109 research outputs found
Energy relaxation dynamics and universal scaling laws in organic light emitting diodes
Electron-hole (e-h) capture in luminescent conjugated polymers (LCPs) is
modeled by the dissipative dynamics of a multilevel electronic system coupled
to a phonon bath. Electroinjected e-h pairs are simulated by a mixed quantum
state, which relaxes via phonon-driven internal conversions to low-lying
charge-transfer (CT) and excitonic (XT) states. The underlying two-band polymer
model reflects PPV and spans monoexcited configuration interaction singlets (S)
and triplets (T), coupled to Franck-Condon active C=C stretches and
ring-torsions. Focusing entirely upon long PPV chains, we consider the
recombination kinetics of an initially separated CT pair. Our model
calculations indicated that S and T recombination proceeds according to a
branched, two-step mechanism dictated by near e-h symmetry. The initial
relaxation occurs rapidly with nearly half of the population going into
excitons ( or ), while the remaining portion remains locked in
metastable CT states. While formation rates of and are nearly
equal, is formed about twice as fast in concurrence with
experimental observations of these systems. Furthermore, breaking e-h symmetry
suppresses the XT to CT branching ratio for triplets and opens a slow CT
XT conversion channel exclusively for singlets due to dipole-dipole
interactions between geminate and non-geminate configurations. Finally, our
calculations yield a remarkable linear relation between chain length and
singlet/triplet branching ratio which can be explained in terms of the binding
energies of the respective final excitonic states and the scaling of
singlet-triplet energy gap with chain length.Comment: For IJQC-Sanibel Quantum Chemistry Symposium, 200
Theory of the singlet exciton yield in light-emitting polymers
This paper presents a possible explanation for the enhanced singlet exciton
yield in light emitting polymers. We propose a theory of electron-hole
recombination via inter-molecular inter-conversion from inter-molecular weakly
bound polaron pairs (or charge-transfer excitons) to intra-molecular excitons.
This theory is applicable to parallel polymer chains. A crucial aspect of the
theory is that both the intra-molecular and inter-molecular excitons are
effective-particles, which are described by both a relative-particle
wavefunction and a center-of-mass wavefunction. This implies two electronic
selection rules. (1) The parity of the relative-particle wavefunction implies
that inter-conversion occurs from the even parity inter-molecular
charge-transfer excitons to the strongly bound intra-molecular excitons. (2)
The orthonormality of the center-of-mass wavefunctions ensures that
inter-conversion occurs from the charge-transfer excitons to the lowest branch
of the strongly bound exciton families, and not to higher lying members of
these families. The inter-conversion is then predominately a multi-phonon
process, determined by the Franck-Condon factors. These factors are
exponentially smaller for the triplet manifold than the singlet manifold
because of the large exchange energy.Comment: To appear in Physical Review B, vol 70, 15 Oct 200
Multiphonon emission model of spin-dependent exciton formation in organic semiconductors
The maximum efficiency in organic light-emitting diodes (OLEDs) depends on
the ratio, , where () is the singlet (triplet) exciton
formation rate. Several recent experiments found that r increases with
increasing oligomer length from a value in monomers and short
oligomers. Here, we model exciton formation as a multi-phonon emission process.
Our model is based on two assertions: (i) More phonons are emitted in triplet
formation than in singlet formation. (ii) The Huang-Rhys parameter for this
phonon emission is smaller in long oligomers than in short ones. We justify
these assertions based on recent experimental and theoretical data.Comment: 8 pages, 7 figure
Self-trapping of excitons, violation of condon approximation, and efficient fluorescence in conjugated cycloparaphenylenes
Cycloparaphenylenes, the simplest structural unit of armchair carbon nanotubes, have unique optoelectronic properties counterintuitive in the class of conjugated organic materials. Our time-dependent density functional theory study and excited state dynamics simulations of cycloparaphenylene chromophores provide a simple and conceptually appealing physical picture explaining experimentally observed trends in optical properties in this family of molecules. Fully delocalized degenerate second and third excitonic states define linear absorption spectra. Self-trapping of the lowest excitonic state due to electron-phonon coupling leads to the formation of spatially localized excitation in large cycloparaphenylenes within 100 fs. This invalidates the commonly used Condon approximation and breaks optical selection rules, making these materials superior fluorophores. This process does not occur in the small molecules, which remain inefficient emitters. A complex interplay of symmetry, Ï€-conjugation, conformational distortion and bending strain controls all photophysics of cycloparaphenylenes.Fil: Adamska, Lyudmyla. Los Alamos National Laboratory. Los Alamos; Estados UnidosFil: Nayyar, Iffat. Los Alamos National Laboratory. Los Alamos; Estados UnidosFil: Chen, Hang. Boston University; Estados UnidosFil: Swan, Anna K.. Boston University; Estados UnidosFil: Oldani, Andres Nicolas. Universidad Nacional de Quilmes; ArgentinaFil: Fernández Alberti, Sebastián. Consejo Nacional de Investigaciones CientÃficas y Técnicas; Argentina. Universidad Nacional de Quilmes; ArgentinaFil: Golder, Matthew R.. University of Oregon; Estados UnidosFil: Jasti, Ramesh. University of Oregon; Estados UnidosFil: Doorn, Stephen K.. Los Alamos National Laboratory. Los Alamos; Estados UnidosFil: Tretiak, Sergei. Los Alamos National Laboratory. Los Alamos; Estados Unido
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