The technology uses polaritons to convert light into an electrical charge; may boost solar cell efficiency — ScienceDaily

Researchers have developed a new type of highly efficient photodetector inspired by the photosynthetic complexes that plants use to convert sunlight into energy. Photodetectors are used in cameras, optical communication systems, and many other applications to convert photons into electrical signals.

“Our devices combine long-distance transmission of optical energy with long-distance conversion into electrical current,” said research team leader Steven Forrest of the University of Michigan. “This arrangement, analogous to that seen in plants, has the potential to greatly improve the power generation efficiency of solar cells, which use photodetector-like devices to convert sunlight into energy.”

Photosynthetic complexes found in many plants consist of a large light-absorbing region that delivers molecular excited-state energy to a reaction center where the energy is converted to charge. Although this setup is very efficient, mimicking it requires achieving long-range energy transport in an organic material, which has proven difficult to accomplish.

To achieve this seemingly impossible task, the researchers used unique quasiparticles known as polaritons. in OPTICALOptica Publishing Group’s high-impact research journal, Forrest and colleagues report their new detector that generates polaritons in an organic thin film.

“The polariton combines a molecular excited state with a photon, giving it light-like and matter-like properties that allow energy to be transported and converted over long distances,” Forrest said. “This photodetector is one of the first demonstrations of a practical optoelectronic device based on polaritons.”

Taking a cue from plants

The researchers envisioned the new detector several years ago while looking for ways to make better solar cells. “After observing the long-distance propagation of polaritons in simple structures like a mirror with an organic film on its surface, we thought it might be possible to make a photosynthetic analogue using polaritons,” Forrest said. “However, it was quite difficult to figure out how to build such a device.”

To create a photodetector based on polaritons, the researchers had to design structures that allow polaritons to propagate over long distances in a thin film of organic semiconductor. They also had to figure out how to integrate a simple organic detector into the propagation region in a way that resulted in efficient polariton-to-charge conversion.

“We borrowed from structures we had previously designed to create efficient organic photovoltaic cells,” Forrest said. “It was a bit of a coincidence that these structures allowed efficient harvesting of the energy carried by the polaritons. Polaritons still hold some mystery and this is a new way of using them, so we weren’t sure if it would work.’

Long distance propagation

The researchers analyzed their new device using a special Fourier microscope to observe the propagation of polaritons. Because of the detector’s unusual structure, they had to develop a way to accurately quantify the results and put them in the context of conventional detectors well known to the optical community.

The results show that the new photodetector is more efficient at converting light into electrical current than a comparable silicon photodiode. It can also collect light from areas around 0.01 mm2 and achieve conversion of light into electrical current over extremely long distances of 0.1 nm. This distance is three orders of magnitude greater than the energy transfer distance of photosynthetic complexes.

Until now, most polaritons have been observed as stationary quasiparticles in closed cavities with highly reflective mirrors above and below. The new work revealed important insights into how polaritons propagate in open single-mirror structures. The new device also allowed the first measurements of how efficiently incident photons can be converted into polaritons.

“Our work shows that polaritons, in addition to being interesting science, are also a gold mine of yet-to-be-discovered applications,” Forrest said. “Devices like ours provide an unusual and possibly unique method of understanding the fundamental properties of polaritons and enabling yet unimagined ways to manipulate light and charge.”

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