Generation and decay dynamics of triplet excitons in Alq3 thin films under high-density excitation conditions

We studied the generation and decay dynamics of triplet excitons in tris-( 8- hydroxyquinoline) aluminum ( Alq3) thin films by using transient absorption spectroscopy. Absorption spectra of both singlet and triplet excitons in the film were identified by comparison with transient absorption spectra of the ligand molecule ( 8- hydroxyquinoline) itself and the excited triplet state in solution previously reported. By measuring the excitation light intensity dependence of the absorption, we found that exciton annihilation dominated under high-density excitation conditions. Annihilation rate constants were estimated to be gamma(SS) ( 6 +/- 3) x 10(-11) cm(3) s(-1) for single excitons and gamma(TT) ( 4 +/- 2) x 10(-13) cm(3) s(-1) for triplet excitons. From detailed analysis of the light intensity dependence of the quantum yield of triplet excitons under high-density conditions, triplet excitons were mainly generated through fission from highly excited singlet states populated by singlet-singlet exciton annihilation. We estimated that 30% of the highly excited states underwent fission.