Kyushu researchers exceed solar cell efficiency limit with 130% quantum yield

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By: Dakir Madiha

Kyushu researchers exceed solar cell efficiency limit with 130% quantum yield

Researchers at Japan's Kyushu University have shown a way to generate more energy carriers from sunlight than photons absorbed. They achieved about 130% quantum yield, surpassing a longstanding physical limit in solar energy conversion. The findings, published March 25 in the Journal of the American Chemical Society, offer proof-of-concept for future solar cell designs.

Conventional solar cells face the Shockley-Queisser limit. This theoretical ceiling states each absorbed photon excites one electron at most. Low-energy infrared photons fail to excite electrons. High-energy photons lose excess as heat. Standard cells thus use only about one-third of incoming sunlight.

Kyushu's team, working with Germany's Johannes Gutenberg University Mainz, used singlet fission. This quantum process splits one high-energy exciton into two lower-energy triplet excitons. Some organic materials like tetracene enable singlet fission. The challenge lay in capturing multiplied excitons before energy loss via competing Förster resonance energy transfer.

"We needed an energy acceptor that selectively captures multiplied triplet excitons after fission," said Yoichi Sasaki, associate professor at Kyushu University's Faculty of Engineering.

The team used a molybdenum-based spin-flip emitter. In this metal complex, an electron reverses spin during near-infrared light absorption or emission. This makes it ideal for triplet energy from singlet fission. Fine-tuned energy levels blocked wasteful transfer and extracted multiplied excitons.

Paired with tetracene-based materials in solution, the system hit 130% quantum yield. About 1.3 molybdenum complexes activated per absorbed photon. The theoretical maximum for this method reaches 200%.

Collaboration began with Adrian Sauer, a Mainz exchange student who brought long-studied materials to Kyushu's lab.

The work remains early-stage. Experiments occurred in solution. Next steps target solid-state systems for functional solar cells. Beyond photovoltaics, potential uses include LEDs and quantum technologies.