Surface reactions of singlet excitons in solid films of 8-hydroxyquinoline aluminum (Alq3)

Fluorescence excitation (FE)spectra of Al(III) 8-hydroxyquinoline solid films in contact with quartz, metal and some red-emitting dyes were measured and related to the FE spectrum recorded from the uncovered film surface. Their profiles indicate that singlet excitons are quenched by the surface and near-surface energy acceptors. The quenching efficiency dependence on penetration depth of the eciting light is a result of both diffusion of singlet excitons toward the surface and their reactions with exponential distribution of quenching centres in the films. It is possible to fit the calculated quenching vs. penetration curves to the experimental date if values of the diffusion length of singlet excitons = 30nm or the penetration depth of the quenching centres from the surface into the bulk of the order of 100-1000 nm are used. Possible origin of quenching centres and the quenching effect on quantum electroluminescence yield of organic light-emitting diodes based on Alq3 emitter are briefly discussed.