KEYWORDS: Excitons, Diffusion, Polymers, Monte Carlo methods, Chromophores, Modeling, Molecules, Molecular interactions, Absorption, Molecular energy transfer
We present a combined quantum-chemical and Monte Carlo approach for calculating exciton transport properties in
disordered organic materials starting from the molecular scale. We show that traps and energetic disorder are the main
limitations for exciton diffusion in conjugated polymers. An analytical model for exciton hopping in a medium of sites
with uncorellated energetic disorder gives a quantitative description on the dependence of the diffusion length to both the
energetic disorder strength and temperature. We demonstrate how traps and energetic disorder can pin down the
diffusion length in conjugated polymers to values below 10 nm.
A model of exciton quenching in disordered organic materials is formulated. The model considers the quenching as a diffusion-limited process with the diffusion rate being controlled by energetic relaxation of excitons within the inhomogeneously broadened excitonic density of states. The calculated dependence of the radiative exciton decay rate upon the trap density is used in order to fit experimental data on the trap-induced photoluminescence quenching in methyl-substituted planarized poly-para-phenylene and in alkoxy-substituted poly-phenylenevinylene.
The problem of charge carrier photogeneration in conjugated polymers is related to the question concerning the singlet excitation binding energy. Measurements of the cw- photoconduction in several conjugated polymers as a function of photon energy, electric field, and temperature under different cell geometries indicate that excitons can dissociate at an electrode as well as via sensitization in the bulk. Intrinsic photogeneration can also occur at higher photon energies via dissociation of vibrationally excited singlet excitons. Charge transport, monitored via time-of- flight signals, can be rationalized in terms of a disorder concept except for a ladder-type poly-(para-phenylene) film in which built-in disorder is weak.
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