The Binding Energy of Charge-Transfer Excitons Localized at Polymeric Semiconductor Heterojunctions

Gélinas, Simon; Paré-Labrosse, Olivier; Brosseau, Colin-Nadeau; Albert‐Seifried, Sebastian; McNeill, Christopher R.; Kirov, Kiril R.; Howard, Ian A.; Leonelli, R.; Friend, Richard H.; Silva, Carlos

doi:10.1021/jp200466y

Cited by 135 publications

(166 citation statements)

References 41 publications

(73 reference statements)

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“…Following the identification of three separate species in concentrated TIPStetracene solution, we determine the time evolution of each using singular value decomposition and a spectral deconvolution code (29). This code, based on a genetic algorithm, generates spectra that best reproduce the original transient absorption data and satisfy physical constraints such as spectral shape and population dynamics.…”

Section: Resultsmentioning

confidence: 99%

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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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: Resultsmentioning

confidence: 99%

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.

Self Cite

190

260

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“…This approach utilizes a genetic algorithm to establish the individual spectra and population kinetics of the singlet, triplet, and the unidentifi ed third species (see Supporting Information for details). [ 21 ] The advantage of this approach is that it does not presuppose any particular model connecting the states or any assignment as to their nature. The resulting population kinetics (Figure 3 c, shaded regions) arise naturally and reproducibly from the data and illustrate a clear sequential relationship between S 1 , T 1 , and S*.…”

Section: Geminate Triplet-triplet Annihilationmentioning

confidence: 99%

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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“…Due to the low dielectric constant of these materials (ε γ within 3-4), the charges remain electrostatically attracted across the D/A interface with length scales smaller than the Coulomb capture radius r c = e 2 /4πε 0 ε γ k B T at room temperature [48,49], leading to a high rate of electron-hole encounters. That could produce Coulombic bound "charge-transfer" CT states with the binding energies of several hundred meV [50], without consideration of whether these states eventually dissociate into free carriers or stay bound until recombination [51]. The fundamental question is how the organic heterojunction enables efficient long-range charge separation, especially; some OPV systems work very efficiently with internal quantum efficiency to 100% [52].…”

Section: Hot Exciton Dissociationmentioning

confidence: 99%

Towards High Performance Organic Photovoltaic Cells: A Review of Recent Development in Organic Photovoltaics

2014

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Organic photovoltaic cells (OPVs) have been a hot topic for research during the last decade due to their promising application in relieving energy pressure and environmental problems caused by the increasing combustion of fossil fuels. Much effort has been made toward understanding the photovoltaic mechanism, including evolving chemical structural motifs and designing device structures, leading to a remarkable enhancement of the power conversion efficiency of OPVs from 3% to over 15%. In this brief review, the advanced progress and the state-of-the-art performance of OPVs in very recent years are summarized. Based on several of the latest developed approaches to accurately detect the separation of electron-hole pairs in the femtosecond regime, the theoretical interpretation to exploit the comprehensive mechanistic picture of energy harvesting and charge carrier generation are discussed, especially for OPVs with bulk and multiple heterojunctions. Subsequently, the novel structural designs of the device architecture of OPVs embracing external geometry modification and intrinsic structure decoration are presented. Additionally, some approaches to further increase the efficiency of OPVs are described, including thermotics and dynamics modification methods. Finally, this review highlights the challenges and prospects with the aim of providing a better understanding towards highly efficient OPVs.

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The Binding Energy of Charge-Transfer Excitons Localized at Polymeric Semiconductor Heterojunctions

Cited by 135 publications

References 41 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

Towards High Performance Organic Photovoltaic Cells: A Review of Recent Development in Organic Photovoltaics

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