Identification of a triplet pair intermediate in singlet exciton fission in solution

Abstract

Singlet exciton fission is the spin-conserving transformation of one spin-singlet exciton into two spin-triplet excitons. This exciton multiplication mechanism offers an attractive route to solar cells that circumvent the single-junction Shockley-Queisser limit. Most theoretical descriptions of singlet fission invoke an intermediate state of a pair of spin-triplet excitons coupled into an overall spinsinglet configuration, but such a state has never been optically observed. In solution, we show that the dynamics of fission are diffusion limited and enable the isolation of an intermediate species. In concentrated solutions of bis(triisopropylsilylethynyl)[TIPS]-tetracene we find rapid (<100 ps) formation of excimers and a slower (∼10 ns) break up of the excimer to two triplet exciton-bearing free molecules. These excimers are spectroscopically distinct from singlet and triplet excitons, yet possess both singlet and triplet characteristics, enabling identification as a triplet pair state. We find that this triplet pair state is significantly stabilized relative to free triplet excitons, and that it plays a critical role in the efficient endothermic singlet fission process.T he fission of photogenerated spin-singlet excitons into pairs of spin-triplet excitons is an effective way to generate triplet excitons in organic materials (1, 2). Because the triplets produced are coupled into an overall singlet state, spin is conserved and triplet formation can proceed on sub-100-fs timescales (1, 3-5) with yields of up to 200% (1, 6, 7). Current interest in singlet fission is driven by its potential to improve the efficiency of solar cells by circumventing the Shockley-Queisser limit for single-junction devices (8-10). Incorporating singlet fission material within a lowband-gap solar cell should make it possible to capture the energy normally lost to thermalization following the absorption of highenergy photons (11,12). An external quantum efficiency of 129% (13) and an internal quantum efficiency of >180% (14) have been reported using pentacene as the singlet fission material and fullerene (C 60 ) as electron acceptor. Despite such significant advances, many questions remain about the underlying mechanism of triplet formation, such as the role of intermediate electronic states and the ability of systems to undergo endothermic fission.The basis of most kinetic descriptions of singlet fission is the triplet pair state 1 (TT), which is entangled into an overall singlet and is an essential intermediate for the formation of two free triplet excitons (1, 15). Whether this intermediate state is present only transiently, as expected in exothermic systems such as pentacene, or whether it can be sufficiently long lived to also play a central role in the fission process in slower systems is unclear. Transient absorption measurements of the canonical systems pentacene and tetracene in the solid state allow clear identification of only the singlet and triplet states (3,6,16,17), meaning the character of any intermediate has not been ob...

Triplet transport in thin films: fundamentals and applications

“…Current investigations into the intermediates in singlet fission, e.g. bound triplet pair excimers 99 or strongly interacting triplet pairs, 136 etc., enabled by various monomer or dimer configurations 97,[137][138][139][140][141][142] will shed light on the dielectric environment needed to stabilize triplet excitons. Such interdisciplinary approaches will reveal the fundamental structure-property relationships affecting triplet diffusion in thin films, and ultimately enable the engineering of optoelectronic devices making use of multi-excitonic processes.…”

“…Instead of the tens of microsecond lifetimes commonly observed in isolated molecules, these dimers have triplet lifetimes on the order of nanoseconds because of enhanced recombination. [97][98][99] On the other hand, monolithic blocks of conjugated polymers exclusively comprising either donors or acceptors can have the degree of crystallinity, solubility, polydispersity etc. pre-designed by controlling the side chains and rigidity of the monomers.…”

Synthesis and Exciton Dynamics of Donor-Orthogonal Acceptor Conjugated Polymers: Reducing the Singlet–Triplet Energy Gap

“…We consider that the smaller ΔEST here results in some dynamic equilibrium or mixing between 3 (π-π*) and emissive CT states, analogous to signatures of singlet-mixed triplet-pair states observed in certain small molecule systems. [25][26][27][28] Nonetheless, on the longest timescales eventual conformational relaxation in the 3 (π-π*) makes back-transfer to CT thermally inaccessible and emission is quenched. Similar results are obtained in the solid state, with only slight shifts in the characteristic PIA bands of each excited state.…”

Localization length scales of triplet excitons in singlet fission materials

“…Using two prototypical singlet fission materials, TIPS-tetracene and TIPS-pentacene [29][30][31][32] ( Fig. 1), which display very different solid-state environments, we correlate the observed localization lengthscales with the role of local anisotropy, providing a way of dielectrically characterizing dark triplet exciton states.…”