News

New photodetector inspired by photosynthesis

14.11.2022 - Technology uses polaritons to convert light to electrical charge. New approach could boost solar cell efficiency.

Researchers have developed a new type of high-efficiency photo­detector inspired by the photo­synthetic complexes plants use to turn sunlight into energy. Photo­detectors are used in cameras, optical communication systems and many other applications to turn photons into electrical signals. “Our devices combine long-range transport of optical energy with long-range conversion to electrical current,” said research team leader Stephen Forrest from the University of Michigan. “This arrange­ment, analogous to what is seen in plants, has the potential to greatly enhance the power generation efficiency of solar cells, which use devices similar to photo­detectors to convert sunlight into energy.”

The 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 a charge. While 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 polari­tons. The new detector generates polaritons in an organic thin film.

“A polariton combines a molecular excited state with a photon, giving it both light-like and matter-like properties that allow long-range energy transport and conversion,” said Forrest. “This photo­detector is one of the first demons­trations of a practical optoelectronic device based on polaritons.” The researchers envisioned the new detector several years ago while looking for ways to make better solar cells. “After observing polariton propa­gation over long distances in simple structures such as a mirror with an organic film on its surface, we thought it might be possible to make a photo­synthetic analog using polaritons,” said Forrest. “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 polariton propagation over long distances in an organic semiconductor thin film. They also had to figure out how to integrate a simple organic detector into the propagation region in a way that would produce effi­cient polariton-to-charge conversion. “We borrowed from structures that we previously designed to create efficient organic photo­voltaic cells,” said Forrest. “It was a bit fortuitous that these structures allowed efficient harves­ting of the energy carried by polari­tons. Polaritons still hold some mysteries, and this is a new way of using them, so we weren’t sure if it would work.”

The researchers analyzed their new device by using a special Fourier plane micro­scope to observe polariton propagation. Due to the unusual structure of the detector they had to develop a way to accurately quantify the results and put them in the context of conven­tional detectors well known to the optics community. The results showed that the new photo­detector is more efficient at converting light to electrical current than a comparable silicon photodiode. It can also gather light from areas about 0.01 mm2 and achieve conversion of light to electrical current over exceptio­nally long distances of 0.1 nano­meters. This distance is three orders larger 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 on both top and bottom. The new work revealed important insights into how polaritons propagate in open structures with a single mirror. The new device also allowed the first measure­ments of how effi­ciently incident photons can be converted to polaritons. “Our work shows that polaritons, in addition to being interes­ting science, are also a goldmine of applications yet to be discovered,” said Forrest. “Devices such as ours provide an unusual, and possibly unique, method to understand the funda­mental properties of polaritons and to enable yet to be imagined ways to manipulate light and charge.” (Source: Optica)

Reference: B. Liu et al.: Photocurrent generation following long range propagation of organic exciton-polaritons, Optica 9, 1029 (2022), DOI: 10.1364/OPTICA.461025

Link: Optoelectronic components & materials group, Dept. of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, USA

Digital tools or software can ease your life as a photonics professional by either helping you with your system design or during the manufacturing process or when purchasing components. Check out our compilation:

Proceed to our dossier

Digital tools or software can ease your life as a photonics professional by either helping you with your system design or during the manufacturing process or when purchasing components. Check out our compilation:

Proceed to our dossier