Bimolecular reactions of singlet excitons in tris(8-hydroxyquinoline) aluminum

We have studied singlet excited state dynamics in thin films of tris(X-hydroxyquinoline) aluminum (Alq3), a well known emitter for organic electroluminescent devices. Our experimental results show that bimolecular recombination dominates the singlet exciton decay in pristine films at high intensities, thus decreasing the photoluminescence quantum efficiency and the singlet lifetime. Because of the relatively low carrier concentration at which light emitting diodes operate the electroluminescence efficiency is not affected. The measured rate constant of singlet-singlet annihilation in AIq3 films is gamma(ss)=(3.5+/-2.5)x10(-11) cm(3) s(-1). The diffusion coefficient and the diffusion length of singlet excitons estimated from gamma(ss) are D-s=(1.2+/-0.8)x10(-5) cm(2) s(-1) and L=98+/-40 Angstrom, respectively. (C) 1996 American Institute of Physics.