Activated Singlet Exciton Fission in a Semiconducting Polymer

Musser, Andrew J.; Al‐Hashimi, Mohammed; Maiuri, Margherita; Brida, Daniele; Heeney, Martin; Cerullo, Giulio; Friend, Richard H.; Clark, Jenny

doi:10.1021/ja405427j

Cited by 152 publications

(266 citation statements)

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“…A similar effect is observed in some polyene-type systems, in which the doubly excited 2A g state can be identified as a bound triplet pair (34). In poly(3-dodecylthienylenevinylene), for instance, the 2A g state is substantially lower in energy than the threshold for singlet exciton fission, and indeed offers a rapid decay channel for triplet pairs (35). Multiexciton states with A g symmetry have also been invoked to explain fission in calculations of crystalline pentacene, although in that system there is no evidence of any significant energetic stabilization in this state (36).…”

Section: Resultssupporting

confidence: 55%

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

Stern

Musser

Gélinas

et al. 2015

Proc. Natl. Acad. Sci. U.S.A.

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189

258

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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...

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Section: Resultssupporting

confidence: 55%

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

Stern

Musser

Gélinas

et al. 2015

Proc. Natl. Acad. Sci. U.S.A.

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189

258

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“…The triplet decay observed in the rigid matrix and polar solvent is markedly faster than the typical bimolecular TTA time scales measured in acenes in the solid state. [ 27 ] On the other hand, it is much slower than the geminate TTA observed in polyenes such as poly(3-dodecylthienylenevinylene), [ 19 ] where the presence of a low-lying excited state of the same symmetry allows rapid coupling to the ground state. We see no direct evidence for lowlying excited states in DP-Mes that would facilitate TTA, but detailed theoretical studies are called for to investigate the possibility more fully.…”

Section: Discussionmentioning

confidence: 99%

“…The same photophysical behavior was observed at 1.0 and 0.1 mg mL −1 ( Figure S17, Supporting Information), confi rming that our results only describe intramolecular processes. Triplet sensitization was performed using established procedures, [ 19 ] with a mixed toluene solution of 1 mg mL −1 DP-Mes and 3 mg mL −1 N -methylfulleropyrrolidine. All solution measurements were performed in 1 mm light-path quartz cuvettes (Hellma Analytics).…”

Section: Methodsmentioning

confidence: 99%

“…[ 6,18 ] We have confi rmed this assignment using an established triplet sensitization experiment with a fullerene donor (Figure 3 b, blue dashed; full spectra in Figure S19, Supporting Information). [ 19 ] Intriguingly, the shape of the sensitized triplet spectrum differs in one key respect: the PIA in the NIR is 50% weaker, relative to the GSB, than in the spectrum following direct excitation. In other words, direct excitation of DP-Mes yields twice as much triplet exciton PIA for the same quantity of excited dimer (GSB).…”

Section: Intramolecular Sefmentioning

confidence: 99%

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Tuneable Singlet Exciton Fission and Triplet–Triplet Annihilation in an Orthogonal Pentacene Dimer

Lukman

Musser

Chen

et al. 2015

Adv Funct Materials

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5452 wileyonlinelibrary.com proceed on ultrafast (≈100 fs) time scales, allowing it to out-compete other decay channels and achieve high effi ciencies. [ 3 ] The essential condition for effi cient SEF is the energetic alignment of the singlet and triplet states, such that 2 E (T 1 ) ≤ E (S 1 ). A recent combined theoretical and experimental study of SEF rates in a range of acene solids has demonstrated that the rate of SEF is also greatly affected by the strength of intermolecular coupling within the fi lm. [ 4 ] In the canonical system, pentacene, triplet pair formation is exothermic and the intermolecular coupling is strong, resulting in SEF with an 80 fs time constant and nearly 200% yield. [ 5 ] Though most experimental studies of SEF have involved crystalline, polycrystalline or amorphous solids, the most basic unit capable of SEF is a pair of chromophores. Indeed, it was recently demonstrated in concentrated solutions of TIPS-pentacene that singlet fi ssion can proceed at high efficiency through bimolecular diffusional interactions. [ 6 ] However, early attempts to directly control the interaction between chromophores through the use of covalent dimers have not been as successful. The most notable systems in this regard are tetracene and 1,3-diphenylisobenzofuran. These materials are found to exhibit effi cient SEF in the solid state, but their covalent dimers achieved triplet yields of only a few percent. In both of these studies, [ 7 ] the two SEF chromophores were joined by a range of linkers to modify the strength of the electronic coupling between them, with the aim of tuning the rate and effi ciency of SEF. The impact was subtle, and it thus remains unclear why covalent dimers have proved ineffi cient to date. Current models suggest that dimers should be asymmetric or contain signifi cant cofacial interaction between chromophores to attain high triplet yields. [ 2,8 ] Interestingly, a recent study of pentacene dimers separated by a phenyl spacer unit achieved triplet yields above 100% in spite of using the same symmetric bonding motifs of the earlier tetracene dimers. [ 9 ] In this work, we report highly effi cient intramolecular SEF in a new type of covalent dimer, with triplet yields of up to 192 ± 3%. The molecule used in this study, 13,13′-bis(mesityl)-6,6′-dipentacenyl (DP-Mes, Figure 1 a), consists of two pentacenes directly bonded through a single C C bond with two bulky mesityl groups at the meso -positions. The geometry of the dimer, with two nearly orthogonal pentacene cores, is unlike Tuneable Singlet Exciton Fission and Triplet-Triplet Annihilation in an Orthogonal Pentacene DimerSteven Lukman , Andrew J. Musser , Kai Chen , Stavros Athanasopoulos , Chaw K. Yong , Zebing Zeng , Qun Ye , Chunyan Chi , Justin M. Hodgkiss , Jishan Wu , * Richard H. Friend , and Neil C. Greenham * Fast and highly effi cient intramolecular singlet exciton fi ssion in a pentacene dimer, consisting of two covalently attached, nearly orthogonal pentacene units is reported. Fission to triplet excitons from...

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“…1, 2 Decades after its first observation in anthracene crystals, SF has been recently experimented in a wide variety of crystalline organic materials, [3][4][5][6][7][8][9][10] polymers, 11 quantum dots, 12 and in solution. 13 One of the reasons behind a renewed interest on this physical phenomenon is given by the opportunity to exploit it, in order to overcome the Shockley-Queisser theoretical limit of conversion efficiency for single junction solar cells, 14 a possibility that has been verified in few instances 4,15 but it is still under debate.…”

Section: Introductionmentioning

confidence: 99%

Singlet fission in linear chains of molecules

Ambrosio

Troisi

2014

The Journal of Chemical Physics

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We develop a model configuration interaction Hamiltonian to study the electronic structure of a chain of molecules undergoing singlet fission. We first consider models for dimer and trimers and then we use a matrix partitioning technique to build models of arbitrary size able to describe the relevant electronic structure for singlet fission in linear aggregates. We find that the multi-excitonic state (ME) is stabilized at short inter-monomer distance and the extent of this stabilization depends upon the size of orbital coupling between neighboring monomers. We also find that the coupling between ME states located on different molecules is extremely small leading to bandwidths in the order of ~10meV. This observation suggests that multi-exciton states are extremely localized by electron-phonon coupling and that singlet fission involves the transition between a relatively delocalized Frenkel exciton and a strongly localized multi-exciton state. We adopt the methodology commonly used to study non-radiative transitions to describe the singlet fission dynamics in these aggregates and we discuss the limit of validity of the approach. The results indicate that the phenomenology of singlet fission in molecular crystals is different in many important ways from what is observed in isolated dimers.

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Activated Singlet Exciton Fission in a Semiconducting Polymer

Cited by 152 publications

References 43 publications

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

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

Tuneable Singlet Exciton Fission and Triplet–Triplet Annihilation in an Orthogonal Pentacene Dimer

Singlet fission in linear chains of molecules

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