A limit long considered insurmountable in solar physics has developed cracks. Researchers at Kyushu University in Japan and Johannes Gutenberg University Mainz have jointly demonstrated a process that achieves 130 percent quantum yield. That means 1.3 energy states are generated per absorbed photon, exceeding the physically expected maximum of 1.0. The work was published on March 25 in the Journal of the American Chemical Society. It is a laboratory demonstration in liquid solution, not a product. But the foundation for solar cells significantly more efficient than anything available today has been laid.
The Limit at Stake
Conventional solar cells run into what is known as the Shockley-Queisser limit, named after physicists William Shockley and Hans-Joachim Queisser who calculated it in 1961. Under this limit, an ideal single-junction solar cell can convert a maximum of about 33.7 percent of incoming solar energy into electricity. Commercial silicon panels today reach 20 to 22 percent. The problem: high-energy photons release their excess energy as heat rather than converting it into current.
One theoretical solution is singlet fission: a high-energy photon is split into two lower-energy excited states (triplets). From one photon, two potentially usable charge carriers can emerge. This allows quantum yields above 100 percent in theory and makes it possible to surpass the Shockley-Queisser limit. Previous attempts failed because these triplet states could not be harvested efficiently before the energy was lost.
The Experiment
The team led by Yoichi Sasaki of Kyushu University and Katja Heinze of Johannes Gutenberg University Mainz solved this harvesting problem using a molybdenum-based metal complex acting as a spin-flip emitter. The complex couples to tetracene, an organic material known for singlet fission. Acting as an energy intermediary, the molybdenum captures the generated triplet states and makes them usable through a quantum spin flip before they decay.
In the experiment, each absorbed photon produced 1.3 excited molybdenum complexes instead of 1.0, corresponding to a quantum yield of 130 percent. This is the first experimental demonstration that singlet fission can be used for efficient energy harvesting.
What This Does Not Mean
130 percent quantum yield is not the same as 130 percent solar cell efficiency. That would violate the law of conservation of energy. Quantum yield describes how many usable excited states arise per absorbed photon, not how much of the total incoming solar energy is converted into electricity. What the overall efficiency of a future solar installation would actually be depends on many additional factors.
There is a further constraint: the laboratory demonstration works only in liquid solution. Transferring the approach into a stable solid-state layer, as required for real solar cells, is described by the research team itself as a significant challenge. How many years it would take to reach a prototype remains open.
The Potential
Despite these limitations, the significance of the result is substantial. If the transfer to solid-state cells succeeds, the researchers project realistic efficiencies of 35 to 45 percent for practical solar cells. That would be more than double today's commercial panels. For comparison, the most expensive multi-junction solar cells used in space applications currently reach around 47 percent but cost many times more than standard cells.
Beyond solar energy, the researchers see application potential in high-efficiency LEDs and quantum technology, where similar spin mechanisms are relevant. That Kyushu and Mainz are advancing this fundamental research together also demonstrates that key technologies for the energy transition are not emerging solely from American or Chinese laboratories. The team's next step is developing a solid-state version of the system.