A research team led by Japan’s Kyushu University, in collaboration with Germany’s Johannes Gutenberg University (JGU) Mainz, claims to have achieved a light conversion efficiency of approximately 130% in solar cells, exceeding the 100% barrier.

This efficiency, which can enable higher performance in solar cells, was achieved after using a molybdenum-based metal complex called a “spin-flip” emitter to harvest multiplied energy from singlet fission (SF) for light conversion.

Photons from sunlight pass their energy to electrons after striking a semiconductor, in this case, a solar cell. The electrons are activated, driving an electric current.

However, the efficiency of this process varies under sunlight. Lower-energy infrared photons cannot excite electrons. Meanwhile, higher-energy photons that do excite electrons, such as in blue light, lose their excess energy as heat. This results in solar cells using only one-third of the sunlight.

This utilization ceiling is known as the Shockley–Queisser limit.

Yoichi Sasaki, Associate Professor at Kyushu University’s Faculty of Engineering, noted that there are two main strategies to break the Shockley–Queisser limit. The first strategy involves converting lower-energy photons to higher-energy photons. The second strategy, employed by the research team, involves using SF to generate two excitons from a single exciton photon.

An exciton is a neutral, bound quasiparticle that is formed when a photon is absorbed by a semiconductor material, causing an electron to jump to a higher energy state, leaving behind a positively charged hole.

Typically, one photon can generate a maximum of one spin-singlet exciton after electronic excitation, which is a process in which an electron in an atom or molecule absorbs energy and jumps from a lower to a higher energy level without leaving the atom.

The SF technology can split the high-energy singlet exciton into two lower-energy spin-triplet excitons. This process can, in theory, double the number of usable excitations. The research team stated that while some organic semiconductors, such as tetracene, exhibit this process, capturing the SF-born excitons has remained a challenge.

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

To select an appropriate energy acceptor for harvesting the excess energy, the research team opted to use metal complexes: molecules whose structures can be flexibly designed. It discovered that a molybdenum-based “spin-flip” emitter serves as an ideal harvester.

Electrons in such molecules flip their spin during the absorption or emission of near-infrared light. This enables the solar system to accept the triplet energy produced in SF. The researchers suppressed the wasteful FRET process by carefully tuning the energy levels. This allowed the multiplied excitons from the SF to be selectively extracted.

The research team harvested energy by pairing the metal complex with tetracene-based materials in solution, thereby achieving quantum yields of roughly 130%. This means that approximately 1.3 molybdenum-based metal complexes were excited per photon absorbed, indicating that the solar system generated and harvested more energy carriers than photons received.

The team said the discovery will help establish a new design strategy for exciton amplification. However, it noted that the latest tests were at the proof-of-concept stage.

The research team plans to bring the molybdenum-based metal complex “spin-flip” emitter and the tetracene-based materials together in a solid state to enable efficient energy transfer and eventual integration into working solar cells.

Electrons can jump across solar materials in approximately 18 femtoseconds, less than 20 quadrillionths of a second, according to researchers from the University of Cambridge’s St John’s College. The researchers claim that this speed of movement is almost the fastest possible in nature, with charge separation observed within a single molecular vibration.

This March, scientists at the University of California in the U.S. developed a molecular system that can capture, store, and release solar energy without relying on conventional batteries or grid infrastructure, addressing a key limitation in renewable energy deployment.