Researchers successfully capture
singlet-fission–amplified excitons with a molybdenum-based emitter, achieving
130% quantum yield and opening a path beyond solar cell efficiency limits.
Credit: Percy Gonzalo Sifuentes-Samanamud / Tokyo University
In
the fight against climate change, solar power is a promising alternative to
fossil fuels. Every second, Earth receives an enormous amount of energy from
the sun. Yet solar cells capture only a fraction of it, constrained by a
"physical ceiling" that seemed impossible to break.
In a paper published in the Journal of the American Chemical Society, a research team led by Kyushu University in Japan,
in collaboration with Johannes Gutenberg University (JGU) Mainz in Germany,
used a molybdenum-based metal complex called "spin-flip" emitter to
harvest multiplied energy from singlet fission (SF)—a "dream
technology" for light conversion. This technology pushes energy conversion
efficiency to about 130%, surpassing the 100% barrier and opening new
possibilities for higher-performance solar cells.
To picture how a solar cell generates
electricity, imagine a relay race among tiny particles. Photons from sunlight
strike a semiconductor and pass their energy to electrons, activating them and
driving an electric current.
But the "runners" in sunlight
vary in ability. Lower-energy infrared photons cannot excite electrons, while
higher-energy ones, like blue light, lose their excess as heat. As a result, solar cells can use only about one-third of the sunlight.
This ceiling, known as the Shockley–Queisser limit, has long challenged
scientists.
"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."
Normally, one photon can generate at
most one spin-singlet exciton after electronic excitation. SF can split this
high-energy singlet exciton into two lower-energy spin-triplet excitons,
theoretically doubling the energy. While some organic semiconductors like tetracene exhibit this process, capturing
the SF-born excitons remains challenging.
"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."
The team turned to metal
complexes—molecules whose structures can be flexibly designed—and discovered
that a molybdenum-based "spin-flip" emitter serves as an ideal
harvester. In such molecules, an electron flips its spin during absorption or emission
of near-infrared light, enabling the system to accept the triplet energy
produced in SF.
By carefully tuning the energy levels,
the researchers suppressed the wasteful FRET process, allowing the multiplied
excitons from SF to be selectively extracted.
"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.
By pairing this complex with
tetracene-based materials in solution, the team successfully harvested energy,
achieving quantum yields of around 130%, meaning roughly 1.3 molybdenum-based
metal complexes were excited per photon absorbed. This exceeds the conventional
100% limit, indicating that the system generated and harvested more energy
carriers than photons received.
This work establishes a new design
strategy for exciton amplification, though the team notes that current
experiments remain at the proof-of-concept stage. Looking ahead, they plan to
bring the two types of materials together in the solid state, aiming for
efficient energy transfer and eventual integration into working solar cells.
Meanwhile, they hope the study will
inspire further exploration at the intersection of singlet fission and metal
complexes, with potential applications ranging from solar cells and LEDs to
next-generation quantum technologies.
Provided by Kyushu University
by Kyushu
University
edited
by Sadie Harley, reviewed by Robert
Egan
Source: 'Spin-flip'
in metal complexes opens a path beyond solar cell efficiency limits
