Singlet Fission Mechanisms in Organic Photovoltaic Materials

Singlet fission is a photophysical process in which a single photoexcited singlet exciton splits into two triplet excitons, potentially doubling the number of charge-generating states and enhancing the efficiency of organic solar cells. The initial step involves ultrafast formation of a correlated triplet pair, often mediated by a charge-transfer character that mixes Frenkel and inter­molecular states. The efficiency of this primary conversion depends critically on energetic alignment—namely that twice the triplet energy lies below the singlet state—and on the degree of electronic coupling determined by molecular packing and orbital overlap. Subsequent separation of the triplet pair into free triplets can be accelerated or hampered by intermolecular geometry, crystal phase purity and side-group engineering. Advances in time-resolved spectroscopy and electron paramagnetic resonance have illuminated the role of transient intermediates, revealing that both direct and virtual charge-transfer pathways can drive fission. Control over spin polarisation, exciton localisation and energy level matching is now guiding the design of new chromophores and device architectures with improved triplet harvesting and stability.

Research from Nature Portfolio

Recent studies have applied time- and angle-resolved photoemission spectroscopy to crystalline pentacene, directly visualising the hybridisation of Frenkel and charge-transfer states in the initially excited singlet and confirming that a charge-transfer-mediated mechanism underpins the ultrafast production of a correlated triplet pair. Momentum-resolved measurements have mapped orbital character, enabling decomposition of overlapping excitonic states and clarifying localisation differences between singlet and bitriplet species. In parallel, time-resolved electron paramagnetic resonance on a rigid acene dimer has demonstrated the preparation of a nearly pure quintet spin state directly from singlet fission. By exploiting non-adiabatic transition theory with minimal spectroscopic parameters, this work quantifies spin-selective relaxation pathways and reveals long-lived coherences at elevated temperatures, indicating new opportunities for quantum-coherent control in organic materials.

Singlet Fission Mechanisms in Organic Photovoltaic Materials publication trend

The graph below shows the total number of articles in singlet fission mechanisms in organic photovoltaic materials across all publications each year (not limited to Nature Index journals).

Technical terms

Singlet exciton: An excited electronic state with paired electron spins (total spin zero).

Triplet exciton: An excited state with two unpaired electrons having parallel spins (total spin one).

Charge-transfer state: An excited configuration in which an electron and hole are localized on different molecular units.

Frenkel exciton: A tightly bound electron–hole pair confined to a single molecule or chromophore.

Correlated triplet pair: A transient bi-exciton complex formed during singlet fission, often with mixed spin character.

Quintet state: A spin manifold of total spin two arising from coupling of two triplet excitons.

References

1. Orbital-resolved observation of singlet fission. Nature (2023).
2. Molecular Control of Triplet-Pair Spin Polarization and Its Optoelectronic Magnetic Resonance Probes. Accounts of Chemical Research (2023).
3. Enhancing Organic Semiconductor Molecular Packing Using Perovskite Interfaces to Improve Singlet Fission. Advanced Functional Materials (2023).
4. Entangled spin-polarized excitons from singlet fission in a rigid dimer. Nature Communications (2023).
5. Efficient Singlet Fission and Triplet-Pair Emission in a Family of Zethrene Diradicaloids. Journal of the American Chemical Society (2017).