585 research outputs found
Theory of Exciton Migration and Field-Induced Dissociation in Conjugated Polymers
The interplay of migration, recombination, and dissociation of excitons in
disordered media is studied theoretically in the low temperature regime. An
exact expression for the photoluminescence spectrum is obtained. The theory is
applied to describe the electric field-induced photoluminescence-quenching
experiments by Kersting et al. [Phys. Rev. Lett. 73, 1440 (1994)] and Deussen
et al. [Synth. Met. 73, 123 (1995)] on conjugated polymer systems. Good
agreement with experiment is obtained using an on-chain dissociation mechanism,
which implies a separation of the electron-hole pair along the polymer chain.Comment: 4 pages, RevTeX, 2 Postscript figure
Temperature and Field Dependence of the Mobility in Liquid-Crystalline Conjugated Polymer Films
The transport properties of organic light-emitting diodes in which the
emissive layer is composed of conjugated polymers in the liquid-crystalline
phase have been investigated. We have performed simulations of the current
transient response to an illumination pulse via the Monte Carlo approach, and
from the transit times we have extracted the mobility of the charge carriers as
a function of both the electric field and the temperature. The transport
properties of such films are different from their disordered counterparts, with
charge carrier mobilities exhibiting only a weak dependence on both the
electric field and temperature. We show that for spatially ordered polymer
films, this weak dependence arises for thermal energy being comparable to the
energetic disorder, due to the combined effect of the electrostatic and thermal
energies. The inclusion of spatial disorder, on the other hand, does not alter
the qualitative behaviour of the mobility, but results in decreasing its
absolute value.Comment: 9 pages, 8 figures, submitted to Phys. Rev.
The size of electron-hole pairs in pi conjugated systems
We have performed momentum dependent electron energy-loss studies of the
electronic excitations in sexithiophene and compared the results to those from
parent oligomers. Our experiment probes the dynamic structure factor
S(q,omega)and we show that the momentum dependent intensity variation of the
excitations observed can be used to extract the size of the electron-hole pair
created in the excitation process. The extension of the electron-hole pairs
along the molecules is comparable to the length of the molecules and thus maybe
only limited by structural constraints. Consequently, the primary
intramolecular electron-hole pairs are relatively weakly bound. We find no
evidence for the formation of excitations localized on single thiophene units.Comment: RevTex, 3 figures, to appear in Physical Review Letter
Theory of Electric Field-Induced Photoluminescence Quenching in Disordered Molecular Solids
The dynamics of excitons in disordered molecular solids is studied
theoretically, taking into account migration between different sites,
recombination, and dissociation into free charge carriers in the presence of an
electric field. The theory is applied to interpret the results of electric
field-induced photoluminescence (PL) quenching experiments on molecularly doped
polymers by Deussen et al. [Chem. Phys. 207, 147 (1996)]. Using an
intermolecular dissociation mechanism, the dependence of the PL quenching on
the electric field strength and the dopant concentration, and the time
evolution of the transient PL quenching can be well described. The results
constitute additional proof of the distinct exciton dissociation mechanisms in
conjugated polymer blends and molecularly doped polymers.Comment: 4 pages RevTeX, 3 Postscript figure
A theoretical investigation of the low lying electronic structure of poly(p-phenylene vinylene)
The two-state molecular orbital model of the one-dimensional phenyl-based
semiconductors is applied to poly(p-phenylene vinylene). The energies of the
low-lying excited states are calculated using the density matrix
renormalization group method. Calculations of both the exciton size and the
charge gap show that there are both Bu and Ag excitonic levels below the band
threshold. The energy of the 1Bu exciton extrapolates to 2.60 eV in the limit
of infinite polymers, while the energy of the 2Ag exciton extrapolates to 2.94
eV. The calculated binding energy of the 1Bu exciton is 0.9 eV for a 13
phenylene unit chain and 0.6 eV for an infinite polymer. This is expected to
decrease due to solvation effects. The lowest triplet state is calculated to be
at ca. 1.6 eV, with the triplet-triplet gap being ca. 1.6 eV. A comparison
between theory, and two-photon absorption and electroabsorption is made,
leading to a consistent picture of the essential states responsible for most of
the third-order nonlinear optical properties. An interpretation of the
experimental nonlinear optical spectroscopies suggests an energy difference of
ca. 0.4 eV between the vertical energy and ca. 0.8 eV between the relaxed
energy, of the 1Bu exciton and the band gap, respectively.Comment: LaTeX, 19 pages, 7 eps figures included using epsf. To appear in
Physical Review B, 199
Ab-initio calculation of the electronic and optical excitations in polythiophene: effects of intra- and interchain screening
We present an calculation of the electronic and optical excitations of an
isolated polythiophene chain as well as of bulk polythiophene. We use the GW
approximation for the electronic self-energy and include excitonic effects by
solving the electron-hole Bethe-Salpeter equation. The inclusion of interchain
screening in the case of bulk polythiophene drastically reduces both the
quasi-particle band gap and the exciton binding energies, but the optical gap
is hardly affected. This finding is relevant for conjugated polymers in
general.Comment: 4 pages, 1 figur
Excitons in quasi-one dimensional organics: Strong correlation approximation
An exciton theory for quasi-one dimensional organic materials is developed in
the framework of the Su-Schrieffer-Heeger Hamiltonian augmented by short range
extended Hubbard interactions. Within a strong electron-electron correlation
approximation, the exciton properties are extensively studied. Using scattering
theory, we analytically obtain the exciton energy and wavefunction and derive a
criterion for the existence of a exciton. We also systematically
investigate the effect of impurities on the coherent motion of an exciton. The
coherence is measured by a suitably defined electron-hole correlation function.
It is shown that, for impurities with an on-site potential, a crossover
behavior will occur if the impurity strength is comparable to the bandwidth of
the exciton, corresponding to exciton localization. For a charged impurity with
a spatially extended potential, in addition to localization the exciton will
dissociate into an uncorrelated electron-hole pair when the impurity is
sufficiently strong to overcome the Coulomb interaction which binds the
electron-hole pair. Interchain coupling effects are also discussed by
considering two polymer chains coupled through nearest-neighbor interchain
hopping and interchain Coulomb interaction . Within the
matrix scattering formalism, for every center-of-mass momentum, we find two
poles determined only by , which correspond to the interchain
excitons. Finally, the exciton state is used to study the charge transfer from
a polymer chain to an adjacent dopant molecule.Comment: 24 pages, 23 eps figures, pdf file of the paper availabl
Electron correlation effects in electron-hole recombination in organic light-emitting diodes
We develop a general theory of electron--hole recombination in organic light
emitting diodes that leads to formation of emissive singlet excitons and
nonemissive triplet excitons. We briefly review other existing theories and
show how our approach is substantively different from these theories. Using an
exact time-dependent approach to the interchain/intermolecular charge-transfer
within a long-range interacting model we find that, (i) the relative yield of
the singlet exciton in polymers is considerably larger than the 25% predicted
from statistical considerations, (ii) the singlet exciton yield increases with
chain length in oligomers, and, (iii) in small molecules containing nitrogen
heteroatoms, the relative yield of the singlet exciton is considerably smaller
and may be even close to 25%. The above results are independent of whether or
not the bond-charge repulsion, X_perp, is included in the interchain part of
the Hamiltonian for the two-chain system. The larger (smaller) yield of the
singlet (triplet) exciton in carbon-based long-chain polymers is a consequence
of both its ionic (covalent) nature and smaller (larger) binding energy. In
nitrogen containing monomers, wavefunctions are closer to the noninteracting
limit, and this decreases (increases) the relative yield of the singlet
(triplet) exciton. Our results are in qualitative agreement with
electroluminescence experiments involving both molecular and polymeric light
emitters. The time-dependent approach developed here for describing
intermolecular charge-transfer processes is completely general and may be
applied to many other such processes.Comment: 19 pages, 11 figure
Photoinduced absorption and photoluminescence in poly(2,5-dimethoxy-p- phenylene vinylene)
We report a study of the photoexcited states in the conjugated polymer poly(2,5-dimethoxy-p-phenylene vinylene). Photoluminescence due to radiative recombination of singlet excitons is observed at energiesjust below the onset of the pi-pi* absorption band at 2.1 eV. Photoinduced absorption at 80 K shows bands peaking at 0.68, 1.35, and 1.80 eV. The features at 0.68 and 1.8 eV are associated with the same excited state which we propose is a doubly charged bipolaron, while the third at 1.35 eV is unrelated. We assign this 1.35-eV absorption to a triplet-triplet transition of a triplet exciton. The bipolarons are long lived with significant numbers surviving in excess of 100 ms at 80 K, and have a weak temperature dependence such that photoinduced absorption is readily detectable even at room temperature. The triplet exciton has a lifetime of order 2.5 ms at 80 K but this falls rapidly at higher temperature and the response is not detected at room temperature. We contrast these results with those obtained previously for the related poly(arylene vinylene) polymers poly(p-phenylene vinylene), and poly(2,5-thienylene vinylene) and for other conjugated polymers, and draw attention to the important role played in the photophysics of these materials by neutral excited states
Ab-initio prediction of the electronic and optical excitations in polythiophene: isolated chains versus bulk polymer
We calculate the electronic and optical excitations of polythiophene using
the GW approximation for the electronic self-energy, and include excitonic
effects by solving the electron-hole Bethe-Salpeter equation. Two different
situations are studied: excitations on isolated chains and excitations on
chains in crystalline polythiophene. The dielectric tensor for the crystalline
situation is obtained by modeling the polymer chains as polarizable line
objects, with a long-wavelength polarizability tensor obtained from the
ab-initio polarizability function of the isolated chain. With this model
dielectric tensor we construct a screened interaction for the crystalline case,
including both intra- and interchain screening. In the crystalline situation
both the quasi-particle band gap and the exciton binding energies are
drastically reduced in comparison with the isolated chain. However, the optical
gap is hardly affected. We expect this result to be relevant for conjugated
polymers in general.Comment: 15 pages including 4 figures; to appear in Phys. Rev. B, 6/15/200
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