Photoluminescence spectroscopy of pure pentacene, perfluoropentacene, and mixed thin films

Anger, F.; Ossó, J. O.; Heinemeyer, U.; Broch, Katharina; Scholz, Reinhard; Gerlach, Alexander; Schreiber, Frank

doi:10.1063/1.3677839

Cited by 84 publications

(130 citation statements)

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Section: S!tt!t+tmentioning

confidence: 99%

“…T T n are denoted by µ S0S1 and µ T T,T Tn , respectively. Following experimental and theoretical studies of the strong triplet excited state absorption, we take µ T T,T Tn = 2.5µ S0S1 [17,18].…”

Section: S12 Vibronic Model For Singlet Fissionmentioning

confidence: 99%

“…Recent studies propose that the `indirect' coupling, mediated by the mixing of the singlet and triplet excitations with charge-transfer excitons dominates the interaction, leading to reported coupling values in the range 0.02-0.11 eV (200-900cm -1) . 14,18,24 There is some debate over the energies of the charge-transfer states and whether they participate as actual Here we use ultrafast 2D electronic photon echo spectroscopy (2DES) to access both the electronic and coupled vibrational dynamics of SF in a series of pentacene derivatives.The unique transition-pathway selectivity and high time resolution (<15 fs) of this technique allow us to observe a number of previously experimentally inaccessible intermediates in the SF process. Employing a theoretical model, which reproduces the 2DES data, we show that vibrational modes serve to couple the 'bright' S and 'dark' TT excitons, generating a vibronic manifold of mixed singlet/triplet-multiexciton states that mediate the ultrafast SF process.…”

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Real-time observation of multiexcitonic states in ultrafast singlet fission using coherent 2D electronic spectroscopy

et al. 2015

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Abstract:Singlet fission is the spin-allowed conversion of a spin-singlet exciton into a pair of spintriplet excitons residing on neighbouring molecules. To rationalise this phenomenon, a multiexcitonic spin-zero triplet-pair state has been hypothesised as an intermediate in singlet fission. However, the nature of the intermediate states and the underlying mechanism of ultrafast fission have not been elucidated experimentally. Here, we study a series of pentacene derivatives using ultrafast 2D electronic spectroscopy and unravel the origin of the states involved in fission. Our data reveal the crucial role of vibrational degrees of freedom coupled to electronic excitations that facilitate the mixing of multiexcitonic states with singlet excitons.The resulting manifold of vibronic states drives sub-100-fs fission with unity efficiency. Our results provide a framework for understanding singlet fission and show how the formation of vibronic manifolds with a high density of states facilitates fast and efficient electronic processes in molecular systems. 2" "Singlet fission (SF) is an exciton multiplication process in organic semiconductors that allows one photogenerated spin-singlet excited state to be converted to two spin-triplet excitons.1 !The two generated spin-triplet excitons are initially correlated to form an overall spin-singlet state, making SF a spin-allowed process in contrast to intersystem crossing that involves a spin flip. For systems where the energy of the lowest lying singlet exciton (S) is close to double the energy of the triplet state (T), such as pentacene and its derivatives, SF can occur on a sub-100fs timescale with every singlet being converted to two triplets.2 SF has attracted great attention lately as it enables photovoltaic devices to overcome thermalisation losses by generating two electron-hole pairs per high-energy photon absorbed, potentially allowing single-junction devices that could beat the Shockley-Queisser limit on power conversion efficiency 3 . The first steps towards this goal have been taken with the demonstration of organic solar cells based on pentacene, that show external quantum efficiencies above 129%, the highest for any solar technology to date. 4,5 Despite advances in the experimental characterization of SF in several molecular systems 6-13 as well as extensive theoretical work, 1, 14-22 the fundamental mechanism of ultrafast SF remains unclear. In the kinetic model proposed by Merrifield and co-workers 23 ! the process can be represented as S!TT!T+TWhere: S is the lowest singlet excited singlet state, T is the molecular triplet state and T+T is a pair of fully independent T states. TT corresponds to a doubly excited pair of spin-correlated triplets, forming an overall spin singlet. The TT state, often referred to as the multiexciton state, is considered a dark state that cannot be optically populated from the ground state g, but serves as an intermediate to the formation of free independent triplets T+T.Current theoretical models for SF focus on characterising t...

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Section: S!tt!t+tmentioning

confidence: 99%

Section: S12 Vibronic Model For Singlet Fissionmentioning

confidence: 99%

mentioning

confidence: 99%

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Real-time observation of multiexcitonic states in ultrafast singlet fission using coherent 2D electronic spectroscopy

et al. 2015

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“…For pentacene, the molecular transition dipole moments are oriented along the short molecular axes, 3,46 and the applied vector components of µ m are accordingly extracted from the crystal structure. 37 The transition dipole moment operatorM acts exclusively on the electronic subspace, effectively inducing a "vertical" transition, and as such mixes vibrational states through vibrational overlap factors.…”

Section: A Linear Absorption and Excited State Admixturesmentioning

confidence: 99%

Vibronic exciton theory of singlet fission. I. Linear absorption and the anatomy of the correlated triplet pair state

Tempelaar

Reichman

2017

The Journal of Chemical Physics

116

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Recent time-resolved spectroscopic experiments have indicated that vibronic coupling plays a vital role in facilitating the process of singlet fission. In this work, which forms the first article of a series, we set out to unravel the mechanisms underlying singlet fission through a vibronic exciton theory. We formulate a model in which both electronic and vibrational degrees of freedom are treated microscopically and non-perturbatively. Using pentacene as a prototypical material for singlet fission, we subject our theory to comparison with measurements on polarization-resolved absorption of single crystals, and employ our model to characterize the excited states underlying the absorption band. Special attention is given to convergence of photophysical observables with respect to the basis size employed, through which we determine the optimal basis for more expensive calculations to be presented in subsequent work. We furthermore evaluate the energetic separation between the optically prepared singlet excited state and the correlated triplet pair state, as well as provide a real-space characterization of the latter, both of which are of key importance in the discussion of fission dynamics. We discuss our results in the context of recent experimental studies.

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“…15 Both molecules exhibit high structural order and a well-defined growth mode in pure PEN films [16][17][18][19] and pure PFP films. [20][21][22] The two compounds have been shown to form a 1:1 co-crystal with strong charge transfer and excitonic coupling, 23 but PEN and PFP crystals are not continuously miscible. 24,25 In this paper, we show that stable organic superlattice structures consisting of alternating pentacene (PEN) and perfluoro-pentacene (PFP) monolayers are formed using OMBD with in situ and real-time X-ray thickness monitoring.…”

Section: Introductionmentioning

confidence: 99%

Identification of an organic semiconductor superlattice structure of pentacene and perfluoro-pentacene through resonant and non-resonant X-ray scattering

et al. 2015

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Identification of an organic semiconductor superlattice structure of pentacene and perfluoro-pentacene through resonant and non-resonant X-ray scattering Highly crystalline and stable molecular superlattices are grown with the smallest possible stacking period using monolayers (MLs) of the organic semiconductors pentacene (PEN) and perfluoro-pentacene (PFP). Superlattice reflections in X-ray reflectivity and their energy dependence in resonant soft X-ray reflectivity measurements show that PFP and PEN MLs indeed alternate even though the coherent ordering is lost after ∼ 4 ML. The observed lattice spacing of 15.9 Å in the superlattice is larger than in pure PEN and PFP films, presumably because of more upright standing molecules and lack of interdigitation between the incommensurate crystalline PEN and PFP layers. The findings are important for the development of novel organic quantum optoelectronic devices. C

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Photoluminescence spectroscopy of pure pentacene, perfluoropentacene, and mixed thin films

Cited by 84 publications

References 43 publications

Real-time observation of multiexcitonic states in ultrafast singlet fission using coherent 2D electronic spectroscopy

Real-time observation of multiexcitonic states in ultrafast singlet fission using coherent 2D electronic spectroscopy

Vibronic exciton theory of singlet fission. I. Linear absorption and the anatomy of the correlated triplet pair state

Identification of an organic semiconductor superlattice structure of pentacene and perfluoro-pentacene through resonant and non-resonant X-ray scattering

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