A research team at Kyushu University in Japan has reported a breakthrough that could steer photovoltaic technology past long‑standing efficiency barriers by harnessing a quantum process known as singlet fission (SF).
Singlet exciton fission is an effect seen in certain materials whereby a single photon can generate two electron-hole pairs as it is absorbed into a solar cell rather than the usual one. The effect has been observed by scientists as far back as the 1970s and though it has become an important area of research for some of the world’s leading institutes over the past decade; translating the effect into a viable solar cell has proved complex.
Singlet fission solar cells can produce two electrons from one photon, making the cell more efficient. This happens through a quantum mechanical process where one singlet exciton (an electron-hole pair) is split into two triplet excitons. By pairing SF with a specially designed spin‑flip molybdenum‑based complex, the scientists demonstrated energy conversion and harvesting in solution with an effective quantum yield of around 130%.
“The applications of this work in solar cells will require integrating singlet-fission (SF) materials with spin-flip emitters in solid-state systems,” Nobuo Kimizuka, lead author of the study, told pv magazine. “As fundamental research, our first step is to develop high-efficiency SF and spin-flip emitters with well-controlled energy levels and luminescence quantum yields in solid-state environments, and then evaluate the performance of these integrated systems.”
“We are actively working on building a higher-performance solid-state system,” he added. “Achieving robust performance in solid-state solar cells remains a challenge, but we expect the efficiency to surpass that of conventional SF technology alone. This approach, which multiplies photons and converts otherwise ‘dark’ triplet excitons into light, could open the door to new quantum technologies such as quantum sensors and exciton circuits, while also contributing to the design of next-generation quantum materials.”
The team developed a molybdenum-based spin-flip emitter that selectively captures the energy of triplet excitons before they dissipate. Its molecular design allows electron spin to flip during near-infrared (NIR) light absorption or emission, enabling more efficient harvesting of the multiple excitons generated by singlet fission.
Further analysis showed that sensitization efficiency depends heavily on the structure of the linker connecting tetracene units. The linker dictates not only the spatial arrangement and electronic coupling of the chromophores but also the exchange interaction within the correlated triplet pair. Variations in linker length, rigidity, and conjugation can significantly affect the rate and yield of triplet energy transfer to the spin-flip emitter, influencing both efficiency and the dynamics of the singlet fission process.
“The methodology we developed for assessing doublet yields provides a practical way to estimate triplet yields of SF dimers, even in systems with complex energy-transfer pathways involving both correlated and free triplets,” Kimizuka explained. “Reducing losses from correlated triplet-pair recombination requires either rapid separation into long-lived multiexcitons or faster triplet transfer to an acceptor molecule, achievable through careful energy-level design in oligomers or solid-state structures.”
“With a versatile selection of central metals, including chromium, molybdenum, and vanadium, and tunable ligands informed by Tanabe–Sugano diagrams and ligand-field theory, spin-flip emitters show strong potential as NIR-emitting materials for efficient triplet extraction, especially with recent advances in air-stable designs,” he added.
The interface design will be critical for converting triplet excitons generated by tetracene singlet fission into charge carriers on the silicon solar cell surface. “In SF-sensitized silicon cells, one major source of energy loss is transfer from the SF molecule to silicon via its excited singlet state,” Kimizuka noted. “Our proof-of-concept method blocks these loss pathways, enabling selective extraction of the excited triplet states originating from singlet fission.”
The research findings are available in the study “Exploring Spin-State Selective Harvesting Pathways from Singlet Fission Dimers to a Near-Infrared-Emissive Spin-Flip Emitter,” published in the Journal of the Chemical American Society.
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