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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