Singlet exciton interactions in solid films of Alq3

The relative fluorescence yield of vacuum-evaporated films of tris-(8-hydroxyquinolinato) aluminum (III) (Alq3) as a function of steady-state exciting light intensity has been measured. The yield decreases, due to singlet exciton–exciton interaction, with increasing intensities, to a half of the low-intensity yield at 5 × 1021 ph/cm2/s. The second order singlet–singlet annihilation rate constant is found to be γSS = (1.1 ± 0.5) × 10−10 cm3/s. If the reaction is diffusion-limited, this rate implies an average singlet exciton diffusivity DS = (6 ± 2) × 10−5 cm2/s, and permits to rule out singlet–singlet interaction as the reason for the observed large-current density roll-off in the electroluminescence efficiency of Alq3-based light emitting diodes.