Polaron-pair generation in poly(phenylene vinylenes) (original) (raw)
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Photoexcitation in poly(p-phenylene vinylene)
Synthetic Metals, 1989
We contrast the generation and decay of charged and neutral photoexcitations in oriented samples of poly(p-phenylene vinylene). Charge separation is the result of interchain electron motion and is detected through measurement of photoinduced absorption (PA). In contrast, photoexcitation within a single chain results in a short-lived neutral polaron-exciton which is observed by its luminescence if it decays radiatively. We fred big differences between the bipolarons detected in the PA experiments (which are extensively delocalized along the chain and are weakly confined) and the polaron-excitons (which show very strong confinement and a small Stokes' shift in luminescence).
Physical Review B, 2000
We have performed in the present work time-resolved experiments on poly͑3-dodecyl-thiophene͒ ͑P3DDT͒ and poly͑2,5-dioctyloxy-p-phenylene vinylene͒ ͑OO-PPV͒ films by directly probing the formation of charge carriers responsible for the cw photoconductivity within the time domain of-10 ps to 1 ns. Laser light pulses of 400 nm wavelength, 150 fs width, induced photoconductivity in a sample with a frequency 1 kHz. Red 800 nm light pulses delayed in respect to blue ones were revealed to affect the photoconductivity. The effect of the second pulses increased with the delay time. Red light induced changes of the photoconductivity were positive in OO-PPV, and negative in P3DDT. These results are rationalized as an evidence of delayed not immediate formation of free charge carriers. The carriers seem to be formed within 10 ps after the pumping pulse. A mechanism of formation of free polarons from polaron pair is suggested, which has permitted to explain main feature of the results including different signs of the effect of the red light in different polymers.
Synthetic Metals, 1996
Recent optically detected magnetic resonance studies of poly (p-pbenylene vinylenes) (PPYs) and poly(3-alkyl thiophenes) (P3ATs) suggest that excitation at visible wavelengths Aex largely generates trapped polarons and polaron pairs. but UY excitation and carrier injection in LEDs generate free polarons. The photoluminescence (PL)-enhancing polaron resonance at long Acx is attributed to non-radiative recom• bination of trapped polaron pairs. which reduces their population and consequently the rate at which they non-radiatively quench singlet excitons. Interchain coupling. some defects induced by structural disorder. and sites adjacent to C 60-counterions apparently enhance the generation of these trapped polarons as well as intersystem crossing from the singlet to the triplet manifold. At short Acx a PL-quenching polaron resonance emerges. and is assigned to fusion of photogenerated free polarons to bipolarons. which also quench singlet excitons. These interpretations imply that the liB. exciton binding energy. defined as its dissociation energy to free polarons. is the difference between the absorption edge and the excitation energy at which the PL-quenching polaron resonance emerges. This difference is-0.8 eY in poly(2.5dioctoxy-p-phenylene vinylene) and poly (3-hexyl thiophene).
Intrachain photogeneration of polarons in poly(3-methylthiophene)
Synthetic Metals, 1993
We report detailed measurements of the excitation profile of the absorption spectrum due to photoexcited polarons and bipolarons in the non-degenerate ground state conjugated polymer poly(3-methylthiophene). Two new sharp peaks at 2.10 and 2.25 eV are observed. The separation of the two peaks, which corresponds to an intrachain vibration, is taken as evidence for a channel of photogeneration of polarons via an intrachain process in poly(3-methylthiophene).
Excitons, polarons, and laser action in poly(p-phenylene vinylene) films
The Journal of Chemical Physics, 2003
We have used a multitude of linear and nonlinear cw optical spectroscopies to study the optical properties of water precursor poly(p-phenylene vinylene) (PPV) thin films. These spectroscopies include absorption, photoluminescence, photoinduced absorption and their respective optically detected magnetic resonance, and electroabsorption spectroscopy. We have studied singlet and triplet excitons, polarons, and laser action in PPV films. We found that the lowest-lying absorption band is excitonic in origin. It consists of two absorption components due to a bimodal distribution of the polymer chain conjugation lengths. Electroabsorption spectroscopy unambiguously shows the positions of the lowest-lying odd parity exciton 1Bu at 2.59 eV and two of the higher-lying even-parity excitons, namely, mAg at 3.4 eV and kAg at 3.7 eV. From these exciton energies we obtained a lower bound for the exciton binding energy in PPV, Eb(min)=E(mAg)−E(1Bu)=0.8 eV. The quantum efficiency spectrum for triple...
Temperature dependence of charge carrier creation in poly(p-phenylene vinylene) [PPV]
Synthetic Metals, 1999
The photocurrent decays measured in poly@-phenylene vinylene) [PPV] from room temperature (RT) down to 77 K in the presence of air traces have been studied. We have observed long-lived decays, between RT and-200 K, which can be fitted to the well known Kohlrausch's law: i&t) = i&O) exp-(t/T)a characteristic of dispersive transport. A detailed temperature dependence study has been carried out to investigate the behavior of the exponent p and the relaxation time 7 in order to understand the origin of the observed complex transport mechanism. At lower temperature, especially below 160 K, a fast component is detected alone, where the slow one is frozen. The associated interpretation is based on a recent work of simulated yield of geminate pair dissociation in an energetically random disorder. Modulated experiments show that both components are superimposed at enough high temperature. In our model, the long-lived contribution is interpreted as being extrinsic due to dissociation of polaron pairs assisted by electronegative defects and the fast one as being intrinsic due to dissociation of excitons assisted by the intrinsic energetic disorder, created by the distribution of conjugation length in PPV.
Polaronexcitons and electronvibrational band shapes in conjugated polymers
The Journal of chemical physics, 2003
The neutral excitations in poly(p-phenylenevinylene) are studied in conjunction with the vibronic structure of the lowest optical transitions. Combining the configuration interaction of Wannier-localized electron-hole pairs with an empirical description of electron-phonon coupling, we obtain the potential energy surfaces of monoexcited states and the Condon electron-vibrational spectra in absorption and emission. The S 1 →S 0 luminescence band shape is found compatible with self-localization of S 1 within about 10 monomers, driven exclusively by electron-phonon coupling. The singlet and triplet polaron-excitons are exchange-split by about 1eV and differ substantially in terms of average electron-hole separation. ________________________ 1
We present an extensive density functional theory (DFT) study on the neutral and charged electronic excitations in oligophenylene vinylenes including lowest singlet (S 1 ) and triplet (T 1 ) excitons and positive (P þ ) and negative (P -) polarons. We investigated the vibrational and electronic properties of molecules using five different DFT functionals from pure GGA to long-range-corrected hybrids and found an explicit correlation between the spatial extent of the state and the fraction of the orbital exchange. While solvent effects are found to be negligible for neutral (S 1 and T 1 ) excitons, they play an important role for charged (P þ and P -) species. S 1 states are observed to be spatially less localized compared to the polaronic wave functions (P þ and P -). This is in contrast to the T 1 states, which exhibit more spatial confinement in comparison to P þ and Pstates.
Dynamics of Photogenerated Polarons in Conjugated Polymers
Physical Review Letters, 2004
Within a tight-binding electron-phonon interacting model, we investigate the dynamics of photoexcitations to address the generation mechanism of charged polarons in conjugated polymers by using a nonadiabatic evolution method. Besides the neutral polaron exciton which is well known, we identify a novel product of lattice dynamic relaxation from the photoexcited states in a few hundreds of femtoseconds, which is a mixed state composed of both charged polarons and neutral excitons. Our results show that the charged polarons are generated directly with a yield of about 25%, which is independent of the excitation energies, in good agreement with results from experiments. Effects of the conjugation length are also discussed.
Singlet exciton binding energy in poly(phenylene vinylene)
Proceedings of the National Academy of Sciences, 2001
The exciton binding energy (Eb) and the band gap energy (Eg) of poly(phenylene vinylene) are determined by high-resolution measurements of the photoconductivity excitation profile as a function of light polarization, applied electric field, and temperature. At high applied electric fields, a peak in the photoconductivity is observed when the sample is pumped at a photon energy just below the onset of the band-to-band -* absorption. This peak is interpreted as resulting from field ionization of a weakly bound exciton with E b Ϸ 60 meV. The binding energy is obtained from the energy of the exciton peak relative to the band edge and independently from analysis of the dependence of the exciton dissociation on field and temperature.