Meta stable excitations in conjugated organic systems

Abstract

This thesis reports the properties of meta stable excitations such as triplet excitons occurring in light emitting conjugated polymers by means of subnanosecond time- resolved luminescence and transient absorption investigations. Triplet diffusion and energy relaxation within a prototypical conjugated polymer has been studied by observing the bimolecular annihilation delayed fluorescence dependent on temperature, excitation dose and concentration. The results are analyzed employing the concept of dispersive hopping in a Gaussian distribution of states. After pulsed excitation the triplet migration is initially dispersive accompanied by a rapid energy relaxation. The subsequent classical regime is described by a time-independent diffusion constant and a invariable average triplet energy. Further, it is clearly found that triplet hopping in conjugated polymers is both an intra- and interchain process. Both the singlet-to-triplet generation branching ratio and the recombination layer thickness in a working polyspirobifluorene device are determined by a relative comparison of the singlet and triplet densities after electrical and optical excitation. The singlet generation rate is a field independent universal constant, the value of which agrees with the quantum-statistical 1:3 limit. Evidence is provided that the charge carrier recombination does not occur homogeneously throughout the device. Instead a certain exciton generation layer exists which, for polyspirobifluorene, is about 6 nm thick. Also singlet excitons are quenched by charge carriers. The latter might be formed after triplet- triplet annihilation in a sequential process, which involves the formation of an intermediate geminate pair prior to the full charge carrier dissociation. Finally, a new type of intramolecular delayed fluorescence is observed in the nanosecond time domain in luminescent polymers and also found to occur in a large number of conjugated organic systems. This luminescence is isoenergetic to the prompt fluorescence and shows the same excitation energy dependence, but rapidly decays with an algebraic law. In dilute solutions the overall emission quantum yield is a fraction of a percent only. Since all potential origins, including geminate pairs and triplet-triplet- annihilation, cannot describe the experimental results, the existence of a novel type of indirect singlet excitation is proposed.