We characterized the photoluminescence properties of an amorphous tris(8-hydroxyquinoline) aluminum (Alq3) thin film heavily doped with fac tris(2-phenylpyridine) iridium [Ir(ppy)3] at 8 K. Not only green fluorescence but also red phosphorescence from Alq3 was clearly observed, where Ir(ppy)3 plays the important role as a phosphorescent sensitizer for Alq3. The triplet energy of Alq3 was estimated to be 2.03 eV from the highest energy peak of the phosphorescence spectrum. The fluorescence intensity was proportional to the excitation power. On the other hand, the deviation from the linearity of the phosphorescence intensity to the excitation power was observed above ∼0.01W∕cm2. This nonlinear phosphorescence behavior is well explained by the simple triplet-triplet annihilation theory. It was demonstrated that the efficient triplet energy transfer from Ir(ppy)3 enables us to observe triplet-triplet annihilation in Alq3.
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Tanaka et al. (2005) studied this question.
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