In research published in the Journal of the American Chemical Society on March 25, scientists from Kyushu University in Japan, working with collaborators at Johannes Gutenberg University (JGU) Mainz in Germany, developed a new way to push past this barrier. They used a molybdenum-based metal complex known as a “spin-flip” emitter to capture extra energy generated through singlet fission (SF), often described as a “dream technology” for improving light conversion.

With this approach, the team achieved energy conversion efficiencies of around 130%, exceeding the traditional 100% limit and pointing toward more advanced solar technologies.

How Solar Cells Work and Why Energy Is Lost

Solar cells produce electricity when photons from sunlight hit a semiconductor and transfer energy to electrons, setting them in motion and creating an electric current. This process can be compared to a relay, where 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 their extra energy as heat. Because of this, solar cells can only utilize about one-third of incoming sunlight. This constraint is known as the Shockley-Queisser limit and has remained a major challenge.

Singlet Fission Offers a Way To Multiply Energy

“We have two main strategies to break through this limit,” says Yoichi Sasaki, Associate Professor at Kyushu University’s Faculty of Engineering. “One is to convert lower-energy infrared photons into higher energy visible photons. The other, what we explore here, is to use SF to generate two excitons from a single exciton photon.”

Under normal conditions, each photon produces only one spin-singlet exciton after excitation. With SF, this single exciton can split into two lower-energy spin-triplet excitons, which could effectively double the available energy. Although certain materials such as tetracene can support this process, capturing these excitons efficiently has proven difficult.

Overcoming Energy Loss From FRET

“The energy can be easily ‘stolen’ by a mechanism called Förster resonance energy transfer (FRET) before multiplication occurs,” Sasaki explains. “We therefore needed an energy acceptor that selectively captures the multiplied triplet excitons after fission.”

To address this issue, the researchers turned to metal complexes, which can be precisely engineered. They identified a molybdenum-based “spin-flip” emitter as an effective solution. In this system, an electron changes its spin during absorption or emission of near-infrared light, allowing it to capture the triplet energy generated by SF.

By carefully adjusting the energy levels, the team minimized losses from FRET and enabled efficient extraction of the multiplied excitons.

Collaboration and Experimental Success

“We could not have reached this point without the Heinze group from JGU Mainz,” Sasaki says. Adrian Sauer, a graduate student from the group visiting Kyushu University on exchange and the paper’s second author, brought the team’s attention to a material long studied there, leading to the collaboration.

When combined with tetracene-based materials in solution, the system successfully harvested energy with quantum yields of about 130%. This means that roughly 1.3 molybdenum-based metal complexes were activated for every photon absorbed, exceeding the usual limit and demonstrating that more energy carriers were produced than incoming photons.

Future Solar and Quantum Technology Applications

This research introduces a new strategy for amplifying excitons, although it is still at the proof-of-concept stage. The team aims to integrate these materials into solid-state systems to improve energy transfer and move closer to practical solar cell applications.

The findings could also encourage further research combining singlet fission and metal complexes, with potential uses not only in solar energy but also in LEDs and emerging quantum technologies.