Singlet fission is the photophysical process of splitting one singlet state into two triplet states. It is an example of a photon amplification process (two photons/excitons for the price of one!) which has seen increased interest in the literature as it is one way of converting high energy above band-gap photons – which would otherwise lose energy through thermalisation – into multiple lower energy photons closer to the band edge. In this way, the Schockley-Quiesser limit of single-junction solar cells can be overcome.
The first requirement for singlet fission is that the first excited singlet state of the chromophores of interest must be roughly equivalent to 2 x the triplet energy as shown in b). If 2(ET1) is lower in energy that ES1 then singlet fission is exothermic. The difference between ES1 and 2(ET1) is lost in the process. If 2(ET1) is higher in energy than ES1, singlet fission is endothermic and energy is gained from the surroundings in the process.
There is still debate in this research field as to which intermediate species are involved in the singlet fission process. The process may proceed directly or via a CT state (as shown in a)).
Singlet fission proceeds via a correlated triplet pair intermediate. This intermediate is two triplet states (on two chromophores) coupled into an overall singlet – distinguishing this process from intersystem crossing in that spin is conserved (c)).
As singlet fission requires two chromophores, the minimum possible unit is a bichromophoric dimer. Dimers are often studied as they are simpler systems in many ways. Intermolecular processes can be avoided as well as issues due to film defects. Because they can be studied in solution, the nature of charge-transfer states can be probed. However, dimer systems also present some issues. The back reaction (triplet-triplet annihilation) can compete as chromophores are in close proximity and triplets generated have nowhere to “go”. Molecules are not fixed rigidly into position so there are many degrees of freedom in terms of molecular configuration that can complicate spectral characterisation.
In recent years we have investigated new systems for exploiting singlet fission and studying the photophysical properties.1-2 Our results have shown how the intermolecular coupling in a crystalline system can give rise to more efficient or longer-lived triplets.3 The other regime of interest in our group is exploring singlet fission in nanoparticles like TIPS-pentacene and diketopyrrolopyrrole (DPP) and investigating how singlet fission properties will change with the morphological evolution of nanoparticles.4
Singlet-fission shows a lot of potential in the future of photovoltaics. What’s left is for us to fully understand the factors driving the process. There are many interesting discoveries left to be made in this exciting field.
- Masoomi-Godarzi, S.; Liu, M.; Tachibana, Y.; Goerigk, L.; Ghiggino, K. P.; Smith, T. A.; Jones, D. J., Solution-Processable, Solid State Donor–Acceptor Materials for Singlet Fission. Adv. Energy Mater. 2018, 8, 1801720.
- Masoomi-Godarzi, S.; Liu, M.; Tachibana, Y.; Mitchell, V. D.; Goerigk, L.; Ghiggino, K. P.; Smith, T. A.; Jones, D. J., Liquid Crystallinity as a Self-Assembly Motif for High-Efficiency, Solution-Processed, Solid-State Singlet Fission Materials. Adv. Energy Mater. 2019, 9, 1901069.
- Masoomi-Godarzi, S.; Hall, C. R.; Zhang, B.; Gregory, M. A.; White, J. M.; Wong, W. W. H.; Ghiggino, K. P.; Smith, T. A.; Jones, D. J., Competitive Triplet Formation and Recombination in Crystalline Films of Perylenediimide Derivatives: Implications for Singlet Fission. J. Phys. Chem. C. 2020.
- Tayebjee, M. J. Y.; Schwarz, K. N.; MacQueen, R. W.; Dvořák, M.; Lam, A. W. C.; Ghiggino, K. P.; McCamey, D. R.; Schmidt, T. W.; Conibeer, G. J., Morphological Evolution and Singlet Fission in Aqueous Suspensions of Tips-Pentacene Nanoparticles. J. Phys. Chem. C. 2016, 120, 157-165.