Direct Observation of Ultrafast Field-Induced Charge Generation in Ladder-Type Poly(Para-Phenylene)

Graupner, W.; Cerullo, Giulio; Lanzani, Guglielmo; Nisoli, M.; List, Emil; Leising, G.; Silvestri, S. De

doi:10.1103/physrevlett.81.3259

Cited by 133 publications

(106 citation statements)

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“…As polaron photogeneration happens during the thermalization process, the tunneling probability is integrated over the thermalization time, where 0 -assumed to be a constantestablishes the connection between the time and energy scale. This tunneling model is in agreement with the directly determined time evolution ͑with 100 fs resolution͒ of the exciton dissociation probability in mLPPP, 42 which is one order of magnitude higher during hot exciton thermalization as compared to thermalized excitons. The following values for 0 , W, and lͱm e * are extracted from the fit obtained to both P (E) spectrum and magnitude shown in Fig.…”

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confidence: 73%

Photogeneration and recombination processes of neutral and charged excitations in films of a ladder-type poly(para-phenylene)

et al. 1999

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We introduce a version of the cw photomodulation technique, measured far from the steady state, for obtaining the quantum efficiency, , of long-lived photoexcitations in -conjugated polymers. We apply this technique to films of a ladder-type poly͑para-phenylene͒ ͓mLPPP͔ for studying the photogeneration action spectra, (E), and recombination kinetics of photogenerated neutral and charged excitations such as singlet and triplet excitons and charged polarons. Whereas the (E) spectrum for singlet excitons shows a step function increase at a photon energy, E, close to the optical gap (Ӎ2.6 eV), both triplet and polaron (E) spectra show, in addition, a monotonous rise at higher E. The rise for triplets is explained by singlet exciton fission into triplet pairs, and from a model fit we get the triplet exciton energy (Ӎ1.6 eV). For polarons this rise is modeled by an electron intersegment tunneling process. The electroabsorption spectrum is also measured and analyzed in terms of Stark shift of the lowest lying exciton, 1B u , and enhanced oscillator strength of the important mA g exciton. A consistent picture for the lowest excited state energy levels and optical transitions in the neutral ͑singlet and triplet͒ and charged manifolds is presented. From both the exciton binding energy of Ӎ0.6 eV and the singlet-triplet energy splitting of Ӎ1 eV, we conclude that the e-e interaction in mLPPP is relatively strong. Our results are in good agreement with recent ab initio band structure calculations for several -conjugated polymers. ͓S0163-1829͑99͒13531-8͔

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confidence: 73%

Photogeneration and recombination processes of neutral and charged excitations in films of a ladder-type poly(para-phenylene)

et al. 1999

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“…In order to detect small field-induced changes of ⌬T, we apply a modulated voltage to the device and reference our detection to the field modulation ͑MF͒ frequency. 11 In this way we detect only field-induced changes 12 in the ⌬T/T spectra, which we call (⌬T/T) MF . Following our nomenclature, (⌬T/T) MF ϭ(⌬T/T) F Ϫ⌬T/T.…”

Section: Methodsmentioning

confidence: 99%

“…The initial fast rise of the field-induced polaron population (PA pol ) is correlated to the singlet exciton (PA sing ) quenching within the noise level indicating a direct dissociation of singlets into polarons. 12 The field-induced triplet population (PA trip ) shows a much slower build-up dynamics, excluding a direct reaction from singlets to the triplets. Therefore, we propose a stepwise formation process: ͑1͒ breaking of hot excitons into spin-1 2 polarons; ͑2͒ nongeminate bimolecular recombination of polarons into triplet and singlet excitons with a ratio of 3:1 following the spin statistics.…”

Section: B Electric Field-assisted Pump-probe Spectroscopymentioning

confidence: 99%

Ultrafast photogeneration mechanisms of triplet states inpara-hexaphenyl

et al. 1999

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We present femtosecond pump-probe measurements, both conventional and electric field-assisted, on organic light-emitting devices based on para-hexaphenyl. The dominant triplet exciton generation mechanism is assigned to nongeminate bimolecular recombination of photogenerated, spin-1 2 polarons. This process is active within a few hundred femtoseconds after photoexcitation and involves about 20% of the initially excited states. At higher photoexcitation densities, we observe an additional triplet generation mechanism, which occurs in the 10-ps time domain, due to fusion of singlet excitons and subsequent fission into correlated triplet pairs. The latter decay on the 10 2 -ps time scale by geminate recombination. ͓S0163-1829͑99͒01721-X͔

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“…[10][11][12][13][14][15][16][17][18][19][20][21] For example, in polydiacetylene, the primary excitation is a neutral exciton with strong exciton binding energy. 10,11 In contrast, the primary excitation in poly(p-phenylenevinylene) (PPV) and its derivatives is debated to be a neutral exciton [12][13][14][15][16][17] or a charged polaron. [18][19][20][21] By contrast, chemically doped conjugated polymers, which show high electrical conductivity at room temperature, have clearly charged defect (polaron) states that are extrinsically introduced in the bandgap by chemical treatment.…”

Section: Introductionmentioning

confidence: 99%

Ultrafast Dynamics of Polarons in Conductive Polyaniline: Comparison of Primary and Secondary Doped Forms

2008

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Time-and wavelength-resolved pump-probe measurements are performed on the conductive, primary and secondary doped, forms of polyaniline in solution to investigate the relaxation dynamics of photoexcited polarons. Contrasting dynamics observed in the two forms allow investigation of electronic (and structural) relaxation of this material. Pump pulse at 800 nm photoexcites an electron from the valence to polaron band, and subsequent relaxation dynamics are probed by a white light continuum pulse. Three distinct features are observed in the primary doped sample: (1) absorption near time zero in the 900-1025 nm probe wavelength region; (2) delayed absorption from 850 to 1025 nm; (3) pronounced oscillations with frequencies of 165 and 210 cm -1 . The first two features are associated with intraband absorptions to higher-lying states in the polaron band from the initial excited and conformationally changed intermediate states. The oscillations reflect torsional motions associated with photoexcitation and relaxation. In the secondary doped material, only bleaching and stimulated emission are observed throughout the whole spectral region; neither transient absorption signals nor oscillatory dynamics are observed. Kinetic modeling is performed to establish the mechanism of relaxation. We propose that the excited polaron relaxes nonradiatively to the ground state through an intermediate state(s) with a twisted geometry. Our measurements and analysis allow us to describe the structure of the polaron bands for primary and secondary doped polyaniline; they are in the small and large polaron limits, respectively, and are consistent with a bandgap for the primary doped form and without a bandgap (i.e., metallic) for the secondary doped material.

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Direct Observation of Ultrafast Field-Induced Charge Generation in Ladder-Type Poly(Para-Phenylene)

Cited by 133 publications

References 20 publications

Photogeneration and recombination processes of neutral and charged excitations in films of a ladder-type poly(para-phenylene)

Photogeneration and recombination processes of neutral and charged excitations in films of a ladder-type poly(para-phenylene)

Ultrafast photogeneration mechanisms of triplet states inpara-hexaphenyl

Ultrafast Dynamics of Polarons in Conductive Polyaniline: Comparison of Primary and Secondary Doped Forms

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