to improve the performance of their polariton-generating scheme. One possibility would be to optimize the geometry of the device by surrounding a large-area organic film with a ring-shaped detector. Such a design would mimic a plant, with the AC and RC roles played by the film and detector, respectively. Forrest speculates that such a device could have higher power conversion efficiency than current solar cells with their non-plant-like design.

Montreal Polytechnic polariton researcher Stéphane Kéna-Cohen says the new design “enables researchers to harvest light from a larger effective area than would otherwise be possible.” He likens the polariton photodetector to a luminescent solar concentrator — a Light-gathering device that uses a plastic material to collect sunlight and direct it to a small solar cell. Both technologies are easy to integrate with electronics, but concentrators can have a larger collection area – thanks to the 10 cm propagation length of photons within the plastic material. For polariton-based devices to be competitive, researchers need to improve the propagation length of polaritons beyond 100, says Kéna-Cohen

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which Forrest’s team was watching.

– Michael Schirber

Michael Schirber is Corresponding Editor for magazine for physics based in Lyon, France.

## references

1. B. Liu et al.“Photocurrent generation after long-range propagation of organic exciton-polaritons”, optics 91029 (2022).

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