- Spin-flip metal complexes capture duplicated excitons produced by singlet fission
- Proof-of-concept experiments have achieved quantum efficiency of over 110% to around 130%
- Integration of semiconductors remains necessary before their use in practical solar devices
Japanese researchers have found a way to capture extra energy from sunlight using a metal-based system that reduces heat loss during conversion.
The work focuses on a chemical structure known as a spin-flip emitter, built from molybdenum, which captures the multiplied energy created during a process called singlet fission.
The research was carried out by Kyushu University in Japan, in collaboration with the Johannes Gutenberg University (JGU) Mainz in Germany. The results were published in the Journal of the American Chemical Society.
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Energy is easily “stolen”
Solar cells already convert sunlight into electricity, but only some of the available energy ends up being usable, forcing scientists to look for ways to make more use of the same incoming light.
A long-known ceiling arises from the mismatch between photon energies and the way semiconductors react, meaning that some photons fail to trigger electrons while others lose their excess energy as heat.
This efficiency ceiling, known as the Shockley-Queisser limit, has pushed researchers to explore methods to reuse wasted energy instead of letting it dissipate.
“We have two main strategies to overcome this limit,” said Yoichi Sasaki, associate professor at Kyushu University’s faculty of engineering. “One is to convert low-energy infrared photons to higher-energy visible photons. The other, which we are exploring here, is to use SF to generate two excitons from a single exciton photon.”
Singlet fission, described by the researchers as a “dream technology” for light conversion, plays a central role in the experiment because it allows one high-energy excitation to split into two lower-energy excitations, theoretically doubling the number of usable energy vectors.
Capturing these duplicated excitons has been the most difficult problem, because competing energy transfer processes can redirect the energy before it becomes useful.
The team solved this bottleneck by combining singlet fission materials with a molybdenum-based near-infrared spin-flip emitter, tuned to absorb specific triplet energy states.
“Energy can be easily ‘stolen’ by a mechanism called Förster resonance energy transfer (FRET) before multiplication occurs,” Sasaki said. “We therefore needed an energy acceptor that could selectively capture the triple-multiplied excitons after fission.”
Experiments using tetracene materials in solution produced quantum efficiencies ranging from just over 110% to around 130%, meaning that more energy vectors were generated than incoming photons absorbed under laboratory conditions.
The results remain limited to testing solutions rather than all-solar devices, meaning practical application still depends on translating the chemistry into solid materials compatible with functional panels.
Future work will focus on combining these materials into solid-state systems where energy transfer efficiency can be tested under conditions closer to actual solar cell operation.
The researchers point to possible applications beyond solar panels, including lighting technologies such as OLED, where managing exciton behavior plays a key role in performance.
Via University of Kyushu
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