Intramolecular Charge Transport along Isolated Chains of Conjugated Polymers:  Effect of Torsional Disorder and Polymerization Defects

Grozema, Ferdinand C.; Duijnen, Piet Th. van; Berlin, Yu. A.; Ratner, Mark A.; Siebbeles, Laurens D. A.

doi:10.1021/jp021114v

Cited by 196 publications

(221 citation statements)

References 32 publications

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“…The importance of this inter-ring torsion on the total reorganization energy was evaluated by Hutchison et al (2005) for a series of heterocycles (including oligothiophenes). Grozema et al (2002) also studied theoretically the effect of the inter-ring torsion on the hole mobility along polythiophene and polyphenylenevinylene chains. The lower mobilities found experimentally in polythiophene with respect to those in polyphenylenevinylene were attributed to larger deviations from planarity in the former system (Grozema et al 2002).…”

Section: Introductionmentioning

confidence: 99%

“…Grozema et al (2002) also studied theoretically the effect of the inter-ring torsion on the hole mobility along polythiophene and polyphenylenevinylene chains. The lower mobilities found experimentally in polythiophene with respect to those in polyphenylenevinylene were attributed to larger deviations from planarity in the former system (Grozema et al 2002).…”

Section: Introductionmentioning

confidence: 99%

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Hole-vibronic coupling in oligothiophenes: impact of backbone torsional flexibility on relaxation energies

Filho

Coropceanu

Fichou

et al. 2007

Phil. Trans. R. Soc. A.

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Density functional theory calculations together with highly resolved gas-phase ultraviolet photoelectron spectroscopy have been applied to oligothiophene chains with up to eight thiophene rings. One of the important parameters governing the charge transport properties in the condensed phase is the amount of energy relaxation upon ionization. Here, we investigate the impact on this parameter of the backbone flexibility present in oligothiophenes as a result of inter-ring torsional motions. With respect to oligoacenes that are characterized by a coplanar and rigid backbone, the torsional flexibility in oligothiophenes adds to the relaxation energy and leads to the broadening of the first ionization peak, making its analysis more complex.

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Hole-vibronic coupling in oligothiophenes: impact of backbone torsional flexibility on relaxation energies

Filho

Coropceanu

Fichou

et al. 2007

Phil. Trans. R. Soc. A.

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“…Using the formalism introduced in Ref. [21] we can simulate the conductivity of a charge along a polymer chain. This model is based on the tight-binding approximation combined with static torsional disorder -deviations from planar alignment of the chaindetermining the effective conjugation length of the polymer chain.…”

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confidence: 99%

Efficiency of Exciton and Charge Carrier Photogeneration in a Semiconducting Polymer

et al. 2004

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We determine the efficiencies for the formation of excitons and charge carriers following ultrafast photoexcitation of a semiconducting polymer (MEH-PPV). The simultaneous, quantitative determination of exciton and charge photoyields is achieved through subpicosecond studies of both the real and the imaginary components of the complex conductivity over a wide frequency range. Predominantly excitons, with near-unity quantum efficiency, are generated on excitation, while only a very small fraction ( < 10 ÿ2 ) of free charges are initially excited, consistent with rapid ( 100 fs) hot exciton dissociation. These initial charges are very short lived, decaying on subpicosecond time scales. DOI: 10.1103/PhysRevLett.92.196601 PACS numbers: 72.80.Le, 71.35.Aa, 73.50.Gr, 73.61.Ph Since their discovery in the 1970s, semiconducting conjugated polymers have received considerable interest owing to their potential in technological applications, particularly in electronics [1]. These materials have many advantages over conventional semiconductors: They are low cost, easy to process, lightweight and malleable, and have shown significant promise in lightemitting diodes [2], photovoltaics, and laser optics [3]. Despite their widespread optical applications, the nature of the photoexcitation physics in these materials is currently subject to intense debate [4 -6]. One of the key questions that has remained controversial is whether, initially upon photoexcitation, excitons or charge carriers are primarily formed. From photoconductivity measurements [7], it is evident that free charges are formed, but the mechanism and efficiency of free charge formation remains polemical: Some studies suggest that excitons (Coulombically bound electron-hole pairs) are the primary photoexcitation product, which may dissociate into free charges after migration to electron (or hole) accepting defects, by absorption of a second photon or by bimolecular exciton-exciton annihilation [8,9]. Other studies suggest electron-hole pairs as the direct and predominant initial excitation product [10]. This apparent contradiction may be related to the selective sensitivity of different experimental approaches: For example, photoconductivity (PC) measurements [7] are sensitive only to free charges (responsible for real conductivity), whereas transient absorption (TA) and stimulated emission (SE) measurements [11][12][13] can probe excitons (bound electron-hole pairs responsible for imaginary conductivity). However, the simultaneous detection of the real and imaginary conductivity with sufficient time resolution is necessary to allow a quantitative comparison of both free and bound charges upon photoexcitation. The relatively recent technique of terahertz time domain spectroscopy [14] (THz-TDS) provides this opportunity.In this Letter we present the first investigation of a semiconducting polymer using THz-TDS. This technique allows us to monitor the evolution of free and bound charges on subpicosecond time scales following photoexcitation, through the time-depen...

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“…On longer time scales (in the ps to ns range), an estimate of this accuracy can be made by comparing mobilities measured in the microwave ͑30 GHz͒ region 27 to those predicted by the model at the same frequency. 25 From this comparison, it is apparent that the error in the model due to additional nontorsional defects is approximately a factor of 10, though this is certainly much smaller on the subpicosecond time scales, on which the THz radiation is emitted, where defects will play a much less important role. Hence, it is clear that only a small fraction of photoexcitations in the polymer results in the ultrafast generation of free charges.…”

Section: Fig 1 Electric Fields Radiated From (A)mentioning

confidence: 97%

“…23 and 25, respectively, plotted in The time-dependent mobility of the MEH-PPV sample is calculated following the formalism in Ref. 25. This model is based on the tight-binding approximation combined with static torsional disorder-i.e., deviations from planar alignment of the polymer chain determining its effective conjugation length.…”

Section: Fig 1 Electric Fields Radiated From (A)mentioning

confidence: 99%

Ultrafast charge generation in a semiconducting polymer studied withTHzemission spectroscopy

et al. 2004

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We study the ultrafast charge generation in a semiconducting polymer (MEH-PPV) by measuring the radiated THz field after photoexciting the biased polymer with a femtosecond visible pulse. The subpicosecond temporal characteristics of the emitted wave reflects the ultrafast photoconductivity dynamics and sets an upper limit for charge generation of 200 fs following photoexcitation, and reveals the dispersive nature of charge transport in MEH-PPV. A comparison of the fields radiated from MEH-PPV and the well-characterized model semiconductor system ͑GaAs͒ allows for an accurate estimate of the quantum efficiency for charge generation in the polymer, found to be less than 1%. Both observations are consistent with ultrafast charge generation in semiconducting polymers through hot exciton dissociation. Semiconducting conjugated polymers have recently received much interest owing to their potential in technological applications, particularly in electronics. 1 Despite this fact, the fundamentals of the photoexcitation physics in these materials have remained controversial: there is ongoing debate whether a band structure description is appropriate to these materials. 2,3 In this picture, interband excitations directly generate free charges, which may relax to lower energy bound states. This strongly contrasts the exciton model, in which molecular excitons are formed upon photoexcitation, so that secondary processes such as exciton-exciton annihilation 4 or hot exciton dissociation 5,6 are required to generate free charges.Although the observation of significant photoconductivity 7 demonstrates the presence of free charges, it is unclear whether carriers are generated directly on ultrafast time scales or produced on longer time scales through, for example, exciton-exciton annihilation. Unfortunately, conventional transient photoconductivity measurements 8,9 lack the resolution to resolve the ultrafast excitation process. Using time-domain THz spectroscopy, 10 we have recently shown that photoexcitation of the polymer poly(2-methoxy-5-(2Ј-ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV) leads to the generation of charges within 300 fs. 11 Additional information about the time scale of charge carrier generation and nature of the transport can be obtained by probing the radiation dispersed by the accelerating charges. 12,13 When changes in photocurrent occur on (sub) picosecond time scales, THz radiation is emitted. The shape of the THz pulse is determined by the rise and decay of the photocurrent, and thus provides a direct probe of the ultrafast transient photoconductivity. In this manner, information regarding the rate of charge carrier generation and time-dependent mobility can be obtained. This technique is ideally suited to the study of charge generation and cooling on sub-picosecond time scales, and has been successfully applied to study the photoexcitation dynamics in materials such as low-temperature grown GaAs. 13 Previous measurements on semiconducting polymer systems have also proven useful, 12 but suffered from...

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Intramolecular Charge Transport along Isolated Chains of Conjugated Polymers: Effect of Torsional Disorder and Polymerization Defects

Cited by 196 publications

References 32 publications

Hole-vibronic coupling in oligothiophenes: impact of backbone torsional flexibility on relaxation energies

Hole-vibronic coupling in oligothiophenes: impact of backbone torsional flexibility on relaxation energies

Efficiency of Exciton and Charge Carrier Photogeneration in a Semiconducting Polymer

Ultrafast charge generation in a semiconducting polymer studied withTHzemission spectroscopy

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