Yoichi Sasaki's Team Makes One Photon Do Double Duty

Before the headlines arrive: no, this does not violate thermodynamics. A 130% quantum yield in a solar cell means that for every photon absorbed, the material produces more than one exciton — a quasiparticle representing an electron-hole pair — through a quantum effect called singlet fission. It does not mean the cell puts out more energy than it takes in. The distinction matters, because the number is real, the physics is sound, and the practical implications are genuinely interesting without requiring a rewrite of the laws of physics.

A research team from Kyushu University in Japan and Johannes Gutenberg University Mainz (JGU Mainz) in Germany has demonstrated a 132% quantum yield in solution-phase experiments using a molybdenum-based spin-flip emitter paired with tetracene-based materials, according to a paper published March 25, 2026 in the Journal of the American Chemical Society (JACS). The work, led by Yoichi Sasaki, an associate professor at Kyushu University's Faculty of Engineering, and Katja Heinze's group at JGU Mainz, represents a proof-of-concept demonstration of a mechanism that has been theorized for years but has proven difficult to implement in practice.

The physics behind it is not new. Singlet fission — the process by which a high-energy singlet exciton splits into two lower-energy triplet excitons — has been understood since the 1960s. The theoretical ceiling for the process is 200% quantum yield: two triplet excitons per absorbed photon, double the single exciton that conventional solar cells produce. What has eluded researchers is a practical way to actually collect those triplet excitons before they dissipate.

The problem is a process called Förster resonance energy transfer, or FRET. In typical singlet fission materials, FRET steals energy from the excited state before the fission event can multiply it into two excitons. "The energy can be easily stolen by a mechanism called Förster resonance energy transfer before multiplication occurs," Sasaki said in an interview with EurekAlert. "We therefore needed an energy acceptor that selectively captures the multiplied triplet excitons after fission."

The Kyushu-JGU Mainz team's solution was to introduce a molybdenum-based spin-flip emitter — a material that undergoes a spin-state transition when photoexcited — as an energy acceptor that selectively harvests triplet excitons from tetracene, a polycyclic aromatic hydrocarbon commonly used in organic electronics. Adrian Sauer, a graduate student from Heinze's group who was visiting Kyushu University on exchange and is the paper's second author, brought the molybdenum material to the team's attention, initiating the collaboration.

The team tested three bridging units connecting the tetracene dimer to the spin-flip emitter: phenylene, 2,5-methylphenylene, and p-terphenylene. The results varied meaningfully: phenylene gave 112±6% quantum yield, 2,5-methylphenylene gave 132±2%, and p-terphenylene gave 128±4%, according to the paper. The best performer — the 2,5-methylphenylene bridge — is not dramatically better than the others, but all three exceeded 100%, which is the meaningful threshold: more than one exciton per photon, confirmed and reproducible.

For context, the Shockley-Queisser limit — the fundamental efficiency ceiling for a single-junction solar cell under standard conditions — caps conventional solar cells at 33.7% efficiency, meaning the maximum fraction of incident solar energy that can be converted to electrical power. Singlet fission-based cells operating at 200% quantum yield could, in principle, break that ceiling in a specific way: by generating two charge carriers from one high-energy photon instead of one, they could capture more of the photon energy that conventional cells waste as heat. This does not create energy from nothing. It recovers energy that would otherwise be lost as excess heat in conventional cells when a photon carries more energy than the bandgap can use.

The current work is limited in an important way: it was conducted in solution, not in a solid-state solar cell device. The team explicitly notes that the next step is bringing the two material systems — the tetracene singlet fission layer and the molybdenum spin-flip emitter — together in solid state, which introduces entirely different physics around charge transport, interfacial stability, and film morphology. Sasaki told EurekAlert that moving to solid-state integration is the plan, but solution-phase results do not automatically translate to working devices.

The timeline from proof-of-concept to deployable solar cell is, as always, uncertain. The authors are not making predictions. What they have demonstrated is a new design strategy for exciton amplification that the JACS peer reviewers found credible enough to publish. The 132% quantum yield is not a laboratory curiosity — it is a measurable, reproducible result that points toward a real physical mechanism for extracting more energy from sunlight than conventional physics allows from a single-junction cell alone.

The authors of the paper are Percy Gonzalo Sifuentes-Samanamud, Adrian Sauer, Aki Masaoka, Yuta Sawada, Yuya Watanabe, Ilias Papadopoulos, Katja Heinze, Yoichi Sasaki, and Nobuo Kimizuka.

Kyushu University research announcement | EurekAlert news release | JACS paper DOI: 10.1021/jacs.5c20500