Radiative recombination mechanisms in aluminum tris(8-hydroxyquinoline): Evidence for triplet exciton recombination

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Abstract
The photoluminescence of aluminumtris(8-hydroxyquinoline) (AlQ) has been studied as a function of temperature and excitation wavelength. It was found that as the temperature and excitation energy is reduced the peak of the photoluminescence moves to longer wavelengths and broadens significantly. The photoluminescencespectra obtained at all temperatures and excitation energies can be deconvolved into three distinct peaks originating from three levels within the molecule. A rate-equation approach has been used to model the observed behavior and to obtain the relative lifetimes of the three processes responsible for the photoluminescence. From this we infer that at low temperatures and excitation energies the radiative recombination of triplet excitons is responsible for a significant amount of the photoluminescence of AlQ. It is this process which is responsible for the low energy tail seen in the photoluminescence of AlQ but which is not present in the electroluminescence.