Solar power plays an important role in efforts to reduce dependence on fossil fuels and combat climate change. The sun supplies Earth with enormous amounts of energy every second, but modern solar cells can capture only a tiny fraction of the energy. This limitation is due to a “physical ceiling” that has been difficult to overcome for many years.
In a study published in Journal of the American Chemical Society On March 25, scientists from Japan’s Kyushu University, in collaboration with collaborators from Germany’s Johannes Gutenberg University (JGU) Mainz, developed a new way to overcome this barrier. They used a molybdenum-based metal complex known as a “spin-flip” emitter to capture the extra energy produced by singlet fission (SF). This is often referred to as a “dream technology” for improving light conversion.
With this approach, the team achieved an energy conversion efficiency of approximately 130%. This exceeds the traditional 100% limit and shows the potential of more advanced solar power technology.
How solar cells work and why energy is lost
Solar cells generate electricity when photons from sunlight strike a semiconductor, transferring energy to electrons, moving the electrons and creating an electric current. This process can be compared to a relay in which energy is passed from one particle to another.
However, not all photons are equally useful. Low-energy infrared photons do not have enough energy to activate electrons, while high-energy photons, such as blue light, lose excess energy as heat. Because of this, solar cells can only utilize about one-third of the incoming sunlight. This constraint, known as the Shockley-Quisser limit, remains a major challenge.
Singlet fission offers a way to increase energy
“There are two main strategies to overcome this limit,” says Yoichi Sasaki, an associate professor at Kyushu University’s Faculty of Engineering. “One is to convert a low-energy infrared photon into a higher-energy visible photon. The other, which we study here, is to use SF to generate two excitons from one exciton photon.”
Under normal conditions, each photon produces only one spin singlet exciton after excitation. Using SF, this single exciton can be split into two lower-energy spin triplet excitons, effectively doubling the available energy. Although certain materials such as tetracene can support this process, efficiently capturing these excitons has proven difficult.
Overcoming energy loss with FRET
“Energy can be easily ‘stealed’ before propagation occurs by a mechanism called Förster resonance energy transfer (FRET),” Sasaki explains. “Therefore, we needed an energy acceptor that would selectively capture the triplet excitons that multiplied after nuclear fission.”
To address this issue, researchers turned to metal complexes that can be precisely processed. They identified a molybdenum-based “spin-flip” emitter as an effective solution. In this system, the spin of the electron changes during the absorption or emission of near-infrared light, allowing the capture of the triplet energy produced by the SF.
By carefully adjusting the energy levels, the researchers minimized FRET losses and enabled efficient extraction of the multiplied excitons.
Successful collaboration and experimentation
“We could not have reached this far without the Heinze Group at JGU Mainz,” says Sasaki. The paper’s second author, Adrian Sauer, a graduate student in the group who was visiting Kyushu University on exchange, drew the team’s attention to material that had been studied at Kyushu University for many years, leading to the collaboration.
When combined with tetracene-based materials in solution, the system successfully recovered energy with a quantum yield of approximately 130%. This means that about 1.3 molybdenum-based metal complexes were activated for each absorbed photon, exceeding the normal limit and indicating that more energy carriers were generated than incident photons.
Future solar and quantum technology applications
This work introduces a new strategy to amplify excitons, although it is still in the proof-of-concept stage. The team aims to integrate these materials into solid-state systems to improve energy transfer and bring them closer to practical solar cell applications.
The discovery could also encourage further research combining singlet fission and metal complexes, which could be used not only for solar energy but also for LEDs and new quantum technologies.
