Reduction of photoluminescence quantum yield by interchain interactions in conjugated polymer films

Jakubiak, Rachel; Rothberg, Lewis J.; Wan, Wai Chou; Hsieh, B.J.

doi:10.1016/s0379-6779(98)00215-x

Cited by 58 publications

(66 citation statements)

References 18 publications

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“…A 3% quantum yield is in accord with the ratio of the room-temperature PL and radiative lifetimes for the aggregate determined by Rothberg and co-workers. 46, 47 The emission from the 1.0% w/v CB-cast MEH-PPV film can then be explained as a ∼1:2 exciton/aggregate mix, and the annealed film PL would result from a ∼15:85 ratio of excitons to aggregates. The 1:2 ratio in the CB-cast film implies that ∼20% of the total emission comes from aggregates, in excellent agreement with the magnitude of the long-time tail in the timeresolved SE data presented in Figure 8.…”

Section: Discussionmentioning

confidence: 99%

“…All of the evidence presented in this paper consistently suggests that excitation of MEH-PPV films at low intensity produces a significant number of aggregates, in agreement with the recent conclusions of Rothberg and co-workers. 46,47 Given that the size of the SE tail increases with increasing aggregation in the film, the ratio of aggregate to exciton emission should be even higher in the annealed films compared to the as-cast films. Figure 10 compares the 625-nm SE dynamics of the as-cast CB film (solid curve, same data as in Figures 8 and 9) and the same film after annealing (dotted curve) at a fluence low enough that there should not be significant E-EA (8 µJ/ cm 2 ).…”

Section: Role Of Film Morphology In Exciton-exciton Annihilationmentioning

confidence: 99%

“…By comparing the relative amplitudes of the intra and interchain luminescence, Rothberg and co-workers conclude that the quantum yield for the formation of interchain species in MEH-PPV is near 50%. 46,47 All of these data still leave open the question of the role of interchain species in films of MEH-PPV. It is clear that the photophysics at high excitation intensities are much more complex in conjugated polymer films than in solution.…”

Section: Introductionmentioning

confidence: 99%

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Controlling Interchain Interactions in Conjugated Polymers: The Effects of Chain Morphology on Exciton−Exciton Annihilation and Aggregation in MEH−PPV Films

et al. 1999

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The presence of interchain species in the photophysics of conjugated polymer films has been the subject of a great deal of controversy. In this paper, we present strong evidence that interchain species do form in conjugated polymer films, and that the degree of interchain interactions can be controlled by varying the solvent and polymer concentration of the solution from which the films are cast. Thus, much of the controversy in the literature can be resolved by noting that the polymer samples in different studies had different side groups or were prepared in different ways and thus have different degrees of interchain interaction. The photoluminescence (PL) of poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene), MEH-PPV, changes both its spectral shape and quantum yield when the films are prepared from different solutions or when the morphology is varied by annealing. Increasing the amount of interchain interactions enhances the red portion of the film's PL, a result assigned to a combination of changes in the vibronic structure of the PL of the exciton and increased numbers of weakly emissive interchain species. Photoluminescence excitation spectroscopy shows that excitation to the red edge of the absorption band preferentially enhances the red emission, suggesting that the interchain species are aggregates with a distinct ground state absorption. Scanning force microscopy shows topographic features that correlate with the degree of interchain interactions, verifying that the morphology of conjugated polymer films changes with polymer concentration, choice of solvent, and spin-casting speed. Even at low excitation intensities, photooxidative damage occurs quickly in MEH-PPV films excited in air, and the rate at which damage occurs is sensitive to the packing of the polymer chains. For samples under vacuum at low excitation intensity, a long-lived emissive tail, in combination with excitedstate absorption dynamics that do not match those of the emissive species, provide direct evidence for the production of interchain aggregates. Annealing an MEH-PPV film produces a photophysical signature similar to photooxidation, implying that defects in conjugated polymer films are intrinsic and depend on the details of how the chains are packed. At higher excitation intensities, we find that exciton-exciton annihilation occurs, and that the probability for annihilation can vary by an order of magnitude depending on the degree of interchain contact in the film. Finally, we show that changing the film morphology has a direct effect on the performance of MEH-PPV-based light-emitting diodes. Higher degrees of interchain interaction enhance the mobility of carriers at the expense of lower quantum efficiencies for electroluminescence. Taken together, the results reconcile much of the contradictory literature and provide a prescription for the optimization of conjugated polymer films for particular device applications.

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Section: Discussionmentioning

confidence: 99%

Section: Role Of Film Morphology In Exciton-exciton Annihilationmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Controlling Interchain Interactions in Conjugated Polymers: The Effects of Chain Morphology on Exciton−Exciton Annihilation and Aggregation in MEH−PPV Films

et al. 1999

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“…A tightly coiled molecule allows for interactions between its closely stacked segments, thus facilitating efficient three-dimensional exciton diffusion (17). In addition, interchain stacking interactions appear to be responsible for reduction in luminescence yields and red-shifted spectra characteristic of longer conjugation in packed regions (16,19,22,23).…”

Section: ϫ10mentioning

confidence: 99%

Single chain spectroscopy of conformational dependence of conjugated polymer photophysics

Huser

Yan

Rothberg

2000

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

300

382

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Single molecule confocal fluorescence microscopy was used to perform photoluminescence spectroscopy on single, isolated molecules of the conjugated polymer poly[2-methoxy,5-(2-ethylhexyloxy)-p-phenylene-vinylene] (MEH-PPV). We show that the fluorescence from single chains of this electroluminescent polymer depends strongly on chain conformation. The time evolution of the spectra, emission intensity, and polarization all provide direct evidence that memory of the chain conformation in solution is retained after solvent evaporation. Chains cast from toluene solution are highly folded and show memory of the excitation polarization. Exciton funneling to highly aggregated low energy regions causes the chain to mimic the photophysical behavior of a single chromophore. Chains cast from chloroform, however, behave as multichromophore systems, and no sudden discrete spectral or intensity jumps are observed. These also exhibit different spectroscopy from the folded chromophores. R ecent advances in instrumentation have allowed the detection and manipulation of single molecules at room temperature (for recent reviews see, e.g., ref. 1). These techniques have led to the observation of physical effects and properties of individual molecules that could not be probed by previous ensemble-averaging studies. For example, measurements of inter-and intramolecular binding forces and elastic moduli of biological macromolecules by magnetic beads (2, 3), atomic force microscopy (4), hydrodynamic flow (5), or optical tweezers (6) have provided significant insight to the mechanical properties of DNA and proteins. Single molecule fluorescence detection has seen particular interest, because the fluorescence labeling of DNA, RNA, enzymes, and proteins allows for dynamic studies of macromolecular interactions and function, such as complexation (7). The optical visualization of these molecules, however, requires specific labeling with fluorescent dyes or the use of intercalating dyes.Single molecule studies of intrinsically fluorescent macromolecules, such as photosynthetic light harvesting complexes (8) or the green fluorescent protein (9), have exhibited interesting quantum effects. Protein molecules with several chromophores, for example, were shown to behave as a single quantum system (10). The close packing of chromophores in such proteins leads to strong interchromophore interaction that gives rise to cooperative effects such as intermittent fluorescence (11, 12) and photon-antibunching (10). Single molecule fluorescence studies of green fluorescent protein revealed similar intermittent fluorescence (9).Most remarkably, intermittent fluorescence together with the observation of discrete emission levels was also reported for single molecules of conjugated polymers estimated to consist of over 1000 chromophores (13). Another surprising result is the recent observation of efficient photoluminescence quenching of polymer chains by extremely low amounts of cationic electron acceptors (14), which provides a unique bio-probe scheme with high ...

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“…The primary photophysical signature of interchain species is the presence of a weak, red-shifted emission that is longer-lived than the single-chain exciton emission observed from dilute conjugated polymer solutions. 9,[12][13][14]17 This weak emission could result from either aggregates or excimers, both of which are expected to be lower in energy than the single-chain exciton and have long radiative lifetimes; 27 the red-shifted emission might also result from the thermally activated recombination of polaron pairs. 6,8,21 In films of some conjugated polymers, the presence of a weak absorption band that is red-shifted from that of a single chain provides evidence that interchain species (aggregates) also can form in the ground electronic state.…”

Section: Introductionmentioning

confidence: 99%

The Nature of Interchain Excitations in Conjugated Polymers: Spatially-Varying Interfacial Solvatochromism of Annealed MEH-PPV Films Studied by Near-Field Scanning Optical Microscopy (NSOM)

Schaller¹,

Lee²,

Johnson³

et al. 2002

J. Phys. Chem. B

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The nature of interchain electronic species in conjugated polymers has been the subject of much debate. In this paper, we exploit a novel near-field scanning optical microscopy (NSOM)-based solvatochromism method to spatially image the difference in dipole moment, and hence the difference in degree of charge separation, between the ground and electronic excited states of the emissive interchain species in films of poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV). The method uses NSOM to collect emission from near the surface of solid samples that are placed into contact with liquids of varying polarity. The solvatochromic spectral shifts of the interfacial luminescence are measured as a function of solvent polarity; the results are analyzed with an interfacial dielectric continuum model to determine the dipole moment of emissive excited states. Experiments performed on films of the laser dye trans-4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran (DCM) in poly(methyl methacrylate) (PMMA) demonstrate that our interfacial NSOM solvatochromic method and analysis can successfully reproduce the known dipole change of DCM upon photoexcitation. With the method calibrated, we then apply it to the interchain luminescence from the surface of thermally annealed MEH-PPV films. The interfacial solvatochromic analysis reveals that the dominant interchain species in annealed MEH-PPV films is "excimer-like", exhibiting an ∼4-7 D decrease in dipole moment upon optical excitation. In a few highly localized regions of the film (ca. 1-2 µm in diameter), however, the interchain excited state exhibits a large (∼9-13 D) increase in dipole moment upon excitation, indicative of minority interchain species with a large degree of charge separation, such as exciplexes or polaron pairs. The large variation in excited-state dipole moments observed throughout the film is suggestive of an entire family of interchain species, each characterized by a different degree of charge separation. The fact that the large-dipole interchain species are found in spatially segregated domains implies that interchain charge separation in conjugated polymer films is associated with the presence of defects. When the molecular weight of the polymer is lowered, the large excited-state dipole regions increase in spatial extent, suggesting that the defects that promote charge separation are intrinsic and may be associated with the chain ends.

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Reduction of photoluminescence quantum yield by interchain interactions in conjugated polymer films

Cited by 58 publications

References 18 publications

Controlling Interchain Interactions in Conjugated Polymers: The Effects of Chain Morphology on Exciton−Exciton Annihilation and Aggregation in MEH−PPV Films

Controlling Interchain Interactions in Conjugated Polymers: The Effects of Chain Morphology on Exciton−Exciton Annihilation and Aggregation in MEH−PPV Films

Single chain spectroscopy of conformational dependence of conjugated polymer photophysics

The Nature of Interchain Excitations in Conjugated Polymers: Spatially-Varying Interfacial Solvatochromism of Annealed MEH-PPV Films Studied by Near-Field Scanning Optical Microscopy (NSOM)

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