Time Resolved Fluorescence Quenching and Carrier Generation in Titanyl Phthalocyanine (TiOPc)

We studied the influence of electric field and humidity on photoconductivity and fluorescence in particles of Y-form TiOPc dispersed in polyvinyl butyral (PVB) polymer matrix. Increasing relative humidity from 0% to 42% at room temperature lead to a decrease of the total fluorescence by about 45%. Fluorescence time decays were analyzed by fitting the data to a sum of two exponentials. Qualitative features of fluorescence decay were the same for low and high humidity levels. The fast fluorescence component, assigned to mobile singlet excitons, shows a decrease in both its amplitude (“amplitude quenching”) and lifetime (“rate quenching”) decrease with increasing electric field. The slow component of fluorescence shows mostly electric field induced amplitude quenching and is assigned to extrinsic fluorescence from a trapped exciton. The relative intensities of the two fluorescence components, fast and slow, are in agreement with a theory which describes energy transfer between free and trapped exciton states. These results indicate that carrier generation in Y-TiOPc originates from both relaxed and non-relaxed intrinsic excited singlet states, while the trapped excitons do not lead to significant carrier production. The presence of water strongly influences the amplitude quenching by the electric field of the fast fluorescence component. This indicates that the decay probability of non-relaxed precursor excitons is influenced by water present in the sample. We propose that the non-relaxed excitons are charge transfer excitons which have been previously observed in electroabsorption measurements. Relaxed excitons, which emit fast component of fluorescence, are then identified as first excited singlet states which do not have significant charge transfer character. The overall picture of carrier generation in Y-TiOPc therefore encompasses three different processes: sensitization of carrier generation by water molecules, field assisted dissociation of charge transfer excitons, and finally field assisted dissociation of first excited singlet state excitons.